D E T E R G E N T ACTIOX O F SOAPS BY PAUL HENRY FALL
I. Introduction Soap is a product which has been used by the world from ancient times. Pliny, the great Roman historian, gives the earliest account of it as having been first made by the Gauls from a combination of goat’s suet and the ashes of the beech tree. The word soap is used by the prophets Jeremiah and Malachi in their writings; but it is thought that possibly they referred to the ashes of plants and other such purifying agents. I n 1 9 2 1 there were made in the United States close to 2.5 billion pounds of soap valued a t approximately 2 j o millions of dollars. Chevreul, a French chemist, raised soap making from empiricism to a scientific basis. I t is altogether proper and fitting that this honor should be claimed by a Frenchman, as France for many years was the great soap market of the world.
11. A Survey of Factors proposed by Different Investigators as entering into Detergent Action of Soaps Scientists have been far from unanimous in their explanations as to how and why soap cleans. Innumerable theories have been proposed by innumerable investigators. Those reading this article who are not interested in this phase of the problem are advised to pass on to the next heading. But for the sake of a few, who may be unfamiliar with the variety and number of proposed answers to the question of why and how soap cleans, we will give the more important explanations which have been formulated by various scientists. Berzelius’O believed it to be due to the free alkali liberated by hydrolysis of the soap, Persozll, in 1846, shared this view with Berzelius. Jevons,’* in 1878, observed very pronounced “pedesis” (now known as Brownian movement) of particles suspended in a soap solution. He thought of the soap as loosening and washing away the dirt particles, which really amounts to a description but not an explanation of the problem. K ~ l b e , ’in~ 1880, thought the cleansing action to be due to the saponification of fats present by the hydrolysis alkali, and the entrapment of dirt particles by the foam, which permitted removal of the dirt by a mechanical process. Wright,I4 assumed that the alkali formed by hydrolysis allowed contact of the water with the substance to be cleaned, that is, he thought the fact that a soap solution would “wet” oil while water alone would not was due to the alkali present in the soap solution. Hillyer’ showed the wetting of the oil to be a property of the soap itself and not the of alkali of hydrolysis. LadenburgI6 quotes Knapp‘j as claiming the cleansing action of soap to be due to the property of soap itself of easily wetting oily substances, pene-
802
P A U L HENRY FALL
trating into the capillaries of the goods and acting as a lubricant, making the tissues and impurities less adhesive to one another and in that way promoting the removal of the dirt. C h e v r e ~ l ,Berzelius,Io ~ Persoqll Knapp,ls and others emphasize the emulsifying power of soap solutions and foams toward fats. Some believed this property to be due to the alkali of hydrolysis, others attributed it to undecomposed soap. No experimental evidence was obtained for either view. Plateau" made extensive studies of substances which foam and of those which are emulsifying agents and laid the power of forming bubbles, films, and foams to two factors: high surface viscosity and low surface tension. Quincke'B shared similar views but ascribed the permanence of foam to the mixed character of the liquid, and claimed that no pure liquid would foam. Hirsch,2' in 1898, showed that fatty oils were not more readily emulsified than various other organic liquids, thus showing emulsification must be due to the soap itself and not t o any alkali present. Donna+ showed, in 1899, that emulsifying power and low surface tension (as determined by drop number) went hand in hand. Kraft,23in 1895, insisted that soaps must be in solution in order to show any detergent action. He also believed the soap to be in colloidal, rather than true solution. With only one or two exceptions all of the theories, up to 1903,of detergent action were founded on the belief that the alkali formed by hydrolysis of the soap was the active agent. At this time an excellent and luminous investigation was published by Hillyer.' He showed, as had Hirsch,*l that the emulsifying powers of soap could not be attributed to alkali produced by hydrolysis. He further pointed out that alkali did not possess the property of wetting oily material as did soap. He demonstrated, as had Donnan,22the parallelism of low surface tension and emulsification and showed that saponin emulsified through the formation of a solid surface film instead of through low surface tension. The extent of hydrolysis in soap solutions has been estimated as ranging all the way from neutrality to nearly complete hydrolysis. McBain and Martin,26and McBain and BolamZ7state that the hydroxyl ion concentration is about X/IOOO for most soap concentrations. We are now at a point in the history of the theories of detergency where i t is agreed that the cleansing action cannot be attributed to alkali. I n 1880 Hofmeister28 considered soaps as materials of colloidal nature. But no attention was paid to this suggestion of Hofmeister's until in 1895 Krafft and Wiglow23 pointed out that the more concentrated solutions of soap did not show the freezing-point lowering or the boiling-point elevation that would be expected of true solutions. They therefore stated that soap solutions were colloidal. Further impetus to the development of this colloid notion of soaps, with added proof of the fact, was given by G o l d ~ c h m i dand t ~ ~Mayer, and Schaeffer
DETERGENT ACTION OF SOAPS
803
and Terroines3 in 1908. After 1908 an innumerable number of investigators added weight to the colloid theory and it is now accepted as the main explanation of the detergent action of soaps. According to M ~ B a i n , *“the ~ chemical formulae of soaps are well ascertained, tautomerism does not occur, true reversible reproducible equilibrium is established in all solutions, and finally the definite transition from typical simple electrolyte through colloidal electrolyte to neutral colloid may be observed in all stages. This transition from crystalloid to colloid is exhibited not only in passing from salts of the lower to those of the higher fatty acids, but may be demonstrated in any one of the higher members merely upon change of temperature and concentration. I n alcohol, soaps exhibit a wholly different and much simpler behaviour. The soap here exists in the form of a simple unpolymerised electrolyte in true solution, whereas in most aqueous solutions it is of course a colloidal electrolyte.” Other factors in the detergent action, involving more or less, the colloidal properties of soap solutions have been emphasized by other investigators. Jacksonm called attention to the influence which soap exerts upon the state of subdivision of the dirt. He observed Brownian movement when he examined, under the microscope, soiled fabrics immersed in soap solution. I n I909 Springe1pointed out that all previous workers had been embued with the conception of dirt as being of a fatty or oily nature, or covered with a coating of such a material. He used lamp black which had been freed from all such material by washing with alcohol, ether, benzene, and benzene vapors. He found the detergent action of the soap quite unimpaired, and noticed that the suspensions would pass through filter paper without blackening it. He considered cleansing by soap to be due to the formation of a sorption compound, dirt and soap, in place of the sorption compound, dirt and fabric, by direct substitution. Considering emulsions formed by soaps, Donnan and Potts,B4in 1910, held that interfacial tension lowering was a n important factor in stabilization. Jacksoneo stated that lather was not necessary but was a sign of detergency. While it is true that lowering of surface tension does not always parallel foaming power (saponin produces foam but does not lower surface tension much) yet the alkali soaps lower surface tension markedly and produce foam in abundance. Stericker71 claims that suds seem to lift the dirt out of the wash liquor thereby preventing redeposition, and that they act rn a cushion in the power washer thus preventing injury to the materials being washed. Linder and Zickermann7* claim that wetting power is most closely related to lather tendency. Additions of saponin to the extent of 3y0 to all kinds of detergents tried in Berlir~-Dahlem~~ failed to increase their value. Too great a n emphasis has been laid upon the lathering power of soap. From the discussion of the last several pages it is apparent that various investigators have added to or modified the views of their predecessors in this field, and that in some cases, quite contradictory views are held. But
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PAUL H E S R Y FALL
following quite closely the outline given by McBainZ4,a summary of a number of definite factors in detergent action, as brought out in the foregoing discussion, follows:( I ) The necessity of having the soap in solution. (2) I t is essential in all cases that the soap should be in colloidal form. ( 3 ) Power of emulsification which parallels low surface tension and formation of surface films, and depends, not upon the alkali of hydrolysis, but upon undecomposed soap. (4) Wetting power which also depends upon undecomposed soap. ( 5 ) Lubrication of textures and impurities which enable the latter to be removed easily. This might be considered as the action of the soap in forming non-adhesive colloidal sorption compounds with tissue and impurities due sometimes to acid soap, but more often to soap itself and capable of remaining in stable suspension. (6) Deflocculation (peptization) of dirt particles. This is directly related to the preceding factors. ( 7 ) Foaming power (to some extent). As McBain states,24“comprehensive quantitative work is necessary to complete and coordinate the existing fragmentary wnrk in any one case. Each of these factors is capable of simultaneous determination and quantitative evaluation.” 111. Methods that have been used for measuring Detergent Action Different investigators have worked on various phases of the detergent action of pure soaps, commercial soaps and soaps with known quantities of addition agents, much more work of a qualitative nature than quantitative having been done. The methods that have been employed have naturally been intimately connected with some of the factors that have been recounted. A list of the experimental methods hitherto published, as having more or less bearing upon the question, follows:-(Outline according to McBainZ4) AIeasurement of surface tension against air hy capillary tubes or h) (I) drop numbers or by bubbling or by measuring the amount of froth produced under definite conditions. (Rnsser,” White and JIarden,’j Stericker” and others made use of such methods). 3Ieasurement of surface tension against oil or paraffin oil or benzene (2) by drop numbers or measurment of emulsification. (Krafft,76Donnan,** H i l l j ~ ~Elledpe , ~ ’ and I s h e r ~ o n d ,Briggs ’~ and Schmidt,7QBottazzi,80 Shorter and Ellingworth,81Lenher and BuelljF2Millardjs Stericker7‘ and others have made use of one or both of these methods). ( 3 ) Protective action as measured by gold numbers. (Work of this character has been carried out by Freundlich and Loeb@and Papaconstant’inouE6 and others). (4) Direct washing experiments with specially soiled clothes under controlled conditions of true temperature and concentration. (Such experiments have been carried out by Zhukov and S h e s t a k o ~ ,Stericker’l ~~ and Heermannag).
805
DETERGENT A C T I O S OF SOAPS
( 5 ) Measurement against carbon or other powders by measuring rate of sedimentation or protective action in filtration. (Qualitative work of this character was done by Spring,61while quantitative work was carried out by McBain and his co-workers=). IIcBain and his co-workers8*developed the first quantitative method for measuring detergent action. Up to the present McBain's met'hod was the only one of a quantitative nature. But the procedure he d:vised is far from satisfactory either from a commercial or a scientific point of view.
McBain's Method and it's Limitations LlcBain and his co-workers utilized the discovery of SpringB*that soap solutions were capable of carrying carbon-only that portion stably suspended-through filt,er paper. The extent to which a soap was capable of doing this was considered a measure of it's detergent action. The carbon number of a soap solution represented the number of grams of carbon carried through by one kilogram of soap solution under standard conditions. The amount of carbon carried through the filter paper was determined by either gravimetric or colorimetric means'. Because of the limit of accuracy of an ordinary chemical balance the gravimetric means of determining the peptized carbon gives a possible error for a 0.1normal solution when only I O in the carbon number of about 107~ grams of the filtrate are used. McBain used only I O grams of the filtrate. If more than ten grams are taken the time consumed in filtering and washing is prohibitive. The probability of error from incomplete washing is very great when the amount of carbon on the anslysis filter paper exceeds I O milligrams. Since I O grams of the filtrate were analyzed, this means that the method cannot be relied upon when the I O C.C. of solution contain more than I O milligrams of carbon. Such a method, requiring so high a degree of accuracy and precision plus the great zmount of time is obviously not applicable to universal use. The colorimetric method is shorter. But when the colorimetric and gravimetric methods are compared the devintion is so great as to throw both methods in doubt.
TABLE I Variation of Carbon Xumber with Concentration of Solutions of Seutral Potassium Myrisbate Soap Sormality
cc Conc.
.
Mean carbon numbers GraviColormetric imetric
1.32
0.05
0,Ij
0.21
1.96
0.075
0.47
0.32
2.59
0.IO 0 .I25
0.49
0.41 0,542
3.22
0.jq *O.OI
Difference 0.04 0.16
0.08
-
% Difference 23
5
31.9
16.3
-
7 0.30 0.29 0.30 0.01 3.4 -. 4 0 ' For details of the procedure see reference 88. This indicates the standard upon which all colorimetric readings were based. Since this was determined gravimetrically there is no difference in the two methods in this particular case.
806
PAUL HENRY FALL
The results obtained by the gravimetric method were assumed to be correct in computing the present difference. The fact that the percent difference is not constant is proof that the variation is not entirely a matter of difference in method of analysis. If the gravimetric method is assumed correct, the colorimetric varies from it by 3.4 to 31.9 percent. If the colorimetric method is assumed correct, the gravimetric varies from it by 3.3 to 46.8%. These differences would certainly place the method outside the category of scientific accuracy. I n the colorimetric method it is necessary t o weigh the filtrate from which the carbon number is determined. This is time-consuming and during the weighing some of the carbon tends t o settle out. Thus a true aliquot part of the filtrate may not always be obtained. Soap solutions have a definite color, and therefore as more concentrated solutions are used, a considerable error may be introduced in the reading of the colorimeter. McBain’sS3 method is based on a filtration process. He states, “In Spring’s experiments with lamp black and in ours with carbon black only a portion of the carbon, that stably suspended, is carried through the filter paper. R e utilize this fact and simply determine the amount of carbon blrtck carried through”. We have found this method bad for two reasons. First, there is a tendency for the carbon (or whatever form of dirt is being used) to be adsorbed by the filter paper. But also the adsorbed dirt tends to clog the openings in the filter paper, so that as filtration (with accompanying adsorption) is continued, It becomes increasingly difficult to filter the suspension and increasingly smaller amounts of carbon pass through the filter as the paper becomes more and more clogged. Finally the filtrate may be almost entirely clear (free from suspended dirt) if one continues to use the same filter paper. However, if a fresh, clean filter paper be substituted for the clogged filter, the suspended dirt will at first pass through quite easily but later with increasing difficulty and with less suspended dirt carried through as this new filter becomes clogged. (Undoubtedly this was much less noticed in the work by AlcBain and his co-workers since they filtered only about I O cc. I was trying to filter 50 cc.). That the above is really what takes place was proved by taking a good suspension of carbon in soap and by use of two or three different filter papers, finally getting a filtrate entirely free of unsuspended carbon. But the filtrate was very black due to a good suspension of carbon. Then this good suspension was poured onto a fresh filter paper. The first time it went through quite readily leaving only a very small amount of adsorbed carbon on the filter paper. Then this filtrate was again poured onto the same filter. This process of refiltering through the same filter paper was repeated several times. By the fifth or sixth time the suspension filtered very slowly, the paper became very black and the final filtrate was more translucent. I n general it was ob-
DETERGENT ACTION OF SOAP
807
served that the more concentrated the suspensions started with, the greater was the quantity of dirt adsorbed and the more rapid the clogging. A second factor, causing a still more serious error, is the clogging due to suspended carbon (or whatever form of dirt is used). This error will be introduced in any method which causes unsuspended dirt to come in contact with the filter paper. McBain’s method involves such contact for it calls for shaking the suspension one hour before filtering. I n the interim, the carbon not truly suspended will, undoubtedly, not all settle to the bottom of the tube. Hence, in pouring the suspension on to the filter some of the carbon not in true suspension will accompany that that is in true suspension and the former will clog the filter very quickly and very appreciably so that in a short time most of the carbon that was in true suspension will be retained on the filter. This has been found to be the case in a large number of experiments which I have tried. (Since McBain and his co-workers filtered such small volumes the error introduced by the clogging of the filter paper would be small, but it would be no less real). Since the accuracy of McBain’s method depends upon the use of uniform papers (as well as accurate measurement of carbon suspended) it is apparent that if the openings are changed by the material being filtered, that comparable or concordant results cannot be obtained by this method unless the clogging of the paper is the same in all cases, which it is not. McBain states,88 “Some of the erratic results were undoubtedly due to faulty filter paper since occasionally some of the carbon coming through with the filtrate settled immediately to the bottom of the test tube and a second filtration through another filter paper reduced the carbon number to the expected value.” He also states that a large number of filter papers were tested and Yo. 31 Whatman Filter Paper was finally adopted as being most porous and yet fairly uniform. h further factor against this method is indicated by McBain when he states, “Table VI gives results for various concentrations of potassium oleate and here the filter paper was folded by gentle pressure between finger and thumb, since it had been found by llessrs. Allnutt that increased pressure may cause slower filtration and give inconsistent results.” I t must be evident that, if a difference in pressure in folding the filter paper produces a noticeable or appreciable change in the amount of carbon carried through the paper, this method cannot be considered satisfactory for universal adoption. While McBain’s method appears to be the first quantitative method we have for measuring the detergent action of soap (hence a standard in terms of which the washing power of soaps can be expressed), yet it IS evident, from the above discussion, that the method proposed by him is far from satisfactory. Therefore, before attempting to evaluate different soaps or to make a study of the factors which may be involved in the detergent process, an effort to develop an accurate, rapid and more practical method seemed not only justifiable but also necessary.
808
PAUL HENRY FALL
IV. Method of Attack Dr. Bancroft suggested that if clay suspensions could be coagulated by the addition of small amounts of sodium chloride a quantitative means of measuring detergent action might be developed on this basis. The essence of such a method would be to form suspensions of clay with varying concentrations of soap solution and then determine the amount of sodium chloride which it would be necessary to add in order to cause flocculation of the clay. The suspension requiring the most sodium chloride would have the most clay in colloidal dispersion, and thus would possess the greatest detergent power. If too much salt is necessary. this method would be useless as salt precipitates soap and this would vitiate the results unless a correction for this were made. The conditions necessary for obtaining good suspensions of clay in soap solutions were first investigated. A 10% stock solution of olive oil soap was made (as received and not moisture-free) ; after this, however, all solutions were calculated on a moisture-free basis. By successive dilution the desired concentrations were obtained. j o cc of solution were shaken thoroughly in IOO cc oil sample bottles with I gram of clay and allowed to stand. The solutions ranged from 10.0 to 0.00257C. The best suspensions were obtained a t concentrations ranging from 0.625 to 0.156 percent. Above 0 . 6 2 5 7 ~ the clay settled to the bottom in a compact mass. In concentrations below an 0 . 1 5 6 7 ~ “flakiness and floating” occurred. That is, part of the clay floated on top and the remainder settled to the bottom in large flakes occupying two to three times the volume that it ordinarily would. The solutions in such bottles were perfectly clear and free from unsuspended material. These concentrations were checked and rechecked and always the region of maximum suspension lay between 0.62 j and 0 . 1 5 6 % soap concentration. Flakiness and floating was found to be a function of the dirt-soap ratio. Various amounts of clay were used ranging from 0 . 0 2 to 8.0 grams. When the amount of clay present was large in comparison to the amount of soap present, this phenomenon occurred. With so cc of a 0.1‘; soap it was observed with 2 grams or more of clay. With j o cc of a o . o j q soap it occurred with I gram or more of clay. Another poirt of note is that in no cases, not even when only 0.01 gram was used, was all of the clay taken into suspension. This points definitely t o the fact that particle size must also play a big part in the formation of suspensions of clay! only those particles of an extremely small size being peptized. Because of these facts, when considering a quantitative measure of detergent action it was necessary to use a definite amount of dirt in order for the results obtained to have any significance. When using the salt method spoken of above the following results were secured. A Absolutely no peptization of the clay was obtained when jo cc of sodium chloride solution, varying in concentiation from IO^, to 0.0002 j% were shaken with I gram of clay.
DETERGENT ACTION OF SOAPS
809
B When increasing amounts of a certain strength of sodium chloride solution were added to a good suspension of clay in soap solutions, the amount of clay suspended appeared to decrease as the amount of sodium chloride increased. Finally, on further addition of sodium chloride, the clay became flaky and some of it floated to the top of the liquid. The liquid became practically free from suspended clay. Table I1 will help to make this clear. 0.5 gram portions of clay were weighed out into large test tubes and the quantities of solutions added as shown in the Table. The contents were shaken vigorously, observed, allowed to stand over night a t room temperature and again observed. TABLE I1 Effect of Sodium Chloride on: ( I ) Suspensions of Clay in Soap Solutions; ( 2 ) Soap Solutions alone 0.5 gram portions of clay (where used). 0 . 2 5 V c olive oil soap solution. 5.0% sodium chloride solution. Room temperature. Tube
KO. 16 .I 7
18 I9 20
21 22 23
24 25
26 27
28
29
Vol. of Sol. used KaC1
Results
Clay present in the Following Good suspension for 30-40 hours 20 Clay settled to bottom in short time 0.5 Some clay still in susp. after 1 5 - 2 0 hrs. I .o About the same as S o . 18 2.0 Very little clay in suspension 3.0 Practically no clay suspended 4.0 Solution clear. Clay flaky and floating j.0 Liquid clear. Clay became flaky and floated to top almost immediately after shaking. S o Clay iresent in the Following Solution slightly opalescent. No precip. 2.0 About the same as No. 24 3.0 Opalescence increased but no precip. 4.0 Opalescence still greater. N o precip. 5.0 Milky but no precip. of soap until it had stood for a number of hours. 6.0 Precipitate of soap almost immediately. 1.0
I t was found impossible to determine the amount of clay in suspension from measuring the amount of sodium chloride necessary to precipitate the soap. This is shown to be true by the fact that 50 cc of a suspension of clay sodium chloride solution to flocculate in 0 . 2 5 7 ~soap required 4 to j cc of a 1.7~ the suspension and give flakiness and floating; with a 2 . 5 7 ~soap solution it
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PAUL HENRY FALL
was necessary to add about jo cc of the NaCl solution. But later experiments prove that a 0 . 2 5 % solution will suspend more than twice as much dirt as will a 2 . 5 % solution. I t is noted that the amount of sodium chloride necessary to cause flocculation is in direct proportion to the amount of soap present. I t is clearly seen that the amount of sodium chloride added gives a measure of the amount of soap present and tells nothing of any value ft9 to the amount of clay in suspension. C Experiments were also made in which equal volumes of soap of a given concentration were added to separate bottles containing clay and equal volumes of sodium chloride solutions of different concentrations. I n each case the results pract,ically paralleled those just reported. This was t o have been expected. The results thus obtained show that while sodium chloride can be added in sufficient quantity to flocculate the suspension before enough was added to precipitate the soap, nevertheless, the amount of salt required to flocculate the suspension is in reality a measure of the amount of soap present. Ammonium chloride, sodium acetate, zinc chloride, cadmium chloride, and sodium nitrate were all tried in place of sodium chloride. They all gave results identical with those obtained with sodium chloride. A gravimetric method was investigated with no success. This consisted of pipetting off a certain amount of the suspension and driving off the water a t temperatures below boiling to avoid spattering. The evaporators containing the suspensions were t.hen heated to constant weight. The gain in weight, minus the weight of soap present, gave the actual weight of clay suspended in a definite volume. The results obtained were very erratic and of no value.
V. Development of a Practical Method for Quantitative Measurement of Detergent Action Dirt is an all-inclusive term and may be applied to almost anything depending on the circumstances. However, it can be nicely classified into two physical states, namely, solid and liquid. We mean to confine ourselves in this investigation to the solid dirts. As all attempts a t gravimetric, colorimetric, and precipitation methods for determining solids in suspension have failed to be of any value it occurred to us that real progress might be made if we could use some dirt which could be determined quantitatively by a chemical means. Manganese dioxide was considered. Experiments with clay, lamp black, ferric oxide, and manganese dioxide showed that all four of these dirts behaved very similarly in soap solutions. K i t h olive oil soap and with sodium oleate they all showed maximum suspension in solutions of 0.62 j to 0.1 j 6 percent soap solution. This proves manganese dioxide to be similar to other dirts that have been used by various investigators. This substance is a very genuine dirt. Zsigmondy and Spear,91in discussing the work done on soaps by SpringG1mention lInO2 as an example of a substance free from fat which sticks to another surface. I t is insoluble in
DETERGENT ACTION OF SOAPS
811
water and soap solutions and cannot be washed off by pure water; but is removed easily if soap is added. I t can be analyzed for, quantitatively, by use of excess of ferrous ammonium sulphate, in the presence of sulphuric acid, and a subsequent titration with a standard solution of potassium permanganate. This is the method used in analyzing pyrolusite-the mineral MnOn.
Experimental Materzals. The materials used in the development of a quantitative method for measuring detergent action were the same as those used in the evaluation of different soaps (to be reported later) since the method and its application went hand in hand. (a) “Dzrts”. I n an effort to obtain concordant results different kinds of MnOZ were used. At the beginning of the work the stock-room supply of the oxide (commercial grade) was used. Later some C. P. MnOz from Scientific Materials Company was tried out. Then in the effort to get better results some of the stock-room supply was put through the Premier Colloid Mill using water as the suspending medium. After the dirt had settled to the bottom of the containing vessel, the supernatant water was decanted and the residue was washed five or six times by stirring and subsequent decantation of the wash water. Then by means of specially constructed apparatus the washed suspension was levigated and thus the finest particles were obtained separate from the coarser. Operation of Apparatus Shown in Fig. 1
A t the start screwclamp H is closed The big jacket tube J ( 5 cm diam.) is first filled with distilled water flowing through tube D which is connected with bottle W by means of rubber tube C. Then screw-clamp H is opened to allow aqueous suspension of NnOn from bottle S to flow down the glass tube A ( 1 3 mm diam. and 1 2 0 cm long). Bulb B rests loosely against the end of the tube A and serves to “spread out” the suspension flowing down tube A. This “spread out” suspension is met by an upward current of distilled water (also “spread out” by bulb B) which tends to carry the finest particles up toward the top of jacket J and out through the overflow tube 0. The degree of separation of fine particles from the coarser depends upon the relative rates of flow of suspension from bottle S and of distilled water from bottle W. The rates of flow can easily be adjusted by means of the screw-clamps H and K. The overfiow (suspension of fine particles) empties into a large tall cylinder. The particles usually settle out on standing over night. Then the supernatant water can be siphoned off and the wet MnO2 can be filtered by suction and dried. Small equal volumes of the some of the levigated aqueous suspension were uspd in several determinations but this proved unsatisfactory because
812
PAUL HEKRY FALL
of difficulty in controlling the amount of dirt taken for a determination. It also made difficult to control accurately the concentration of the detergent solution.
FIG.1 E. Inlet for air to keep suspension stirred up F. Loose cotton plug in glass tube. S.Aquous suspension of MnO.. W. Distilled water. H and K. Screw clamps to control rate of flow. 0. Overflow tube. A and D. Inlet tubes (the arrow abreast of A represents flow inside the tube.) 3.Glass bulb to spread out liquids. J. Jacket tube. $1. Cylinder for collecting fine particles. C. Rubber tube. L. Outlet for drawing off settled out mass of larger particles.
Finally some of this levigated aqueous suspension was filtered, dried in an oven a t 6o-7o0C, ground in a mortar, put through a 160 mesh sieve, thoroughly mixed and bottled as a stock supply. This was designated as “Batch B-I, colloid mill Illn02”. This furnished more than I O O percent more suspendible particles than an equal weight of the untreated M n 0 2 .
D E T E R G E N T ACTION OF SOAPS
813
L-nfortunately, before all of the determinations given in this work were completed, this stock supply was exhausted. But another portion of the same levigated aqueous suspension was similarly filtered, dried, pulverized, sifted and mixed and a dirt giving very similar results to Batch B-r was obtained. This was designated as “Batch B-2, colloid mill MnOzJ’. Then still later, a shipment of Kahlbaum’s “Precipitated LInO?” was received from Germany and this was used in a number of different determinations. (b) Soaps. Solutions of different soaps-olive oil soap, palm oil soap, tallow soap and sodium oleate of various concentrations were used. The analyses of the first three soaps named will be given later; the sodium oleate was a Powers Weightman product, labeled “Xeutral.” In most cases the dilute solutions were made by proper dilution of more concentrated or “stock” solutions. In other cases the solutions were made up directly from the solid soap. The “stock” soap solutions were kept in glass-stoppered bottles, the stoppers of which had been well vaselined, because of McBain’sS8indication that unless the stoppers were treated in this manner the COn from the air entered and had a deleterious effect on the soap solutions. (c) Reagents. C. P. ferrous ammonium sulfate from Scientific Materials Company and from Kahlbaum was used. Also 5 I O solutions (standardized) of potassium permanganate, KS\In04, made from C. P. crystals, obtained from the chemical store-room were used. These solutions were always allowed to stand in a dark cupboard 4 - j days after being made up, and mere then filtered through asbestos before being standardized and used in subsequent titrations. When not in use, the solutions were kept in a dark cupboard. 6 5 solutions of sulfuric acid were made from the concentrated C. P. supply.
Procedure. The method of procedure, to be described is of course a product of development, being quite different from the procedure as first attempted. I n its present form it has been found very practical and yields quite satisfactory, though not ideal, results as will be explained. (a) Making the suspensions. One gram portions of MnOz, weighed within 0 . 2 - 0 . 3 percent, are placed in separate IOO cc oil sample bottles. Then 50 cc portions of the solution, of the kind and strength whose detergent value it is desired to determine, are added to each bottle. (It has been the custom to make triplicate determinations). Since most of the determinations are C than that of the room, the bottlesmade at temperatures I ~ O - ~ O O higher six at a time-of dirt and detergent solution are placed in a metal container (a gallon can provided with a tight-fitting cover) lined with heat-insulating packing and covered with asbestos. The container, with its contents, is laid in a motor-driven, horizontal, shaking machine and shaken the standard time-five minutes-at an average rate of 130 shakes per minute. At the end of this period of shaking the bottles are removed quickly and allowed to stand a t room temperature, or, by means of spring clips which firmly grasp the necks of the bottles, the latter are freely suspended in a 40’ or 75OC thermostat. Here they remain undisturbed for a period of four hours.
814
PAUL HENRY FALL
(b) Separatzon of true suspenszon for analysts. At the termination of the four-hour period, foam, if present, is dissipated by means of an electrically heated spiral of nichrome wire. Then by means of a special capillary siphon tube, 43 mm. depth of suspension, as measured from the surface, is sucked over into a large test tube connected to the delivery end of the bent siphon tube. By means of a threaded collar, which slips over the exposed portion of the siphon tube, the latter is adjustable to any desired vertical distance within a range of 2-3 cm. With this arrangement the intake (bottom of the capillary siphon tube) is always adjusted to be exactly 43 mm. from the surface of the suspen+on, the total average depth of which is 65-68 mm. Thus the intake of the siphon tube is 2 2 - 2 5 mrn. from the bottom where the unsuspended dirt has settled. When suction is applied to the test tube, all of the suspension for a depth of 43 mm, as measured from the surface, is sucked over into the test tube.
Description of Fig. 2. Tightly fitted into cork K is a female-threaded brass tube L. H is a slightly smaller brass tube with male threads to match threads in L. H is just large enough to permit the glass siphon tube (capillary) S to be moved up or down as desired. By means of the collar J which rests on the brass tube H, S can be adjusted and fixed at any height desired, since J has a set screw. The threads on H and L permit vertical adjustment of S for 2-3 cm. without resetting collar J. Cork K fits tightly into chimney C, which is wide a t the base and large enough throughout to slip over a I O O cc oil sample bottle B, which contains the suspension to be separated from unsuspended material a t the bottom. Burned into the siphon tube S at exactly 43 mm. from the bottom end (intake) is a small piece of glass, A, which acts as a permanent mark. Closely adhering to the outer walls of the siphon tube S and fastened to it, are fine, insulated copper wires, W W , which terminate in platinum points PI and PB. The latter extends 3-4 mm. below mark A while the end of PI is exactly even with mark A. The wires W W pass through tiny holes in cork K and connect in series with a dry cell and galvanometer. T is a large test tube which fits over the two-holed rubber stopper containing the delivery end of siphon tube S and the bent glass tube R. By means of rubber tubing the latter is connected with a Chapman suction pump through a small separatory funnel F, which acts as a safety trap. Operation of Apparatus shown in Fig. 2. Chimney C is slipped over the bottle B while siphon tube S and the closely adhering and fastened wires W W pass through the mouth of the bottle. (It is best to have H screwed up high enough so that mark A comes above the surface of the suspension, when the bottom of the chimney rests on the table top and collar J rests on H). After S is inserted H is turned down until P, just touches the surface of the liquid. This is determined by observing the galvanometer. Since soap solution conducts the electric current, the instant PI touches the surface the circuit is completed (since PZreached
815
DETERGENT ACTION O F SOAPS
the surface before PI) and the galvanometer deflects. This gives a very sensitive and accurate means of determining when S is “set” so that the intake is exactly 43 mm. from the surface of the suspension. Furthermore, the chimney C becomes covered with “fog” when siphoning hot solutions and this makes it impossible to see whether mark A is exactly at the surface. Since the galvanometer is very sensitive, foam must be expelled for it oon-
II
R -
I
Apparatus used for separating true suspension from unsuspended material.
ducts sufficiently to cause the galvanometer to deflect before the wire PI reaches the surface of the liquid. When the siphon tube is “set”, glass stop-cock M in the separatory funnel F is opened and suction left on until no more suspension comes over into the test tube T. This happens when the surface of the liquid in the bottle comes below the intake of the siphon tube. The stop-cock M is closed, tube T and bottle B are removed and others put in their places. The method is rapidsuspensions can be separated at the rate of one per minute. The volume of true suspension so obtained varies between 33-38 cc depending on the cross section of the bottle. And thus easily, quickly, and completely, true suspen-
816
PAUL HENRY FALL
sion to be analyzed is separated from dirt not in true suspension. The efficacy of this method for separating true suspension from unsuspended dirt will be discussed later. (c) Determznatzon of the amount of X n 0 2 present. The suspension in the test tube is shaken to insure uniform distribution of suspended particles. Then, by means of a pipette, 2 5 cc of the homogeneous suspension are removed and added to a 500 cc. Erlenmeyer flask containing a known amount, but more than enough, ferrous ammonium sulfate (dissolved in 6N H2SOa) to react with the Rho2present in the 2 j cc of suspension. (It has been the custom t o use 2 5 - 5 0 cc of a sulfuric acid solution of the ferrous salt, made by dissolving 40 grams of the salt in enough 6K acid to make one liter of solution. Hence each 2 j cc of the solution contains one gram of the dissolved salt). The soap reacts with some of the acid giving a fatty acid, and perhaps some insoluble iron soap, and the MnOn is reduced by the ferrous iron, forming, in the presence of the sulfuric acid, manganese sulfate and ferric sulfate. I t is not necessary t o heat the mixture (checked by comparative tests) as is the custom when analyzing pyrolusite, because the suspended MnOe is in such a finely divided state that it readily reacts with the ferrous salt. The mixture is diluted to about 2 5 0 cc and the excess ferrous salt present is determined by titrating with N,/Io K M n 0 4 solution. In some cases, as will be pointed out later, it may be necessary to filter off the fatty acid before making the titration with the K M n 0 4 . A blank is run using strengths and volumes of ferrous iron solution and detergent solution equal to that used when determining the amount of MnOz in the suspension. (d) Calculatzon of results. Then we have the simple relation.-cc N IO K M n 0 4 for blank nitnus cc N,/Io KAlnOc for suspension equals cc W r o K M n 0 4 equivalent to the amount of MnOA present in z j cc of suspension. Since one cc of W I OK M n 0 4 is equivalent to 0.434j centigrams of hInO2, the number of cc. of X / I OK M n 0 4 equivalent to the M n 0 2 in 2 5 cc. of suspension multiplied by 0.4345 gives the number of centigrams of MnOz present in that volume, and this value, when niultiplied by 40 gives the centigrams of M n 0 2 suspended in one liter. (e) Expresston of results. The centigrams of MnO2 suspended in one liter is taken as the “ M n 0 2value’’ of the detergent under the conditions employed. This amounts to practically the same as calling the “Mn02 value” the number of centigrams of MnO. suspended by one kilogram of soap solution under the specific conditions. This is true because, according to ~McBain,*~ “all soap solutions have approximately the same density as water of the temperature, and this even in extreme concentration”; and because the very minute increase in volume due to suspended MnO2 is negligible. The use of centigrams eliminates the use of decimals that would be necessary if the values were expressed in grams.
DETERGEXT ACTION OF SOAPS
Discussion of the Method and Factors affecting Accuracy
At different times in its development, where MnOp was the “dirt” used, the method was put to practical test in the evaluation of some commercial soaps. But time and again disappointment was the writer’s portion because, with a given solution, concordant results were not obtained. I n attempting to locate and eliminate the trouble and to obtain concordant results, many experiments were made. Most of these will be dealt with in the following topical discussion of various factors involved in the method. I t should be stated, however, that most of these factors would have been involved using any ot’her solid dirt; hence they were not peculiar to MnOz. (a) Reliability of Analysis. When once the suspension is separated from unsuspended dirt, the method of determining the amount of MnO2 in suspension, by use of excess ferrous ammonium sulfate in acid solution and subsequent titration with S / r o KMn04, is very reliable. Using widely differing strengths of suspensions and different kinds of MnOt and different soap solutions, this has been checked no less than a dozen times. The analyses of equal volumes of the same suspension always checked within 0 . 2 - 0 . 2 5 cc of N / I O KMn04. This is equivalent to an error of less than one percent in the volumes dealt with. This also shows that the provoking discrepancies could not be attributed to grossly faulty technique-pipetting the solutions, making titrations, etc. That no “side reactions” were taking place which would vitiate this method of analyzing for h h O p suspended by soap was shown by ( I ) determining the effect of various quantities of soap on the tit’ration value of a definite amount of ferrous ammonium sulfate by the KhlnO4 solution; ( 2 ) determining the effect of various quantities of soap on the titration vaIue of a definite quantity of MnOn, using one gram of ferrous ammonium sulfate and the stock solution of K > l n 0 4 .
For the first experiment, exactly one-gram portions of the ferrous ammonium sulfate were weighed out into separate 500 cc Erlenmeyer flasks. To each was added jo cc of 6K H&Oa and then 50 cc of either pure water or some soap solution of known concentration. Then the solutions were heated to boiling, diluted to about 250-300 cc. and titrated with KR/In04 solution until a faint pink tinge was apparent throughout the mass of the solution. In some cases, where soap concentration was high, the pink color soon disappeared, but usually the end-point was sharp, though not permanent. Results obtained from ( I ) are given in Table 111. This shows that a t soap concentrations under one weight-volume percent (this means that where less than 0.5 grams of soap are present, as j o cc of solution were used) the soap and K M n 0 4 react to such a small extent that it is negligible. Fortunately, concentrations in which we are interested from a washing standpoint all lie beneath one percent. 4 t increased concentrations there is sufficient soap present so that the unsaturated compounds in it react with an appreciable amount of KMn04.
818
PAUL HENRY FALL
TABLE I11 Effect of Various Quantities of Soap on the Titration Value of One Gram of Ferrous Ammonium Sulfate with KMnOl I gram portions of ferrous ammonium sulfate. 50 cc portions of water or soap solutions. Solution of KMnO,, slightly stronger than N/Io. Kind of Soap Percent Conc cc KMnOr to give end point Pure water 22.4 Palm Oil 0.2 22.5 Tallow 0.2 22.5 Sodium Oleate 0.2 22.4 Olive oil 0.2 22.4 Olive oil 0.j 22.j Olive oil 1.25 22.8 Olive oil 2 .j 23.2 Olive oil 5.0 24.0 Olive oil 10.0 2 6 . o End point vanished quickly
As a blank is run on the soap solution and ferrous ammonium sulphate any error that might creep in from the above is eliminated. But when dealing with soap concentrations of around 10% the end-point vanishes so quickly as to make it difficult to tell when to stop the titration. To eliminate this the precipitated soap should be filtered off before adding the KMn04. ( 2 ) By a series of simple experiments it was proved beyond all doubt that the MnOs does not in any way react with the soap. Table IV is included to substantiate the above statement.
TABLE IV Effect of Various Quantities of Soap on the Titration Value of 0.1 Gram Portions of MnOp in the Presence of One Gram of Ferrous Ammonium Sulfate, using K M n 0 4 0.1 gram portions of C. P. Mn02. j o cc portions of pure water or olive oil soap. Percent concentration
Pure water Pure water
Cc IihInOa for end-point 22.4
4.05
0.05
4.00
0.01
3.95
0.02
3.90
0.04 0.6
4.05
0.8 1.0
( S o MnOp was present here)
DETERGE1“-T ACTION OF SOAPS
819
I t was also demonstrated that there was no impuyity in the M n 0 2 which was reacting with the ferrous salt or K M n 0 4 . Therefore, it has been experimentally demonstrated that this method of analyzing for the hlnOz, in the presence of the soap that peptizes it, is reliable and practical, giving a rapid and easy method of determining the amount of “dirt” that a given soap will suspend (peptize or deflocculate) under standard conditions. (b) Method of separatzng T r u e Suspensions from Unsuspended Dart. Filtering the unsuspended dirt from the suspended dirt (which goes through the filter paper) was found to be unsatisfactory no matter how it was executed. Allowing the entire 50 cc to drain through or collecting the first I O cc as did ~ in McBain, gave poor results. Some experiments showed I O O ~difference the amount of MnOz carried through the paper from similarly treated and prepared suspensions. These differences were in all probability caused by the fact that it was impossible to wash the filter paper entirely free of adsorbed LlnOz which should pass into the filtrate were it not adsorbed by the paper fibers. This in itself would cause erroneous results, but as more and more was adsorbed the pores of the filter paper became clogged and kept back more of the suspended material. There is also a possibility that some of the smallest particles, which had settled out during the period of standing were washed through the filter, provided the soap solution started with was strong enough so that dilution by the wash-wate: did not immediately reduce the soap concentration below a value where it was affective in carrying h h 0 2 through the filter. Possibly the MnOa behaves much worse than the carbon black used by hfcBain; but we found lamp black just as “unruly” as 311nOz. Separation by centrifuging and subsequent decantation and filtration were tried with no marked improvement in the results obtained. I n laundry practice as well as for a method of analysis, we are interested in the amount of dirt which a given soap solution can peptize or deflocculate or stably suspend, and not in how much dirt it will rarry through a given filter paper. Therefore, the method of separation by siphoning off the suspension from unsuspended dirt was attempted. The method went through various stages of development but in the form adopted it is simple, easy to duplicate by any other worker, easy to operate; it is rapid, equally applicable a t different temperatures and gives good results as regards separation of true suspension from unsuspended dirt. At first equal aolumes of suspension were removed from the different bottles by having the intake of a siphon tube always at the same distance from the bottom of the bottle. Later several experiments were tried in which the intake was always a t the same distance from the top of the bottle (by using a 2 5 cc pipette in a one-holed large cork fixed at a definite position on the stem of the pipette; this cork rested on ‘.he “lips” of the bottle when 2 5 cc of the suspension were being drawn off). These were merely simple means of having the intake at approximately the same distance from the surkce of each separate suspension. But in the course of the work one cause of failure
PAUL HESRY FALL
820
to get concordant results was found to be due to some differences in cross sections of the IOO cc oil sample bottles used. The narrower the bottle, the greater will be the total depth of suspension for equal volumes of liquid. Since M n 0 2 is gradually settling out (the rate, according to Stokes's Law, being dependent on the size of the particles) the total amount of M n 0 2 remaining in suspension, after the lapse of a definite period of time, will be greater, the greater the total depth of the suspension, even though esactly similar suspensions were used at the beginning. T.4BLE
Showing that the Amount of MnOz remaining in Suspension in Equal Volumes of a Homogeneous Suspension, varies with the Cross Section of the Container j o r c portions of a homogeneous suspension. All stood over night ( 1 5 - 1 z hrs.) at room temperature. A11 were filtered at end of period of standing. z j cc portions of each filtrate analyzed for MnO?. Values given represent the amount of NnO?, expressed in equivalents of X/ I O KILlnOa solution Experiments nere run in triplicate. Container used 500
Total depth of suspension in mrn
cc bottle
cc bottle (oil sample)
IOO
64 6;
25
cc expressed
Average
4.3 3.7 4.5
68
Long narrow tubes
hIn02 suspended in in cc I i l l n O r .
4.2
6.4
6.7 6.6j
562 532
11.3 10.6
528
10.6
6.6
1: 0 . 8
This is strikingly illustrated by the following experiment where admittedly exaggerated conditions obtained. h liter of a suspension was made by using 20 grams of RInO2 and 1000cc of 0.4 percent solution of olive oil soap. The mixture was well shaken and allowed to stand for about an hour. Then about 800 cc were siphoned off from the settled-out material. This was made homogeneous, before removing small aliquot parts, by vigorous shaking immediately preceding the removal of each aliquot part. 50 cc portions of this homogeneous suspension were placed in long narrow tubes or bottles of various sizes. After standing over night, each portion was filtered and then z j cc of each filtrate were analyzed for quantity of MnOz present, using the method already described. I t was necessary to separate true suspension from unsuspended material by means of filtration because the wide difference
82 I
D E T E R G E S T ACTION O F SOAPS
in total depth of equal volumes of liquid made it impossible to siphon off equal tdunies of any appreciable value, by having the siphon intake the same distance from the surface of each separate suspension. The results are made clear in Table T. Hence the removal of equal L'alumes, whether the siphon intake be placed always at the same distance from the surface of the suspension or from the bottom of the bottle, will not give concordant results, if the bottles differ appreciab1)- in cross section. But if we remove all of the suspension for a definite depth, measured from the surface down, we shall be dealing with equal concentrations, provided the suspensions were similar at the start. Then by subsequent analysis of equal volumes of the siphoned-off suspensions we shall obtain concordant results for like suspensions and comparable results for dissimilar suspensions. This is the procedure used in the evaluation of diffrrrnt detergents, to be given later. It ought to be added here that any method requiring the suspensions to stand for a definite period of time, (however the separation of true suspension from unsuspended material is effected) must be a similar procedure unless the T . . I T ~ ~ in ~ P which the suspensions stand are equal in cross section or the dirt used is composed of exceedingly fine particles Jvhich settle out very very SlOl\-l!..
TABLE VI Showing that the Amount of NnO? stably suspended (peptized or deflocculated) increases as Time of Shaking increases I gram portions of Kahlbaum Precipitated or Ratch B-2 colloid mill JInO?. io cc portions of 0 . 4 ~ ; olive oil soap solution. -411 inirtures stood at room temperature four hours after shaking. Siphon method used for separating true suspension from unsuspended material. Values giren represent amount of LInO? in 2 j cc of suspension, expressed in cc S I O Ii31n04.
d l e t h o d and Tiwe of Shaking Colloid Mill M n 0 2 Hand Machine IO
sec.
IO
min.
Hand IO
see.
24,4
26.4
30.6 30.2
25,4 26.6 26.9
30.7 30.3 30.3
24.5
hv. 26. j
30.4
27.4
hahlbaurn's Precipitated 1InO9 Machine 2 min. j min. I O min.
20
min. 32.0
__
26. j 26.8 26.0 26.3 26.0
29. j 29.5 29.4 29.4 28.9
30.4 30.6 30.0 29.6 29.8
31.9 32.i 31.7 31.4
24.9
26.3
29.3
30. I
31.9
25,3 2j.2
822
PAUL HENRY FALL
It is important to not,e that this method can be relied upon to yield a suspension free from unsuspended dirt, unless one is exceedingly careless and stirs up the sediment of 541102 when insert'ing the siphon tube or in the operation of drawing off the suspension. That the suspension is free from unsuspended dirt is shown by the fact that the suspension will pass through a filter paper quite readily. By washing the paper several times only the merest trace of MnO? is left adsorbed on the filter paper. As >In02 is very dense (sp. g. j + ) the unpeptized particles settle readily to the bottom. This makes it an especially valuable dirt to use for this procedure. Carbon would not be good to use as the unpeptized particles do not settle readily. (c) X e t h od of Shaking: -It first, the bottles containing dirt and detergent solutions were shaken vigorously by hand for about ten seconds. But experiments showed that the amount of dirt taken into suspension by a given detergent was increased by increasing the period of shaking. I n general, though not always, more concordant results were obtained by machine shaking for two or three minutes. These facts are well illustrated in Table VI. This suggests the importance of agitation in washing machines. Undoubtedly deflocculation of the dirt takes place more readily in machines that facilitate thorough agitation. Since the method and t'me of shaking the misture of MnOn and detergent solution affects the total amount of dirt stably suspended, the shaking had to be standardized. Shaking by machine can be done more uniformly than shaking by hand. Fire minutes was arbitrarily chosen as a convenient and suitable period of shaking, since, as the average values in the preceding table show, after this period of shaking, the amount of dirt that will be suspended, when employing a given soap and a given quantity and kind of dirt, is close to the masimuni. Also longer periods of shaking would have necessitated special methods of heating the niistures during shaking, when working with solutions at temperatures much above that of the room. With the container described on page 813 the heat lost by the mistures during the fire-minute period of shaking was negligible. (d) Tirrrc of S t a d i u g . JIcBain and his co-workers,8s allowed the suspensions of carbon to stand undisturbed 2 3 hours: then they were shaken vigorously by hand and allowed to stand a further hour before filtering. I n the present work, no more concordant results were obtained when suspensions stood I Z O , 2 4 , or 1 j . j hours than when they stood 4 hours. This latter period was chosen as suitable and convenient and made for economy in working time, as well as ease of temperature control. When shaken with pure water the >In02 all settles out, leaving a clear supernatant liquid, in much less than four hours-usually in less than one hour. Therefore when soap solutions are used, we know that any MnOa that remains suspended for four hours or more is due to the peptizing action of the soap that is present in the solution. (e) Eflect of Diferent Quaritities of Dirt. When o . j , 1 . 0 ~and 2.0 gram portions of l I n O z were treated with 50 cc portions of 0.4 percent olive oil
D E T E R G E N T ACTION OF SOAPS
823
soap and the suspensions analyzed in the usual way, the amounts stably suspended were found to be in practically the same ratio as the amounts of dirt started with. Evidently the ability of the soap to suspend the MnOz stably has not been taxed to the limit, in any of the above cases because still more MnOz is found to be suspended stably if one used 3.0 grams a t the beginning. (But when 4.0 or more grams of the MnOn are shaken with the same quantity and strength soap solution, the phenomenon of flakiness and floating occurs and practically no MnOZ remains in true suspension. This was the case when smaller concentrations of soap were shaken with one gram portions of MnOs and the three other dirts-clay, lamp black, and ferric oxide. At once one suspects that with, say, one-gram portions of the MnOz and j o cc portions of the 0 . 4 percent olive oil soap, the suspendible particles are entirely exhausted since about twice as much MnOp is stably suspended when two gram portions of the oxide are used. This was carefully checked up three or four times with different kinds of MnOs by adding fresh soap solutions to the residues left after siphoning off the suspension. In no case were the suspensible particles exhausted. For example, by three successive exposures of one gram of the colloid mill MnOs to fresh 0.4 percent soap solutions, a t least 66 percent more suspensible particles were found to be present than were suspended by the first 50 cc of the soap-that is, by the regular method of procedure. Furthermore, by successive exposures of a one gram portion of the hInOz to fresh soap solutions ( j o cc portions) there was obtained a total amount of MnOn stably suspended, equivalent to or even greater than the amount suspended by one exposure of a two-gram portion of the dirt to 50 cc of the same soap solution. One wonders why the soap does not take up a t once, that is, on the one exposure as used in the regular procedure, all of the particles it is capable of suspending. Professor Bancroft suggested-what of course seems logical and simple, once it is pointed out to one-that the small particles and the larger particles do not act independently (like a two-phase system) but that the smaller particles are influenced or held back by the larger particles., Clay was found to act in a manner similar to the l l n O s . I t is also interesting to note that McBain and his co-workersSs found a similar action with the carbon they used. They state that “the carbon residues left on the filter paper from a previous filtration were treated with a fresh sample of the original soap solution to see if any further carbon could be extracted, as there was a possibility that all the fine particles had already been taken through. However the carbon number so obtained was invariably increased many fold. For instance, instead of carbon numbers 0.56, 1.14, 0.88, 1.46, 0 . 6 j and 0.36 there were now obtained such values as 5.01, 3 . j 9 , 4.64, 4.39, 5.02 and 4.57 and a third extraction with still further soap still gave quite high results.” Using an amount of MnOp that approaches “saturation” of the given soap solution with the dirt, gave no more concordant results than smaller amounts.
824
PACL HENRY FALL
For example, three or more grams of MnOp with 50 cc of soap solution gave no better results than when one gram was used. The latter amount was retained as the standard amount for determinations made. Discussion of Phenomenon of Flakiness and Floating of the Dirt. This phenomenon has been mentioned a number of times in the preceding pages. Working with different kinds of iLIn02, the relative detergent values of different concentrations of the same soap were found to vary somewhat with tke kind of Mi102 uFeti. This was anticipakd a t the very beginning of this work; but this variation is not peculiar to Mn02 as a form of dirt. Using the “drop number” method, H i l l ~ e found r ~ ~ the relative values of soaps varied with different supplies of kerosene. Papaconstantinouse found the protective action of a given soap solution varied with different gold sols and was different for arsenious sulfide sols from that for the gold sols. With MnO? as the dirt, the difference in values between a given soap solution with two different kinds of MnOs was more pronounced at low concentrations of soap, Le., where the ratio of soap to dirt was relatively smallmuch smaller than is ordinarily met with in practice. For example, with colloid mill NnO? the detergent value of a 0.3 percent solution of olive oil soap differs but slightly from that of a 0.15 percent solution. With Iiahlbaum’s XnO, the detergent value of the 0.3 percent solution is what one would normally expect, but with the 0.1 5 percent solution practically no MnOz remains in true suspension. This is because in this latter case flakiness and floating of the dirt occurs, and whenever this happens practically no dirt remains in true suspension. This is true not only of X n 0 2 but of other solid dirts as well; e.g., clay, lamp black, and ferric oxide. When working with clay as the dirt, it was found that when the ratio of soap to clay was much less than I :zo or 2 j , no good suspensions of clay were obtained but flakiness and floating occurred. There has not been time to devote to a detailed study of this phenomenon but results obtained during the course of the work show that it depends, in general, upon the soap-dirt ratio, but more specifically upon the ratio of fine particles. The specific volume of the Kahlbaum WnOn is 2 - 3 times as great as that of the colloid mill MnO,, indicating a larger percentage of fine particles in the former. Xnd with five different commercial soaps, flakiness and floating occurred with higher concentrations of soap when using the KahlbaumMnO, than when using any other kind of l\InOs or even another kind of dirt. I t is quite possible that the phenomenon of flakiness and floating is due to the exhaustion of soap from the bulk of the solution, due to most of the soap being adsorbed by the dirt which in turn is carried to the interface. And here the dirt is apparently flocculated but instead of falling to the bottom of the bottle, it is held up at the surface of the liquid by entrapped air. Thus there is not left in the bulk of the solution sufficient soap to give an appreciable suspension of dirt. Undoubtedly the solid particles of RInOp stabilize the foam* and so the dirt floats a t the surface indefinitely. I have samples which have been standing two or three months.
* Bancroft: “Applied Colloid
Chemistry”, 364 (1926).
DETERGENT ACTION OF SOAPS
825
That the phenomenon occurs only when the ratio of soap to dirt is relrttively low favors such a theory. Furthermore that it apparently occurs more readily the higher the percentage of fine particles also lends support to the theory. For the greater the surface exposed (per gram of dirt) the greater should be the adsorption of soap, other things being equal. I t is well known that gelatine is a protective colloid quite similar in effectiveness, in a number of cases, to that of soap. And yet* “years ago Schulze pointed out that small amounts of gelatine solution were as effective as lime or alum in causing the rapid sedimentation of clay and that addition of minute quantities of gelatine to barium sulfate simplified the question of filtration and washing very much”. If larger amounts of gelatine are used the flocculation of the clay or many other substances is greatly retarded or prevented. And so we may have a somewhat similar occurrence here when the ratio of soap to dirt is small. Foaming is a factor in the phenomenon. Saponin, which foams very readily and persistently gave true suspensions of MnOp for concentrations of 0.062 to 1.0 percent using, as in the cases discussed above, one gram portions of dirt and jo cc portions of the solutions. But when the saponin-dirt ratio was about 1:60 flakiness and floating of the dirt occurred just as was found with some soap solutions. Incidentally, 0.5 to 0 . 1 2 5 percent solutions of saponin were found to stably suspend about as much MnOz as equal concentrations of soap. As will be seen in some work to be discussed a little later, temperature is also a factor in this phenomenon of flakiness and floating, which is more pronounced a t higher temperatures. Other detergents, such as silicate of soda, trisodium phosphate and sodium carbonate are capable of peptizing (stably suspending) MnOz and other dirts within certain limits of concentration or detergent-dirt ratio. If the ratio of detergent to dirt is too small, we get no true suspension, but neither do we get flakiness and floating. This is because these substances do not foam and therefore no air is entrapped to keep the dirt particles a t the top, but instead they soon settle to the bottom. The rate of settling decreases with the increase in concentration of detergent up to a certain value, and then the rate of settling increases as concentration is increased. I t may be added here that the relative detergent values of soaps are not independent of the nature of the dirt used. I n other words, specific adsorption obtains here just as in other cases of adsorption. This may be somewhat disconcerting-though not entirely unexpectedsince one cannot measure the relative or absolute detergent values of different soaps, using a given dirt, and then affirm that this relation will hold quantitatively for other dirts. But it should be remembered that qualitative experiments with four different kinds of dirt showed them to be very similar in their action with a given soap solution. What McBain8*states relative to the carbon black they used holds just as true for the MnOz used in our work.
* Bsncroft: “Applied
Colloid Chemistry”, 309 (1926).
826
PAUL HENRY FALL
NcBain says: “The action upon carbon black may be t o a certain extent specific and in so far not, quite parallel t o the effect on oily matter, or on emulsification or on frothing but this would merely limit but not destroy the value of the direct information obtained.” The fact stated above, that 0.5 to 0 . 1 2 j percent solutions of saponin (which is about the optimum concentration of soap for suspending MnOz) were found to stably suspend about as much MnOs as equal concentrations of soap, is extremely interesting. This information should be of unusual importance to the soap manufacturer for it means that he can add saponin to his soap in large quantities (1007if‘he so desires) and not decrease appreciably the suspending power of the soap. This does not mean that the resulting soap would be as good a detergent for practical purposes as a soap containing no saponin. There are various other factors to be considered, the more important being the manner in which such a soap would emulsify oils, and the effect it would have upon the skin, and the fabrics. Also there is the possibility that saponin in the presence of soap might act differently than it does alone. This, however, is not likely as it is already used in small quantities as an addition agent in soaps. At least it opens up a field for investigation which promises to be interesting and useful, and offers the soap manufacturer an “addition product” which is not a mere adulterant but’rather a substance, which if thoroughly investigated, might compare quite as favorably with soap for a detergent as the first investigations indicate. (f) Other Factors aflectiny Accuracy of the M e t h o d : For the most part, this method of determining the M n 0 2 in suspension checked itself within z or 37c. In many cases absolute checks were obtained. The slight variations obtained were to be expected as we are dealing with adsorption phenomena, and not with substances which react in stoichiometric amounts. Such being true it ~ would therefore be rather surprising to have obtained results 1 0 0 7 perfect each time. There are also t,wo other factors which may have had some bearing upon the fact that it was absolutely impossible to obtain concordant results in all cases. First, one-gram portions of the dirt may have varied in their content of suspensible particles. But certainly it would be difficult to obtain a m x e uniform dirt than the one used. Second, I n a few cases the upper limit of “settling-out’’ material was not characterized by a horizontal plane but by a wavy surface (the upper limit can be observed only in suspensions of low concentration). Thus, there is the possibility of irregular settling. h method of filtering off the unsuspended material before allowing settling to occur in order to eliminate the above, proved useless, giving results which differed by IOO‘?‘,,. (9) Advantages o j the Method here developed using JfnOZ as the Solid Dirt. It is worth while to summarize the advantages of this method of measuring detergent action and why &In02 is so well adapted for this particular problem.
DETERGENT ACTION OF SOAPS
827
( I ) As has already been stated, M n 0 2is a very genuine though not commnn form of dirt. I t forms a real smudge on the skin, clothing, wood work, etc, which cannot be removed with pure wat’er alone, but can easily be removed by the use of soap. (2) I t is insoluble in water and in soap solutions. It is not even attacked by boiling dilute nitric or sulfuric acid. I t reacts with ferrous ammonium sulfate, in the presence of sulfuric acid, to give an almost colorless solution. (3) It can be determined accurately since it reacts quantitatively with the ferrous ammonium sulfate, in the presence of sulfuric acid. The permanganate used in titrating excess ferrous ammonium sulfate is its own indicator. It gives a sharp, distinct end point, although not very permanent in the presence of soap (the decomposed soap-fatty acid). (4) The particles of MnOz that are stably suspended by the soap are small enough so that when the suspension is poured into the acid solution of ferrous ammonium sulfate, the MnOz reacts immediately and completely, without the necessity of heating the mixture. ( 5 ) It can be analyzed for in the presence of soap. This hits already been explained. (6) The acid needed for the reaction between MnOn and the ferrous ammonium sulfate and between the excess ferrous salt and K31n04 decomposes the suspension (just what is desired) and also throws the soap out of solution, forming afatty acid and perhaps a small amount of insoluble iron soap. ( 7 ) By running “blanks” this method can be used equally well with any soap of almost any concentration. The very high concentrations of soap, however, cause the end-point color to disappear rapidly, thus making it difficult to determine accurately the volume of K M n 0 4 required. But this is true only with concer. .tions of 5.0 percent and above-concentrations which are very seldom found in practical work and also concentrations which fail to suspend dirt to any appreciable extent. (8) MnOp is suspended stably by dilute solutions of silicate of soda and some other alkali salts and thus there is no difficulty in determining the detergent values of a soap just because it happens to have some such “filler” present. (9) When once the K31n04 solutions and the ferrous salt solution, as well as the soap solutions are prepared, one can make two or three score determinations without making any weighings except the one-gram portions of dirt (MnO?) started with. ( I O ) l I n O n is very cheap; it is a common laboratory chemical and the method used for its quantitative determination is rapid. ( I I ) I t has a high specific gravity (+) which proved to be a real asset in the siphon method of separating true suspension from unsuspended dirt. (12) The whole method and procedure ala simple and rapid. S o coniplicated apparatus or special technique are required. If a thrrmostat of sufficient size is available it is easy to make 36-40 determinati s or measurements in a working day. This includes the entire operation liom beginning to end.
828
PAUL HENRY FALL
( 1 3 ) The method is entirely practical and has yielded quite satisfactory results in the evaluation (relative) of different detergents. (h) A Standard for Testing. To make it possible for others to employ this method and obtain results comparable to those given by the writer, it would of course be necessary to establish a standard for testing. This has not been done because it would be very difficult to standardize the Rho2since different supplies of the oxide would vary greatly in degrees of fineness and this would cause the “MnOz value” of a given soap to vary accordingly. The method could probably best be standardized by determining thc MnOn value” of a pure soap (synthesized in the laboratory under specified and easily reproducible conditions, e.g., by the addition of the calculated quantity of sodium hydroxide required to react with a definite weight of a designated pure fatty acid). The value of this soap with a given X n O n , say the colloid mill bInO? that has been used in this work, could then be taken as the standard. Then any investigator, using the MnO? available to him, could determine the “RfnO? value” of some of this specific standard soap (whicl: he could easily synthesize), under the same conditions as used in the original standard. The bInOBavailable to him could then be used for evaluating different detergents. By multiplying the “MnO? values” thus obtained by a proper factor, (found thus: “MnOz value” of standard soap using the M n 0 2 availabl: divided by the standard “MnOg value’’ of the standard soap) the results should be fairly comparable to those obtained in the present work. Such a standard has not been made since, in the present work, no pure soap (synthesized in the laboratory) has been used. Further, with the stock supply of colloid mill M n 0 2 relative values were obtained which served the desired purpose. I n the present work the main purpose has been to develop a practical method of measuring quantitatively the detergent powers of soaps in order to make a study of fundamental fact,orsin detergent action; also to deterniine the relative detergent powers of the different soaps furnished by the Palmolive Company. So much time was consumed in developing the method that the study of different factors in detergent action has been curtailed. This study will no doubt be taken up by the next incumbent of the fellowship, who will a!so no doubt develop a standard for testing.
VI. Application of the Method to the Evaluation of Different Detergents (A). Sodium oleate and some commercial soaps. (B). Silicates of soda of different compositions. (C). Caustic soda and some alkaline salts. EXPERIMENTAL (A). Sodium oleate and some commercial soaps. Materials. (a) Dirls. In practically all of this work the colloid mill MnO? was used. In a series of determinations involving different concentrations of sodium oleate the C. P. M n 0 2 was used.
829
DETERGEST ACTION OF SOAPS
(b) Soaps. Powers-Weightman “Xeutral” sodium oleate was used. Also the Palmolive Company furnished five different kinds of commercial soaps-olive oil, palm oil, tallow, “Green Arrow” and “Green Arrow” containing some silicate of soda. The complete analyses of the last three soaps was not ascertained. The last named contained j 7 c silicate of soda of the ; 8.93Sc; Si02 ~ 7 . 8 3 7 ~ These . following composition: moisture 6 3 . ~ 4 ‘ 7 ~Xa20 two oxides are in the molecular ratio 1:3.2. The tallow soap was guaranteed to be made from Soyc tallow. A11 three were analyzed for moisture before being used, since solutions were made up on the moisture-free basis. The following analyses were reported for the olive oil soap and the palm oil soap: Determination
Moisture Titer of fatty acids Total fatty acid Total alkali as S a 2 0 Free alkali ( S a 2 0 ) Free caustic as carbonate Free fat Glycerine Unsaponifiable matter Saponification value of fatty acid Salt Filler
Olive oil soap
Palm oil soap
33 4 5 7 18 7’C 59 3 c; 6 7 %
36 9 % 44 6’C 57 43% 6 49% 0 03%
Trace 0 14% 0 2j%
0 7 % 3 2 5 I95 0
95%
&-one
0 1 %
Trace Present 1 4 % 203 0 93q
Kone
Procedure. The relative detergent values of the five commercial soaps listed on the preceding page have been determined for seven different concentrations and at temperatures of 40’ and 7 j°C. With the range of concentrations employed, the values of either side of the concentration that gives maximum suspension of dirt are made available. Comparative values a t temperatures less than 4ooC were not determined because of the low solubility a t room temperature of all but the olive oil soap. 2.0 percent solutions of each soap were carefully made up on the basis of dry [moisture-free) soap and the more dilute solutions were made from the 2.0 percent stock solutions by proper dilution. All solutions were fresh or less than a day old. Bottles and dry M n 0 2 contained therein always stood in the thermostat long enough to attain constant temperature before the soap solutions, of thermostat temperature, were added. Six bottles a t a time were shaken for five minutes and then replaced in the thermostat. After standing four hours (accurately timed) the suspensions were separated from unsuspended material and analyzed as has been described on page 816. “MnO2 17alues” of diflerent Concentrations of Sodium Oleate. Before the colloid mill M n 0 2 was obtained and while a I 5 . j hour period of standing was still employed, a quantitatiile study of the effect of concentra-
830
P a c L H E S R Y FALL
tion was made using the c‘. P. MnO? and different concentrations of the sodium oleate. The determinations were made at room temperature, about zs0C. The results are shown in Figs. 3-j. In obtaining the data for the former, soap concentrations ranging from 0.0312 to 8.0 percent were used, the more dilute solutions being made by progressive dilution of some of the 8.0 percent solution. As the curve shows, the optimum concentration of soap for maximum suspension of dirt is 0.4 percent. A negligible quantity is peptized or stably suspended in concentrations greater than 4.0 percent.
&5,!25
I
i
(
. O X 2 ?In
”i.
1.0
COW!
x 2.0
-t.D
FIG 3 Sodium oleate. C. P. 1InO . Room temp.
In order to get more specific data for concentrations in the range near the “peak” of the curve in Fig. 3, the value for concentrations ranging from o . o j to 1.0percent were determined at intervals of every 0 . 2 percent. And here the soap solutions were made by Keighing out the required amount of the dry powder to make 400 ec of each concentration given. These results are shown in Fig. 4 and here again the ‘’peak” of the curve-the optimum eoncentration-is for a 0.4 percent solution.
D E T E R G E N T ACTION OF SOAPS
83 1
These suspensions not only stood a longcr time than in the regular procedure now used, but the C. P. M n 0 2 always gave much lower “l\lnOz values” for a given soap than the colloid mill MnOz. But the relative detergent values of the different concentrations are nevertheless valuable. In Fig. 5 is shown a similar curve for tallow soap solutions, using the colloid mill MnO, and the regular procedure that was used in evaluating all the dii-ferent detergents.
FIG.4 Sodium oleate. r.P. SlnO!. Room temp
Here the actual “ l l n O z values” are much greater than those given in the first two graphs and the actual values are not comparable with those in the first two (where C. P. MnO, was used). But the similarity between the curve in Fig. 5 and that in Fig. 3 is obvious. These curves are very similar in shape t o those obtained by NcRain and his co-workers,88using carbon as the solid dirt. But, as will be emphasued later, the optimum concentrations are very different in the two casez. They found the optimum concentration about ten times greater than that iound in this work. A more thorough discussion of the ‘%In02 values” will be found, later under “Discussion of Results at 4oOC”. Results of Evaluatzon of the Fzte Commerczal Soaps. The relative “MnOn values” (centigrams of MnOz present in one liter of suspension) of the five different soaps for seven different concentrations a t 40°C. are given in Table S’II. Corresponding values a t is°C. are given in Table S’III.
PAUL H E S R Y FALL
TABLE VI1 Comparison of the ‘‘Mn02Values’’ of Various Concentrations of Five Different Commercial Soaps I gram portions of Batch B-I Colloid Mill MnOs. jo cc portions of soap solutions. Values given represent centigrams of ?\In02 present in one liter of suspension as calculated from an analysis of 2 5 cc of suspension. Values given are the average of triplicate determinations. ~i Olive 011 Soap
Conc. 7-c 2 . 0
‘$12
1.4 0.8
440 468 153
0.4 0
2
0
1
435 336 162
0.05
40°c
Tallow Soap
327 427 496 532 539 328 334
500
-FIG.5
Palm Oil Soap
302 377 420
512 5 50 335 2j8
Green Arrow Soap
387 43 5 47 5 522 520 28 j
Silicated Green Arrow
278 384 486 505 j22
323 3 60
DETERGENT ACTIOS O F SOAPS
TABLE
833
T’III
A t 75°C 2.0
322
316
299
325
282
.4
352
372
3 42
3 48
341
0.8
42 7
43 3
409
427
416
0.4
4.5 5
453
460
455
448 457
432
0 . 2
453 416
0. I
I53
322
304
250
450 261
85
123
141
I34
219
I
0.05
Discussion of Results at 40°C (a) Relzabzlzty of values. The values for all of the soaps at 0 . o j percent concentrations are not a t all reliable, because all gave flakiness and floating. When this occurs the method here used is of little value. I t is safe to assume that all of the values for the 0.05 percent concentrations, a t this temperature and with this particular MnO,, are too high. This is true because dirt, not in true suspension, that floats at the top, is mechanically carried over with the small amount of dirt that is in true suspension when the latter is removed from the bottle by use of the siphon tube. All other concentrations gave very consistent and quite concordant values. (b) Optimum concentration. With only one exception, the optimum concentration is in the range 0.2-0.4 percent, with the 0.8 percent a close second. In all cases there is a decided decrease for concentrations below 0 . 2 percentthe values for 0.1 percent solutions being 2 j-40 percent less than those for the 0.2 percent solutions. For one engaged in the business of selling soap it would perhaps be more gratifying to have the optimum concentration in the range found by ILIcBain and his c o - ~ ~ o r k e rather r s ~ ~ than the low value just reported. Using carbon black as the solid dirt, they found the optimum concentration to be about 4.45 percent. I t was stated that adsorption is specific and that two different dirts would no doubt act differently with the same detergent solution. But, as has been stated before, the four different dirts-clay, lampblack, ferric oxide, and MnOs-all come, qualitatively within the range 0.1 56 to 1 . 2 j percent of olive oil soap solution for maximum suspension. Judging by turbidity the optimum concentration for suspension of all four was very close to 0.4 percent. Therefore MnO? is not an isolated case. Further the values found by the above investigators are much higher than those found by any other investigator whose data has been available. (c) Optzmuni concentratzon by other workers. I t is interesting to note that Zhuknov and S h e s t a k o ~ found ~’ that in laundry practice the best results were obtained with 0.2 to 0 . 4 percent soap solutions.
834
PAUL HESRY F A L L
Spring,6‘ who made only qualitative sedimentation tests with lampblack. ferric oxide and potters’ clay found soap solutions of 1.0, 0 .j and 0.03 percent respectively gave most stable suspensions of the three dirts in the order named. Here specific adsorption is distinctly in evidence; but, considering all three kinds of dirt and the optimum concentrations for maximum suspension of each, the range he found agrees much better with the results here reported than with those obtained by NcBain and his co-workers.* Donnan and Potts64found an optimum concentration of S 300 ( = about 0.67, for the soap they used) in the emulsification of paraffin oil by soap solutions. Shorterg2says : “A curve showing the relation between ‘drop number’ and the concentrations of soap solution indicates that so far as surface tension is concerned, it is a waste of soap to use it at a concentration much greater than 0.4 percent.” I n a w r y elaborate and involved process for determining the cleansing effect of washing compounds, Heermann,89used 0.4 percent soap as the normal concentration for wash experiments. Briggs and Schmidtjg found 1.0 percent solutions of sodium oleate to be the optimum amount for emulsification of benzene. PapaconstantinouB6found that I .o percent sohtions of sodium oleate had slightly better protective action on gold sols than 0.1percent solutions and the latter, in turn, considerably better than 0.01percent solutions. From his results-though he gives only three perdifferent concentrations-we can deduce the rather wide range of 1.0-0.1 cent solutions for the optimum value. From what I can learn from the laundrymen here in Ithaca, they use perhaps less than 0.1 percent soap solutions. Lenher and Bue1IJ8*who worked with pure solutions of sodium oleate, invariably gave maximum found K/320 solutions (equal to about 0.17~) suspensions of M n 0 2 , and ferric oxide. With the exception of the very high value, 4 . 4 5 5 , obtained by ?vlcBain88 and his co-workers, the optimum concentrations obtained in this work are in fair agreement with those found by other investigators who have attacked the problem in different ways. This very fact strengthens the validity of the method here developed and used, and increases the value of the results obtained.“ (d) Similarity of all F i r e S o a p s : At first it was quite a disappointment to find that all five soaps acted so similarly. I t seemed that a wide difference in their detergent powers would a t least be more impressive and dramatic. But when one considers that “since adsorption is essentially specific, the amount will necessarily vary with the nature of the adsorbing agent, the liquid, and the substance to be adsorbed,” it is just as dramatic and no less remarkable that all of the soaps are so very similar in their peptizing action on MnO2. The fact that all of the soaps showrd practically the same suspending power in the range of maximum suspension led t o the suspicion that perhaps the same amount of 3h02 was suspended by each soap because the suspensible particles were exhausted before some of the soaps had exerted their
* Since this was written, Vincent has been able to duplicate McBttin’s results by varying the time of standing. He will discuss the subject in his thesis.
DETERGENT ACTIOX O F SOAFS
83 j
maximum suspending power. Of course this suspicion would not arise for concentrations above or below the optimum value, since the amount suspended by the latter was always greater than for concentrations above or below it. This was checked carefully three or four times by adding fresh soap solutions to the residues left after siphoning off the suspensions, made by using the optimum concentration of soap. In no case were the suspensible particles exhausted. This has already been discussed. Since the soaps are similar at equal concentrations, it follows that the optimum concentration will be the same for all, as Tables VI1 and VI11 show (with the exception of the olive oil soap). Here, at a temperature of 4o°C, as in many other determinations made while developing and improving this method and analysis, the olive oil soap proved less effective than the other four soaps a t the optimum concentration. This was always found to be the case. hlso the solutions of sodium oleate always give values very similar to those of the olive oil soap. The four saturated fatty acid soaps form gels at ordinar>-room temperature if the concentration is much in excess of 0 . 2 perccnt. Hence there was a possibility that these four soaps might have greater relative viscosities than the olive oil soap at 4ooC. If so, this might well account for the larger amount of MnOz remaining suspended in them. However, as shown in Table IS, the relative viscosities of 0 . 4 percent solutions of the five soaps were found to be very similar at equal temperatures. For the sake of comparisons the “hInO, values“ of the solutions are included in Table I S .
TABLE IS Relative Viscosities and ‘WnOn Values” of 0.4~; Solutions of all Five Soaps at 4o°C‘ and at i 5 T . “PIIn02 values” represent centigrams of N n O z present in one liter of suspension as calculated from an analysis of 2 j cc of suspension. Relative Viscosities represent time, in seconds, for .j cc of soap solution to pass between the marks on an Ostwald Viscosimeter. Values given are the average of five determinations. Soap
0.4% Conc.
Olive Oil Tallow Palm Oil Green Arrow Silicated G. A.
“MnO2 1-alue” (centigrams)
Relative Viscosity (seconds)
4O0C
j,jT
453
453
64-4 5
532
455
jI2
453 448 43 2
66-2 5 66 66 66-3,’s
522
505
40°C
7j°C
39-1 5 39-115 39-215
39-3 5 39-3 5
As the results indicate, the difference in the ‘%In02 values’’ of the olive oil soap and the other four soaps can be attributed to difference in viscosities. By reference to Table VI1 it will be seen that the olive oil soap apparently excels a t the two highest concentrations employed. But concentrations as great as these will seldom be met with in laundry or toilet practice.
836
PAUL HENRY HALL
(e) Order of detergent power of the soaps. I t might be parenthetically added here that the reason so much time and effort were expended on improving the method of analysis, already discussed, is because the soaps proved to be so similar. This necessitated refinement of method to increase accuracy of results in order to make it possible for one to distinguish any difference between the soaps. S o doubt the four saturated fatty acid soaps differ less among t,hemselves than the limits of accuracy of the method used for measuring their detergent powers. Throughout all the different determinations that have been made, the order of detergent power at 4o°C has not always been the same for these four soaps, but they have always exceeded that of the olive oil soap. Taking the average of all the determinations that have been made, using optimum soap concentration, the order would probably be: tallow palm oil > Green Arrow > silicated Green Arrow > olive oil. But as noticed above, the difference between the first four is almost negligible. While this is true, it is doubtful if the house-wife would choose the palm oil soap because of the very slow rate at which it goes into solution. This is especially true at 4 0 T . I t dissolves much more slowly than any of the other soaps a t this temperature. Once in solution it is as good as any of the others. Here again it is instructive to compare the results with those obtained by other investigators. LIcRain and his co-workers8gused widely different soaps-much more different in composition than the five soaps here used. They say: “Once in solution there is surprisingly little difference between soaps as different as myristate (C18H?;C00fo and oleate (C,;H,,COOK)”. Rasser’l states : “Different soaps shorn the following order in cleansing power: tallow; soaps from liquid vegetable oils or olein; cocoanut, palm-kernel, and rosin soaps.” L. nher and BuellY?state, “.At 100°C sodium palmitate emulsifies olive oil m re efficiently than the oleate or stearate, thus indicating that a t 100°C it is a better detergent than oleate or stearate.” From these results by different investigators, it is evident that the order of detergent power varies u-ith the method used to determine it. But all have found the different soaps to be very similar, regardless of the methods used. (f) C o m p ~ i s o nof Eficierrcy. IVhile the optimum concentration for all of the soaps here studied is in the range 0.2-0.4 percent, yet from the standpoint of efficiency the 0.1percent solutions of soap will win every time, as can be seen from Table V I I . That is, the 0.1percent solutions are capable of suspending considerably more than one-half as much dirt as solutions of two, four, eight or even ten times this concentration. It is quite possible that the 0.05 pe ent solutions would win in an efficiency contest, but as stated on page o j ~ ,the detergent values for this particular concentration are not reliable. Hence any conclusion involving them would be of similar character. Rasser” states that the greatest cleansing efficiency is obtained with 0 . 2 to 0.4 percent soap solutions. But he is really referring to the best concentration for detergent action. This range is in strict agreement with that found here in this work.
DETERGENT ACTION OF SOAPS
83 7
Moreover, it is detergent action in which we are interested. The fact that a
0.17~ solution may suspend more in proportion to its concentration than a
0.2% solution does not warrant its use as a detergent. A 0.2 to 0.4% solution is a relatively weak solution anyway, and if it suspends more dirt than a 0.17~ it is consequently the solution to use.
Discussion of Results at 75°C (a) Relzabzlzty of oalues. The values have been given in Table VIII. At this temperature the values for the 0.1and 0.05 percent concentrations are not reliable because flakiness and floating occurred. That this phenomenon did not occur with the 0.1 percent solutions at 40°C but did a t 75OC is evidence that not only soap-dirt ratio or soap-fine-particles ratio is a factor but that temperature also is a factor in producing flakiness and floating. (b) O p t m u m concentratton and sznitlarzty of soaps. Here, as at 4ooC, the maximum suspending power of the soaps is exerted when concentrations are in the range 0.2-0.4percent, although again, the 0.8 percent solutions give only slightly less values. The 0.1and 0.05 percent solutions have detergent powers decidedly less than the 0.2 percent solutions but just how much less, cannot be determined by the values given in Table VIII, since, for these more dilute solutions, the values are not reliable. Also at this higher temperature the five soaps are very similar, with the similarity being perhaps more striking for the concentrations 0 . 2 and 0.4 percent than it was a t 40°C. (e) E f e c t of temperature. For equal concentrations of soap solution the detergent values at 7 j°C are less than at 4 0 T . This is true of all five soaps a t the seven different concentrations except the 2.0 percent solution of silicated Green Arrow. T o show that the decrease, where ?rho?is the dirt, was aot due to a more or less sintering or stable agglomeration of the M n 0 2 partic s a t this higher temperature, several bottles of the one-gram portions of dry AnOs were kept at a temperature of 7s0C for over four hours. After this they were allowed t o cool to 4ooC and suspensions were made with a detergent a t this temperature. The amount of N n O 2 suspended was practically no different from that found when the MnO? had not been warmed up to a temperature above 4ooC and the same detergent solution used. Thus the difference in detergent powers, a t different temperatures, for equal concentrations of a given soap, must be due to changes in the properties of the soap itself. This decrease in detergent power, as temperature increases, comes somewhat as % surprise until we consider some of the factors involved. Hillyer’ showed experimentally that emulsifying and wetting properties of soap solutions, both of which are factors in detergent action, must be di to soap itself. He ststes: “Low surface tension and emulsifying power are due not to the alkali or t o acid set free by hydrolysis but to undecomposed soap it self .” (d) Results by other znvestzgators. Hillyer found the “drop number” of rosin soap of all concentrations from N I O to N,/320 to be less a t 100°C than
838
PAUL HENRY FALL
in the cold. This decrease was accounted for on the basis of hydrolysis. The latter increases with temperature and this would mean less undecomposed soap for a given concentration at IOO’C. than at room temperature. And he accounted for the decl.ease in “drop number” on the basis of the decrease in amount of undecomposed soap for a given soap solution. If this be true, the results obtained by the use of NnO? as the dirt fit in very well. In the very inclusive article, “Colloid Chemistry of Soap”, McBainZ4says that soap solutions are much more colloidal at lower temperatures than a t the boiling point. He also states that the detergent action parallels the amount of colloidal soap present. Later, in actual measurement of detergent action, using a solid dirt, McBain and his co-workersB8found that increase of temperature greatly diminished the “carbon number” of the soaps they usedpotassium oleate and potassium myristate. With the latter, they found 30 percent less carbon suspended at 60°C than at zs0C. On the other hand, PapaconstantinouS6found the protective action of soaps on gold sols to increase a i t h increase in temperature. Curiously enough he makes use of the conclusions of ?rIcBainZ4to account for this action; but he gives an interpretation which has little meaning-to me a t least. He says: “XIcBain and his co-workers, from a study of the lowering in vaporpressure and the electrical conductivity, have concluded that the ionic micelles of the soap decrease in size with rise in temperature. The increase in mobility (and possibly in number) of the ionic micelles with rise of temperature together with the increase in thermal agitation of the soap mice!les and the gold particles may therefore be connected with the increase in protective action (viewed from a kinetic standpoint).” Then he adds: “If we adopt the theory of Spring, that the washing power of a soap is related t o its power of forming ‘adsorption compounds’ with the dirt particles, it is probable that the relative washing powers of soaps will be connected with their relative protective actions toward a gold sol.” Cndoubtedly there is a parallelism between the detergent powers of soaps and their protective action on gold sols, but to what limits or extent this parallelism may be said to hold, is, to some extent at least, a matter of opinion as me11 as a question requiring still more experimental evidence. We must admit that the method used by PapaconstantinouE6does not give results analogous to those obtained by Hillyer’, by McBain and his coworkersss or to those obtained in the present work. Since the measurements by Papaconstantinou did not involve any form of dirt, while those by the other three involved either a liquid or a solid dirt, it is natural to give more weight to the latter. In some emulsification experiments, Lenher and BuelP found that a definite amount of sodium oleate would emulsify more of an oil a t high ten:perature, than at low; but that the emulsions were not so stable a t the higher temperature. This would seem to lend some support to the results obtained by Papaconstantinou, but it is not in agreement with the results obtained by Hillyer who was using a liquid dirt.
D E T E R G E N T ACTION O F SOAPS
839
Based on the results herein reported we can say that, once in solution, the detergent powers or the suspending powers of all five soaps toward the dirt MnOp is better a t 40°C than a t 75°C. b7here olive oil soap was used at room temperature, the detergent power was found to be greater than at 4ooC. I n view of this work and that of some others noted above, we are justified in drawing a more general conclusion, vis., that if soaps are once in solution, their detergent powers are decreased rather than increased by rise in temperature. However, the saturated fatty acid soaps will never be in demand for cleansing agents a t room temperature, because of the slow rate a t which they go into solution as well as their low solubility a t this temperature.
Risumi of Relative Values of the Five Soaps We may summarize as follows concerning the relative detergent action of the five commercial soaps used:( I ) There is a n optimum concentration (0.2-0.47~) for which the suspending power toward MnOl is a maximum. (2) This range for optimum concentration is the same for all five soapswith the olive oil soap a slight exception. (3) The optimum concentration is the same at the different temperatures used-7 so, 40” and room ( = 2 5°C). (4) All five soaps are surprisingly similar in suspending power at the same temperature and a t equal concentrations, except the olive oil soap which is somewhat less effective than the other four soaps. ( 5 ) Increase in temperature from 40’ to 75OC diminishes the suspending power of all five soaps, an average of about 20 percent. This is equivalent to a decrease of 0.5 to 1.0percent per degree rise in temperature, a t the optimum concentration. (R) Evaluation of Diflerent Kinds of Silicate of Soda Different investigators have made various claims for the effect of “addition agents” on the detergent powers of soaps. Most, of the esperiment,al evidence has been obtained by determining the effect, on surface tension of soap solutions produced by adding a base or an alkaline salt. That “addition agent” which produces the greatest lowering of the surface tension of a given soap solution has been assumed to enhance the detergent power of the soap the most. The quant,it’ativemethod of measuring detergent action developed in this work makes possible the evaluation of these different agents alone as well as soap solutions containing them. Materials and Procedure This work shows that silicate of soda solutions act similarly t o soap in suspending RlnO? stably. Samples of this so-called soap filler of various compositions were furnished by the Philadelphia Quartz Company. The three brands used were designated by letters and had the compositions shown in Table S. The relative detergent values of nine different concentrations were determined at 40’ and at ; 5 T , in a manner exactly similar to that used in
840
PAUL HENRY FALL
evaluating the soaps. Fresh 0.5 percent stock solutions were ma& up on the basis of NanO, SiOz content. The more dilute solutions were made by proper dilution of a known volume of the 0 . j percent stock solutions. The (‘MnOz values” are given in Table XI.
TABLE X Composition of Silicates of Soda used Brand
Sp. Gr.
(IS”
“K” “BW”
%Sal0
%SiOt
6.4
24.7 31.2 30 6
I .31 1.48
11.0
1.68
19.4
I I I
Fhtio NaQO : SiOl Percent Molecular : 3 86 I :3.97 : 2 84 I :2.92 : 1.j8 I : 1.62
TABLE XI Comparison of the “MnOz Talues” of Various Compositions of Three Different Silicates of Soda a t 40’ and a t 75’C I gram portions of Batch B-2 Colloid Mill MnOz 50 cc portions of silicate of soda solutions Values given represent centigrams of MnOZ present in one liter of suspension as calculated from an analysis of 2 5 cc of suspension. These values are the average of duplicate determinations. “8” Brand “Ii” Brand “BIT” Brand Conc. 0
/O
0.5 0.3 0.1;
0.05 0.025 0.012:
0.0062
0.0031
5
0.001
40°C 292 382 396 431 445 436 427 278 36
jjcC
40°C 219
75°C
240
261 287 346 351 348 216 ooo
316 394 430 431 433 297 41
247 300 342 342 334 247
ooo
000
000
205
ooo
40°C 40 219 396 43’ 452 437 404 203 19
7j°C
61 200
300 344 348 313 227
38
ooo
Discussion of Results (a) Reliabnlzty of aulues. Except for a few erratic results for the two lowest concentrations, these values are reliable. However, they are not exactly comparable with those in Tables VI1 and VIII, pp. 832-833, sinceBatch B-z Mn02 gave values about 4.0 percent higher than Batch B-I, with the same kind and concentrations of detergent solution. (b) Szmilartty of action of szlzcaie of soda and saaps. The similarity between the action of silicate of soda solutions and of soap solutions is very striking. Thus for relatively high or low concentrations, very little >\In02 is stably suspended. There is an optimum concentration range, within which the values for different concentrations of the same silicate differ but little. For the silicates the optimum concentration range is 0.0125-0.og percent, whereas for soaps it is 0.2-0.4 percent. The optimum Concentrations of all
DETERGENT ACTIOS OF SOAPS
841
the silicates have “MnOt values” numerically less than all the soaps. This is true for both temperatures. But the values a t 40°C approach very closely t o those for olive oil soap a t the same temperature. (c) E f e c t of composition of the szlicnte. There is a very marked difference between the values for “S” brand and those for “B\F7” brand in the 0.5 and 0.3 percent solutions, while the “K” brand has values intermediate between these two extremes a t these same concentrations. But the values for all three silicates differ only slightly within the optimum range of concentration. By reference to Table X, it will be seen that the ‘3” brand is high in silica, the “BW’” is low in silica ( = high in alkali), while the “K” is of a composition intermediate between these two. As just indicated, there is a corresponding difference in “RlnOl values” for the higher concentrations. At concentrations greater than 0.1j percent, the more siliceous ( = less alkaline) silicate is distinctly superior to a highly alkaline silicate, for suspended MnO?. These results make it easier to understand the conspicuously contradictory results obtained by different investigators-Richardson)% SterickerJ7’Millards3and others-who have studied the detergent action of silicate of soda. I n the first place, different men worked with silicates unlike in NazO-SiO? ratio and in the second place, some worked with only one concentTation of silicate of a particular composition and this was probably too high. Using concentrations of 0.5 percent and greater, Stericker” found that mineral oils (kerosene, Acto, U. S. P.) were more readily emulsified by silicates in which the ratio of silica to soda is relatively high. If emulsification of oils parallels the peptization of MnO?, then the results obtained with this dirt are in exact accord with Stericker’s (for the one common concentration, viz. 0.5 percent). For solutions still more dilute, e.g., within the optimum concentration range (0.0125-0.0jYc), there is little choice between the different silicates as regards their peptization of MnO?. found that an alkaline silicate of soda produced a greater decrease of surface tension when added to a 0.03 percent soap solution, than when a less alkaline silicate was added. He was working with silicate of soda solutions of concentrations ranging from 0.012-0.139 percent and so was within the range found here for maximum suspension of JInOe. The maximum values I obtained are so very similar that it does not seem justifiable to claim superiority for any one particular brand, x h e n used at the o p t i m u m concentratzon. However, it would seem advisable to use a silicate high in SiO,, for it is indeed superior unless the concentration is kept below a certain value. Furthermore, the initial cost should be less than a silicate high in soda. (d) Efect of temperature. Just as rise in temperature was found to decrease the suspending power of the soaps, so also a similar effect was observed when silicate of soda alone was used. Other things being equal, this means that silicate of soda is more advantageous in a cold water soap than in a hot water soap. Within the range for maximum suspension of MnO, the silicates are about 2 2 - 2 5 percent more effective at 4ooC than a t 7 j”C, and they have been found to be about 1 2 - 1 j percent more effective a t room temperature than a t 4ooC.
842
PAUL HENRY FALL
(e) Silicate of soda as a detergent. If this suspending power or peptizing power of a substance toward MnOz is to be taken as a measure of detergent action (and we believe it is, as has already been explained), then the results here obtained are added evidence to the claims made by Stericker71 and others, viz., that silicate of soda is more than a mere filler for soaps; it has detergent properties of its own. It is significant that he showed this to be true for liquid dirts, whereas the present work has demonstrated it to be true for a particular solid dirt, MnOz. S o quantitative measurements have been made, but qualitative tests showed that silicate of soda solutions are capable of suspending other solid dirts, clay and lampblack, as well as different kinds of MnOs. The effect of the silicate of soda on the fiber that is washed in a silicated soap is still an open question. By far the most important result which is arrived at from this investigation of silicate of soda is that it is not just a “tolerant” filler for soaps but that it is an exceptionally good addition agent. Before we can be definitely certain that it is advisable to add silicate of soda t o soaps in large quantities it’s effect on emulsifying oils and on the fabrics and skin must be investigated. But silicate of soda might be added to soap until it forms nearly 1007~ of the cake without greatly decreasing the suspending power of the soap. Using the values for the “S” brand sodium silicate, which appear best, a 0.3 percent solution a t 40’ has almost 85Yc suspending power of olive oil soap, and 72Yc the suspending power of tallow soap. Certainly this fact should cause the heart of every soap manufacturer to beat in wild ecstasy. It is not advised that the manufacturer should start immediately to place a highly silicated soap (approximately 100%) on the market. Before this is done the problem must be further investigated. But no one can now say that such and such a soap is highly adulterated with sodium silicate, for sodium silicate is not an adulterant but rather a detergent. Further investigations may prove that a high content of sodium silicate is desirable from various other standpoints. We are deducing from the fact that sodium silicate alone shows a detergent act,ion on solid dirts comparable to soap that it can be added in large quantities to soap. Experiments in which varying amounts of the soap have been replaced by sodium silicate should be made.
(C) Eialuatio?i of Caustic Soda and Some d l k a l i n e Salts. d c t i o n of different alkalies on different dirts. “Addition agents” other than silicate of soda such as sodium carbonate, borax, sodium hydroxide are often present in some soaps. As experiments of last fall showed, silicate of soda acted very similar to soap in suspending the MnOz used (stock-room supply), but sodium carbonate, borax and sodium hydroxide failed to stably suspend the MnO2. But later, these same substances as well as trisodium phosphate were found to peptize the colloid mill M n 0 2 , if the concentrations were not too
DETERGENT ACTION OF SOAPS
843
strong or too weak. This was true of all except the borax which failed to peptize either kind of dirt even though a wide range of concentrations was employed. The earlier experiments differed from those of later date only in the kind of M n 0 2 used and the method and time of shaking the mixtures of dirt and solutions. Experiment, has demonstrated that the results obtained earlier, with the stock room supply of MnOz, were correct. Shaking the mixtures of this dirt and these different alkalies in the machine for five minutes made no difference. The dirt settled out as quickly from the alkaline solutions as from pure water. Only silicate of soda and soap solutions produced stable suspensions. With the colloid mill MnO2 all of these solutions, except the borax, gave good suspensions. The difference in action of these alkaline solutions on the colloid mill MnOz cannot be attributed to contaminations of the latter with some oil or organic acid, which then reacted with the added alkaline solution to produce a soap, which in turn peptized the dirt, because borax was no more effective than pure water. Other dirts-clay, lampblack, C. P. MnOz, and Kahlbaum ?vlnOz-were subjected to the action of these solutions. Only qualitative tests were made, but all gave fairly good suspensions except the borax and C. P. MnO1. This is another illustration of the specificity of adsorption and emphasizes fineness of particles as an important factor. Since soap and silicate of soda were able to peptize the untreated MnOz, and these other alkaline solutions were not able to do so, the former must be more powerful peptizing agents. This is shown to be true in a quantitative way, Table XII. One may attempt to explain the difference by saying that silicate of soda and soap have greater ability to disengage strongly agglomerated particles ; it hardly seems plausible that these solutions could break up single large particles into smaller, suspending particles, as one might break a stone into smaller pieces with a hammer or with a mortar and pestle. But when the dirt was put through the colloid mill the particles were decreased in size and the number of fine, suspensible particles was increased so that the weaker peptizing agents (caustic soda and the alkaline salts already mentioned) became effective. One wonders why the borax was ineffective toward the colloid mill MnOz and yet was able to peptize the Kahlbaum MnOz. The property common to caustic soda, sodium carbonate, trisodium phosphate and borax is that their aqueous solutions have an alkaline reaction due to excess of hydroxyl ions. Their peptizing action is due no doubt to adsorption of these hydroxyl ions by the solid particles of dirt. If this is the explanation, it is difficult to see why borax “acts so stubbornly” in some cases. Possibly the borate ion or the weakly ionized boric acid exerts a counter effect in some cases. The “MnO2 values” of various concentrations of the alkalies named in the preceding paragraph have been determined just as they were for soaps and the silicates. The values are given in Table X I . For the sake of comparisons, the values for “S” brand silicate of soda are added, Table S I . S o values for borax are included since it formed no stable suspensions.
844
PAUL HENRY FALL
TABLE XI1 Comparison of the “MnOs Values” of Various Concentrations of Different Alkaline Solutions a t 40°C and a t 75OC. I gram portions of Batch B-2 Colloid Mill M n 0 2 . j o cc portions of alkaline solutions. Values given represent centigrams of MnOz present in one liter of suspension as calculated from an analysis of 2 5 cc of suspension. These values are the average of duplicate determinations. Conc. c /c
0.5 0.3 0.Ij
0.05 0.02; O.OI2j
0,0062 0 , 0 0 3I 0.001j
“S” Silicate
40T 292 382
75°C 240 261
396 434 445 436 427
287 346 351 348 216
2;s
36
NaOH 40°C 75°C
XarCOa 40°C 75°C
Na3PO4 40°C 7j”C ooo 40 ooo 140
000
000
000
000
000
000
000
000
17
52
000
I2
70
232 347 373 333
231 269
30
99 196 234 140
350
000
17
ooo
68
17
40
248 113 ooo
ooo
000
000
000
000
000
000
252
140
IIO
284 203
365 273 125
274 290 291
Discussion of Results (a) Reliability of d u e s . I n some cases erratic results were obtained, especially with the ?Ja3P04but these determinations were repeated. So, in general, the values are fairly reliable. Where the figures “ooo” occur, the suspensions became clear in less than four hours and hence there was no MnOn to analyze for. (b) Opti?num concentration, For all of these substances there is a rather narrow range of concentrations that will give stable suspensions. It is much more limited than where soaps or silicates are used. There is an optimum concentration for each in the range 0.012 j-0.025 percent. In almost all cases suspending power decreases with rise in temperature, and in this respect they resemble the soap and silicate solutions. There is not very much difference between the effectiveness of the sodium hydroxide solutions and equal concentrations of the trisodium phosphate. This is to be expected since the latter salt is I O O percent hydrolyzed in aqueous solutions. There is some difference at the higher concentrations and at the higher temperature. This may be due to the influence of the relatively weak acid anion, HPO4”, formed by hydrolysis of the normal salt, but if this is the case, its effect is apparently negligible in the range of optimum concentration. Sodium carbonate is less effective than the other two alkaline substances. As has already been indicated, silicate of soda solutions and soap solutions are better peptizing agents for this particular dirt than are these alkaline solutions. (D). Evaluation of Nlixtzires of Soap and “ A d d i t i o n Agent.” Among those who have claimed that “addition agents’’ lower the surface tension of soap solutions, and thereby enhance the detergent action of the
DETERGENT ACTIOS O F SOAPS
845
latter, are RichardsonJs3Elledge and Isherw~od,’~ and The latter found the reducing effect of different agents on the surface tension of a 0.03 percent solution of soap, to be in the following order:-sodium hydroxide > sodium carbonate > trisodium phosphate > “BIT” silicate of soda > modified soda > “Star” silicate. He states: “The decrease in surface tension produced by an “addition agent ” when a soap solution of mixed concentration is used, has been considered a measure of the cleansing power until some more definite way of measuring this property is established.” Experiments show that this cannot be taken as a true criterion. Hillyer? showed that surface tension is reduced continuously with an increase in concentration of the soap solution, whereas the emulsifying power of such solutions increases with concentration only up to a certain point (a little less than 2.0%) and subsequently decreases. So also Harkins and co-workers66found the surface tension of sodium oleate solutions against benzene decreased as soap concentration increased up to X / I O solution ( ~ 3 . 1 3 % ) . Briggs and Schmidtse, however, found the optimum amount’ of sodium oleate for the emulsification of benzene to be 1.0percent solution, and Donnan and Potts6* found an optimum concentration of about 0.6 percent solution in the emulsification of paraffin oil by soap solutions. I n the present work 0.2-0.4 percent solutions of soap have been found to give maximum suspension of different’ dirts, although according to the experiments of Hillyer and Harkins just mentioned] the surface tension of these solutions is appreciably higher than that of more concentrated solutions. Thus it does not seem justifiable t o claim that a solution with the lowest surface tension will have the greatest detergent power. Referring to Tables TI1 and YIII, it will be seen that we can increase the detergent value of soap solutions of 0.1and less percent concentration by adding more soap, that is, by making a 0 . 2 or 0 . 4 percent soap solution. But we cannot increase the detergent power of a 0.4 percent solution by adding more soap, say sufficient to make a solution of 0.8 or greater percent concentration. But in all of these cases, as we add soap to soap, we are lowering the surface tension of the original solution. Since soap itself is a better detergent than any “addition agent”, and since soap added to a soap solution already a t its optimum concentration for cleansing power, does not enhance the cleansing power of the solution (but does decrease its surface tension), we should not expect an “addition agent” to enhance the cleansing action of a soap solution unless the latter was a t a concentration less than its optimum value. Cnless this latter condition obtains, the criterion used by Millards3 goes by the board. This was found to be the case in actual experiment. The two papers by ChapinQ3were published after this work was finished and written up, so that a discussion of them is not included. Summary X brief history of soap industry has been given. (I) A resume of various factors auggested by different investigators as (2) entering into detergent action, has been given in chronological order.
846
PAUL HENRY FALL
(3) A summary of the methods that have been used for measuring detergent action of soaps has been given. (4) The method developed by McBain and his co-workers has been described and some serious objections to the method of procedure have been pointed out. ( 5 ) h theory of measuring detergent action as proposed by Dr. Bancroft has been given. (6) Qualitative tests showed the optimum concentration of olive oil soap or sodium oleate solutions, for suspending a fine grade of clay, to be in the range 0.1j 6 - 0 . 6 2 5 5 . ( j ) The same range holds true for at least three other kinds of solid dirt-lampblack, ferric oxide, and manganese dioxide. (8) The soap-dirt ratio, or more specifically the soap-fine-particles ratio is of importance in obtaining suspensions of dirt in the detergent solution. If the ratio of soap to dirt is less than I :z5-30 (a condition seldom, if ever, met with in laundry practice) practically no dirt is stably suspended, but “flakiness and floating” occur. An explanation of this phenomenon has been offered. ( 9 ) The method of estimating the cleansing power of soaps by the use of sodium chloride, or a number of other salts, on suspensions of clay in soap solutions, was found not to be feasible. A practical method has been developed for the direct and rapid (IO) determination of the amount of finely divided MnOn which various detergent solutions are able to suspend for a definite period of time. This gives an “JlnO2 value” characteristic of each solution which may be taken as a measure of the detergent value of the material tested. The method of procedure has been outlined and a discussion of the (II) method and factors affecting accuracy of results obtained by it has been given. The advantages of this method have been emphasized. ( I 2) (13) Practical application of the method has been demonstrated in the evaluation of (a) sodium oleate and some commercial soaps, (b) silicates of soda of different compositions, (c) caustic soda and some alkaline salts, mixtures of soap and “addition agents.’! (14) The relative detergent values of soaps differ with different dirts, due to specificity of adsorption. ( I 5 ) The relative detergent powers of various concentrations of five different commercial soaps-olive oil, palm oil, tallow, “Green Arrow’’ and silicated “Green Arrow”-have been determined a t 4ooC. and at 7 j”C. There is an optimum concentration for all five soaps, at both temperatures, in the range of 0.2-0.4 percent. The four saturated fatty acid soaps are surprisingly similar at the same temperature and at equal concentrations. Olive oil soap solutions are less effective than these four a t the optimum value. The detergent values of all the soaps decrease with increase in temperature. (16) A comparison of the results obts.ined here with those obtained by other investigators has been given.
DETERGENT ACTION O F SOAPS
847
(17) Solutions of silicate of soda are similar to soap in their ability to peptize different solid dirts. There is an optimum concentration for all the silicates in the range 0.01~5-o.ogpercent. At concentration; greater than 0.15 percent the silicates relatively high in silica greatly excel those high in soda in their ability to peptize solid dirts. The detergent powers of the silicates are less than the soaps at the optimum concentration of each. Just as with soaps, the detergent action decreases with rise in temperature. (18) Caustic soda, sodium carbonate, and trisodium phosphate solutions are able to peptize some kinds of MnOs but not others. Where they act as suspending or peptizing agents, the optimum concentration for maximum suspension is in the range 0.0125-0.02 j percent. Increase of temperature also decreases the suspending power of these solutions. (19) hlaximum surface tension lowering is not a safe criterion for estimation of maximum detergent action. “Addition agents,” even though they may lower the surface tension of soap solutions, will not enhance the detergent action of soap solution towards Xln02,if the soap is at or above the optimum concentration. If the original soap concentration is less than the optimum value, an “addition agent” (if it itself is able to peptize the dirt) will increase the detergent power of the original soap solution for the reason that they act like soap and adding them is equivalent to adding more soap, thus bringing the total detergent content of the solution nearer to the optimum. Silicates of soda, as “addition agents” to soap solutions whose con(20) centrations are below the optimum value, enhance the detergent powers of the soap more than other alkalies, such as sodium hydroxide, sodium carbonate, and trisodium phosphate because silicate of soda is more nearly like soap than the others. (21) As saponin, and sodium silicate (h’as01Si02= I : 3.86 approximately) have almost BS much power as soap to suspend M n 0 2 it should be possible to use them in exceedingly large amounts (approximately I O O ~ ~as) a substitute for soap in the original cake. As the recipient of the Palmolive Fellowship for two consecutive years, the writer is deeply grateful to the Palmolive Company of Milwaukee for the generous Fellowship, and to the Committee that awarded it. Thanks are given to the Palmolive Company for supplies of different kinds of soap; to the Onondaga Pottery Company of Syracuse for a generous supply of English China Clay; to the Philadelphia Quartz Company for supplies of silicates of soda of different compositions; and to the Premier Mill Corporation of Geneva, N. Y., for disintegrating some of the “dirts” used, by means of their colloid mill. The writer feels greatly indebted to Professor Bancroft, under whom this work was carried out for the interest, helpful criticisms, and encouragement manifested throughout the work. The writer is glad to make known in this more public way his appreciation of the help and encouragement received throughout this work from his wife, Dorothy J. Fall. Cornell Cnit’ersity
8 48
PAUL HEXRY FALL
Bibliography Watt: “The Art of Soap Making” (1884). Carpenter: “Soaps, Candles, Lubricants and Glycerine” (1885). 3 Cristiani: “Tecbnolo y of Soap and Candles” (1881). 4 Lamborn: “Modern 8oapQ Candles and Glycerine” (1906). 5 Chevreul: “Rechercbes czimiques sur les corps gras d’origine animale.” Published 1823. Republished 1889. 8 Encyclopaedia Britannica, 25, 296 (1911 ). Hillyer: J. Am. Chem. Soc.; 25, j I t (19031. 8 Rotondi: Atti R. Accad. Sci. Torino, 19, 146 (1883). Quoted in Reference 9jof this bihliographv. K r a f t a n d Stern: Rer., 27, 1747,.1755 (1894). Berzelius: “Lehrbuch der Chemle”, I1 Aufl (1828) 111, 438, Quoted in Ref. 9 of this bibliography. 11 Persos: “Trait6 t h h i q u e et pratique de l’impression des tissues,” 354 (1846). Quoted in Ref. 9 of this bibliography. l 2 Jevons: Chem. Ztg., 2, 457 (1878). Quoted in Ref. 16 of this bibliography. 13 Kolbe: Org. Chem. I1 +1fl.~J1880). Quoted in Ref. 9. 14 Wright: Quoted in hluir’s, Dictionary of Applied Chemistry.” 3, 41I. 16 I h a p p : Quoted from Reference 16 of this Bibliography. 16 Ladenburg: “Handworterbuch” 10, $7j (I892). “Plateau: Ann. Physik. ( 2 ) 141, 44 (1870). 18Quincke,:Ann. Physik, (3)35, 592 (1888). 1 9 Freundlich: “Kapillarchemie,” 302 (1909). ? o Bancroft: “Applied Colloid Chemistry,” 269 ( 1 9 2 1 ) . 21 Hirsch: Quoted from Reference 24 of this bibliography. 22 Donnan: Z. physik. Chem., 31, 42 (1899). 23Krafft and IVi loa Ber., 28, 257.3 (1895). 24 McBain: ThirfRe.port on Colloid Chem Brit. .Am. Adv. Science (1920). 25 Lewkowitsch: Z. angew. Chem., 20, 951 (1907:. 26 McBain and Martin: J. Chem. SOC.,105,957 (1914). 2 7 McBain and Bolam: J. Chem. Soc., 113, 825 (1918). 2 3 Hofmeister: Arch. exp. Path. Pharm., 25, 6 (1888). 29 Smits: Z.physik. Chem., 45, 608 (1903). 3 0 Merklen: “Etudes sur la Constitution des Savons du Commerce dans ses Rapports avec la Fabrication.” S o t accessible, but salient points quoted in Ref. 31 of this bibliography. 31 Lewkowitseh: ,I. SOC.Chem. Ind., 26,590 (1907). 32 Goldschmidt: liolloid-Z., 2, 193,227 (1908). 33 Mayer, Schaeffer, Terroine: Compt. rend.: 146, 484 (1908,). 34 Leimdorfer: Kolloidchem. Beihefte, 2, 343 (1911);“Beltrage zur Technologie der Seife” (1911). 35 Bottaaai and Victorow: Iiolloid-Z., 8, 220 (1911). 38 McBain and Taylor: 2. physik. Chem., 76, 179 (1911). 3‘Bowden: J. Chem. Soc., 99, 191 (1911). 38 Zsigmond and Bachmann: Kolloid-Z., 11. 145 (1912). 3O McBain, eornish and Bowden: J. Chcm. Soc., 101,2042 (1912). 40 Reychler: Kolloid-Z., 12, 277 (1913). Goldschmidt and Weissmann: Z. Elektrochem. 18, 380 (1912);Kolloid-Z., 12, 18 (1913). 42Farrow: J. Chem. Soc., 101,347 (1912). 43 Moore and Roaf: Kolloid-Z., 13, 133 (1913). 44 Bunbury and Martin: J. Chem. Soc., 105,417 (1914). l5 Kurzmann: Kolloidchem. Beihefte, 5,427 (1914). 46 Arndt and Schiff: Kolloidchem. Beihefte, 6, 201 (1914). 47 Paul: Kolloid-Z., 21, 176 (1917). 4 8 Lifschitz and Brandt: Kolloidchcm. Beihefte, 22, I33 (1918). 49Laing:J. Chem. SOC.,113,4.35 (1918). 5 D McBain, Laing and Titley: J. Chem. SOC., 115, I279 (1919). 5 1 McBain and Salmon: J. Am. Chem. Soc , 42, 426 (1920). 52 McBain ;nd Laing: J. Chem. Soc., 117, 1507 (1920). Soaps and Proteins” (1921). 53 Fischer: 541redale:J. Chem. Soc., 119,625 (1921). 55Prosch: Z. deutsch. Ul-Fett. Ind. 42,410 (1922);Chem. Abs. 16, 3jj1 (1922). 5 6 Salmon: J. Chem. Soc., 121, 711 (1922). 5 7 McBain: Fourth Report on Colloid Chem., Brit. Assoc. Adv. Science (1922). 5 8 McBain, Taylor and Laing: J. Chem. SOC.,121, 621 (1922). 5 s Laing: J. Phys. Chem., 28, 673 (1924).
DETERGEST ACTION OF SOAPS
849
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