SYNDETS AND SURFACTANTS
0
FOSTER DEE SNELL and CORNELIA T. SNELL Foster D. Snell, Inc., New York, N. Y.
DETERGENTS are of practical importance to all of us, since we all use them several times a day. What are popularly known as "detergents" today were first termed "synthetic detergents," then this was abbreviated to "syndets." Syndets are cleansing agents, applied both in the home and to many industrial uses. They are a part of the more general class of surfactants, those compounds which concentrate at the interface. They lower the surface tension if the interface is between air and a liquid or a solid, and they lower interfacial tension if the interface is between two liquids or a liquid and a solid. Such lowering of interfacial tellsion promotes ease of wetting; for example, a solution of a syndet wets soiled garments much more readily than plain water will. This increased ease of wetting has led to the development of a vast number of commercial applications. The great disadvantage of soap is precipitation of insoluble, curdy calcium and magnesium soaps when the soap is dissolved in hard water, or precipitation of insoluble fatty acids in acid solution. This explains the decreasing popularity of soap in hard-water areas of the country, which are extensive, with corresponding increase in popularity of syndets. The latter, in general, are soluble and effective as detergents in hard water, in solutions of strong electrolytes, and in acid media. This explains why syndets and surfactants find so many uses in industry, where soap would be completely ineffective. The change in the sales picture of soap versus syndets in the United States since 1950 is shown in Table 1' with the per cent loss or gain over the previous year. TABLE 1 Sales of Soaps and Syndets in the United States Soap (millions of ibs.)
Loss
%
Syndets millions of lbs.
3000 2480 1865 1645 1450 1390 1325 1189
17 25 12 12 4 5 10
1250 1434 1530 1867 2063 2317 2690 2916
0-
/o
Gain. 13
7 18 10 11 14 8
of the molecule may consist of carboxyl, hydroxyl, ether oxygen, sulfonate, sulfate, phosphate, amino, ammonium, or other polar groups. This part of the molecule is attracted by water rather than by oily phases or air. Innumerable combinations are possible, resulting in wetting agents and emulsifying agents, as well as syndets. When a surfactant concentrates at an interface, it orients so that each section of the molecule is in the phase which attracts it. This reduces interfacial free energy. The presence of unsaturated bonds in a hydrocarbon radical promotes water solubility. Acid groups such as sulfate and sulfonate are usually neutralized, mostly with caustic soda. Basic groups such as amine and substituted ammonium may also be neutralized, mostly with hydrochloric acid. Organic salts are obtained in both cases, the first containing a large anion, the second a large cation. Besides these two classes of anion-active and cationactive surfactants, there is a third, the nonionics. The latter have usually been considered as not ionizing and mostly to have an ether or ester structure. However, a concept has been presented of an equilibrium in solution in which some large cations are counterbalanced by hydroxyls. This would explain synergistic effects with anionics which appear to be related t o the formation of loose complexes of the nonionics and anionics. However, nonionics are much less reactive than either anionic or cationic types, and are sorbed from solution to a lesser degree on solid surfaces. Each of the three classes will be discussed in some detail. Innumerable patents have been issued on specific compositions; the emphasis here will be placed on commercially important products. I n terms of tonnage production or money value, the anionics are the most important general class of detergents. The leader in this group is sodium dodecyl benzene sulfonate. This particular syndet constitutes more than half of all the surfactants produced, typified by Nacconal, Santomerse, and Oronite. Its manufacture involves several steps: (1) production of alkylate, (2) sulfonation of alkylate, (3) neutralization of sulfonate, (4) building and drying.
GENERAL STRUCTURE
A molecule of a surfactant is always highly unsyrnmetrical; one section is polar, the other nonpolar. The latter is commonly a hydrocarbon chain. The nonpolar part is attracted to oily, fatty, or waxy substances, and to air, in preference to water. The polar section From the Assao. of Am. Soap & Glycerine Producers, Ine.
VOLUME 35, NO. 6, JUNE, 1958
A/Q& NaOH
-"S08iia
Because of the importance of this syndet, its production will be given in more detail than for other syndets. 271
Alkylate Production. For several years the alkylate or hydrocarbon portion of the syndet was made by chlorination of a kerosene fraction boiling at 185'275"C., and reacting this with benzene by the FriedelKraft reaction to give an alkyl benzene in which the alkyl group was a mixture of different chain lengths, but with an average of ClrCla. Better control of the chain lengths has been achieved by development of propylene tetramer. This is made by polymerizing purified propylene in the presence of a suitable catalyst, often a fluoride, sometimes phosphoric acid. H H H
2 C=C-CH
H
H'
-
CHa H I H H H C=C-CC-CH H H H H
The dimer which first forms reacts with itself or with more monomer. Fractional distillation of the mixture of products from the polymerization gives the desired dodecene. Ample supplies of this are available as a byproduct fraction from other processes of the petroleum industry. It has long been known that detergency is improved by minimum branching of the side chain or alkyl portion. This is the reason for use of tetrapropylene rather than tributylene. With a straight side chain, maximum foaming is at a chain length of 10-12 carbon atoms, but detergency increases with a somewhat longer carbon chain. Alkylation of benzene by reaction with dodecene is carried out in the presence of a sulfuric acid catalyst or a hydrofluoric acid catalyst. The latter catalyst is used under pressure at about 40°C. With 90%-100% sulfuric acid, the temperature is kept at 0". The acid phase may he recycled. The desired alkylate cut is separated by fractional distillation. Just after World War I1 there was not enough benzene being produced in this country to meet the demand for raw materials for syndets. The petroleum industry then took over to provide synthetic benzene from petroleum to amplify that from the coke oven. It strikes a humorous note to point out that now the coke-oven henzene has to be refined to meet the standards set by the synthetic. Production of dodecyl benzene, alkyl benzene, or "alkylate" is carried out by the petroleum companies, and to a limited extent by others. Some of these process the alkylate into syndets themselves. They also sell the alkylate t o the large soap companiesthe big t h r e e t o a few large chemical companies, to many smaller concerns. A considerable volume of alkylate is exported for processing abroad. Suljmatim of Alkylate. Two methods are in use: sulfonation with 20% oleum (fuming HBOa) and with liquid sulfur trioxide. I n the former method the alkylate is precooled to about 10°C. in a stainless steel, or in a glass-lined reactor. Addition of 20% oleum is made slowly with agitation and cooling to prevent localized overheating. About 1.25-1.3 parts of oleum are required to 1 part of alkylate to give a monosulfonate. Addition of oleum is at a rate such that the temperature can he kept at 25'-30°C. Agitation is continued about 2 hours after addition of the oleum, with the temperature still kept a t about 25'. Only a minimal amount of unreacted alkylate may remain, usually under 2%. The "huildmg" (see later discussion) of syndets with
various chemicals which promote detergent action should be mentioned here, since the method of manufacture depends on the desired type of end product. If a light-duty detergent containing 40y0 or more of sodium sulfate after neutralization is required, spent acid is not necessarily separated. For a heavy-duty detergent to be built with tripolyphosphate and other builders, maximum separation of spent acid is essential. To remove excess acid to give a sulfonated alkylate suitable for heavy-duty building, water is added to the sulfonator with agitation, until the free sulfuric acid is 7070-80%. Ice may he added instead of water, in order to keep the temperature a t 60"-70°C. Temperature control is to avoid further darkening of the product, while at the same time promoting separation of the sulfonic-acid from the aqueous sulfuric-acid layer in which it is insoluble. The mixture is pumped to aseparating tank, where the sulfuric acid layer is allowed to settle for 2-3 hours. A longer settling time tends to darken the upper sulfonic acid layer. Spent sulfuric acid is drawn off, leaving the more viscous sulfonic acid containing 10%-15% of unreacted sulfuric acid. Use of liquid sulfur trioxide is relatively new. Advantages are that excess of acid is not needed to complete the reaction, so that the final product contains little sodium sulfate. The difficulties of removing and disposing of spent acid are avoided. This results in a quicker process. The product is said to avoid the "kerosene" odor of those made by use of oleum. However, liquid sulfur trioxide is not easy t o handle and must be protected from contact with moisture to avoid polymerization to the alpha form. A high heat of reaction requires efficient heat removal to keep temperatures from rising excessively. It is advantageous to pretreat the alkylate with 10T0 of 96% sulfuric acid. Dry sulfur trioxide is vaporized, mixed with 9 parts of moisture-free compressed air, or of dry nitrogen, and added to the alkylate. The reaction mixture circulates in contact with an efficient heat exchanger. Sulfonation is a t 5O0+O0C. Diluent air is removed. About 0.36 pound of sulfur trioxide is required per pound of alkylate. Reaction time is kept to a minimum, which may be about 2 hours; neutralization follows imediately. Neutralizatim. The viscous sulfonic acid is pumped to a stainless steel neutralizer equipped with an external heat exchanger. Commercial caustic soda solution-50yo or 70%-is pumped to a measuring tank and from there run into the neutraliier where it is diluted to 180jo20%. The sulfonic acid is allowed to flow slowly into the caustic solution, with which it is mixed by agitation. The temperature should be kept down to 50"-55°C. The last of the caustic solution is added in accordance with the amount indicated by titration of the still slightly acid mix, in order to give a neutral sulfonate. The final product contains about 50% of water. The amount of sodium sulfate is 10%-15%, with sulfonate made by the oleum process, and practically nil with sulfonate made with sulfur trioxide. Largescale sulfonators are going more and more to continuous sulfonation with sulfur trioxide. Five continuous sulfonation processes have been described. Two plants are manufacturing stabilized sulfur trioxide. Foreign JOURNAL OF CHEMICAL EDUCATION
plants are now producing sulfonates by continuous methods. Building. The term "builder" has been accepted as meaning a compound which itself has little or no cleansing action, but which will greatly enhance the cleansing action of a detergent, whether syndet or soap. From the point of view of economical use, the building of detergents is very important. Much less syndet with builder can be used than would be required to give a comparable cleansing result without a builder. Detergency of soiled cottons can be obtained by mixtures eontaining a builder which could not be obtained with the pure undiluted active agent. For many purposes sodium sulfate and other inexpensive salts can be used to replace a large proportion of the more expensive active detergent. The builders added to give heavy-duty products consist in large part of molecularly dehydrated phosphates such as tetrasodium pyrophosphate and sodium tripolyphosphate. The largest selling syndet cleanser on the American market has been built to the extent of over 50% tripolyphosphate. Another additive which has come into general use is sodium carboxymethylcellulose (CMC). This material serves to promote detergency by preventing redeposition of soil suspended in the wash solution. This property of CMC is particularly noticeable with the syndets, the soil-suspension characteristics of which are relatively poor compared with those of soap, particularly when used in washing cotton textiles. Syndet products have had a corrosive action on the parts of washing machines. This has been greatly reduced by incorporation of sodium silicate in the detergent formula. Soaps are not especially corrosive because a film of calcium soap tends to deposit on the metal parts, by reaction of hard-water salts--calcium and magnesium compounds-with the dissolved soap. Sodium silicate in solution in contact with metal has a similar protective effect. It deposits a thin film of noncrystalline silica which is gelatinous and difficult to see when wet. Such a deposit will form on aluminum, copper, iron, steel, brass, and bronze. This is one reason for including sodium silicate in heavy-duty formulas. Sodium dodecyl benzene sulfonate does not give as voluminous nor as long-lasting a foam as soap does. The presence of soap suds has long been a criterion for determining that enough soap is present in the wash solution to do the work of cleansing. It became a mark of a good product; lots of suds meant satisfactory washing. The foaming of dodecyl benzene sulfonate has been enhanced and made much more stable by inclusion of a fatty amide in the formula. One of the newer compounds for this purpose is N-oleyl-Nmethyl taurate. The presence of a substantive fluorescent dye improves the whiteness of washed linens. This dye, although itself colorless, gives off a blue fluorescence which counteracts the yellow which white materials tend t o develop after repeated washing. The dye makes the material look bluer, or actually in common terms, "whiter." Such fluorescent dyes are now used in all soap and detergent products for the laundry. They have replaced the earlier process of a blueing step in the home laundry. A typical formula of a heavyduty and hence a built syndet i~ given in Table 2. VOLUME 35, NO. 6,
IUNE,1958
TABLE 2 Typical Heavy-duty Syndet Ingredients
% by weight
Dodecyl benzene sodium sulfonete Sodium tripolyphosphate Sodium silicate (1:3.25) Sodium earboxymethylcellulose Fatty amide Sodium sulfate Moisture Fluorescent dye
20 45 5 0.5 2.5 22 5 trace
In a fair-sized plant, the slurry may be run into a second tank for addition of builders, with the two tanks arranged so that they can be used iuterchangeably for neutralization and building. The dry builders are added with heavy-duty, screw-type agitation in a steam-jacketed crutcher, to obtain maximum dispersion and solution. The temperature is kept at 55"60°C., with use of live steam if necessary. Air should not be beaten in as this would cause undesired foaming. Hot water is added t o prevent t,he mix from becoming too viscous. If the product is t o be drum-dried, the water content should be kept to 35y07,-45%; if spraydried, it should be 50%-60%. Drying. Drum drying is used for some industrial applications where shipment over long distances is necessary and bulk density is important. The less expensive spray drier is suitable for household products, as it produces a lighter, dust-free bead. A drum-dried product may range from 40 to 45 pounds per cubic foot, spray-dried from 15 to 28 pounds. For drum-drying, slurry is pumped to a trough formed by two steamheated drums of the drier. As these rotate in opposite directions, each picks up a film of slurry which dries as it is carried around to the doctorblades which scrape it off. The speed of rotation is such that when the doctor-blades are reached, the moisture content will be 1.570-270. A difficultyis that tripolyphosphates may be overheated and revert in part t o orthophosphate, an undesirable change. With spray-drying, the built slurry is pumped a t 82"-99°C. through atomizers into a stream or preheated air or gas. The fine spray, consisting of many small droplets, gives a quite uniform product. Some reversion of tripolyphosphate occurs but can be more readily controlled. Danger of overheating is small if the entering gas temperature is a t 400°480"C. The dried particles are carried along by the gas stream to the product-collection system, where they are discharged to drums or to a screening and packaging unit. The loss of dry product as dust in the exhaust air can be kept below 1% and, if necessary, eliminated by installation of a wet collector. As discharged, the particles are generally not over 65'C., after which they are air-cooled on a vibrating conveyor. If dried to 2yo-3% moisture content, they do not cake in the container but remain free-flowing. Many products approach 10yo of retained moisture. A particle-size analysis from spray-drying should give 91y0 of beads through 20 mesh and on 100 mesh. Material outside this range is mixed with incoming slurry. Built heavy-duty alkyl aryl sulfonates, after spraydrying, have a bulk density of 15-28 pounds per cubic foot. Variations in bulk density can be made by adjustments in operating conditions. The larger the 273
bead, the fluffier and lighter the product. Light-duty sulfonates tend to be of lower density than the heavyduty type.
be petroleum-derived or from other sources. The reaction may be depicted as follows:
fi+cy
A L K n SULFATES
Next in importance to dodecyl benzene sulfonates among the anionics are the alkyl sulfates, or fattyalcohol sulfates, typified by dodecyl sodium sulfate. These were the first syndets to become commercially successful on a large scale, for example, the Duponols and Gardinols. The structure is closelv related to that of soap with -COONa as the waier-attracting group in soap being replaced by -OS03Na. For many years coconut oil was the fat used, but more recently tallow has also been a source for making alkyl sulfates. Inedible tallow is currently in abundant supply because of the great decrease in demand for it in soap making. Over the last several years soaps have fallen to about 30% of the total cleanser market, while syndets have correspondingly climbed to 70%. To make alkyl sulfates, alcohols are fist made from the fats by catalytic reduction of the triglycerides with sodium, or by hydrogenolysis with a chrome catalyst. The alcohols are then esterified with sulfuric acid and neutralized. The active agent may be built and dried in much the same way as dodecyl benzene sulfonate. A study of tallow-alcohol sulfates showed that the properties were a little different, depending on the method of manufacture. Unsaturated groups such as those present in oleic acid are unchanged by sodium reduction; with hydrogenolysis they are changed to saturated groups. The unsaturated product is a better foamer than the saturated. The saturated product appeared to be somewhat more effective than unsaturated for hand dishwashing, but both were in general, good detergents. Alkyl sulfates are even better detergents in hard water than alkyl aryl sulfonates. By changing from the sodium t o some other cation such as ammonium or triethanolamine, products suitable for use in liquid form are obtained. When made of coconut oil, the alkyl sulfates are more expensive than the alkyl aryl sulfonates. With tallow stabilized a t the relatively low price of 7 cents a pound, raw material costs of this are competitive with the latter. Major applications of the alkyl sulfates are in shampoos, cosmetics, and pharmaceuticals although substantial tonnage goes into household syndets. NONIONICS
Practically all of the better-known nonionic detergents depend on a polyoxyethylene chain for the polar portion of their molecules. The hydrocarbon section is frequently an alkyl substituted phenol. Polyoxyethylene ethers of alkyl phenols are analogous to alkyl aryl sulfonates with the polyoxyethylene chain taking the place of the sulfonate group. Another type has a straight-chain mercaptan from petroleum sources, joined to a polyoxyethylene chain. Phenol is ordinarily synthesized from pet.roleum today. The polyoxyethylene group is built up from ethylene oxide molecules and the ethylene oxide in turn derived from petroleum. The alkyl side chain may 274
+ CHaCH,
-+
R~
y
R~
+
OCH2CH90CHsCHnOH
A
I I RW
(continued to any desired chain length.)
I n 1957 the alkyl groups commonly used in making such nonionics commercially included octyl, nonyl, dodecyl, dinonyl, and pentadecyl. The nonionics, when properly used, offer exceptional efficiency in washing operations. Further, they are highly flexible. Because the polar or ethoxy group can he built up step by step to any desired degree and because the alkyl group can be varied, it is possible to tailor-make a large variety of nonionics for special purposes. The ether oxygen portion is the water-attracting group; increasing the proportion of ethoxy groups therefore increases water solubility. When the ethoxy portion is minor, an oil-soluble product is obtained suitable for use as emulsifier in cosmetic creams, etc. When the ethoxy portion is greatly increased, the product becomes water-soluble. The great majority of this type as produced contains 60%-70% of ethylene oxide. The ether linkage makes for chemical stability of the compound-stable in acid, alkaline, and strong-electrolyte solutions. With anionics, such variability is not so easy. There the polar group consists of a single radical and is present on an all-or-nothing basis. Thus for an alkyl aryl sulfonate we must have a t least one sulfonate group, but two sulfonate groups are entirely too much for ordinary purposes. However, with a nonionic of the type described we can have 6, 7 , 8 , 9 , etc., oxyethylene groups in the chain, varying the solubility, wetting power, specific detergent effects, etc. An interesting modification of this type of agent is the sulfated nonionic, or rather, a nonionic converted to an anionic. This is made by attaching a fairly short polyoxyethylene chain to an alkylated phenol, then sulfating to attach a sulfate group via an ester linkage to the terminal hydroxyl in the polyoxyethylene chain. Such detergents retain many of the virtues of the nonionics while avoidinv some of their disadvantages. For example, they are good foamers. Another type of nonionic detergent is that made by reacting ethylene oxide with tall oil. Tall oil is a mixture of fatty and rosin acids derived from pine wood during sulfate pulp manufacture. It is quite low in price and of limited value as a source of fats or oils for many purposes. The use of this derivative has been moderately successful in the laundry. The tall oil-ethylene oxide nonionics offer the advantage of low price. They are low in foaming power, a desirable attribute in some applications of syndets. For example, in certain types of automatic home washers, excessive suds interfere with the cleaning process and decrease the over-all efficiency. For this reason low-sudsing detergents were developed, and competitive products
-
JOURNAL OF CHEMICAL EDUCATION
of this nature are produced by the three major soap syndet producers, namely, Procter and Gamble, Lever Brothers, and Colgate-Palmolive. The tall oil-ethylene oxide nonionics have proved useful in making lov-sudser detergents. Somewhat related is a type of nonionic based on the comhiuation of ethylene oxide with alcohols. These may be mixtures obtained from coconut oil, tallow, or other natural fats, or they may be polyhydric alcohols such as sorbitol. Again the number of ethoxy groups in the molecule will determine the properties and permit tailoring to fit the needs. Highly water-soluble members of this group have proved effective as dyeing assistants; they promote even dyeing hy delaying the rate of dye uptake by fabric. A nonionic which was first produced commercially in lg5' is One by the esterification reaction between sucrose and fattv acids from tallow. Both raw materials are low in cost. The first of the series was sucrose dipalmitate, a fat-soluble syndet highly satisfactory as an emulsifier in margarine. When the water-soluble monotallowate becomes available, it may be competitive with the firmly established dodecyl benzene snlfonate. Since the sugar esters are nontoxic, they are aimed primarily a t the food, drug, and cosmetic industries. As production costs decrease, they may be expected to find more general use. In the past, nonionic detergents have had a rather limited market. However, during the last few years there has been a very marked growth of interest in nonionic detergents and the market appears to be expanding very rapidly. They are combined with anionics in some detereent nroducts. I t is nossihle that they form loose compounds with them mkch like the addition compounds of nonionics with phenols. Nonionics are also compatible with cationics and are combined with them in various cleaning products. CATIONICS
Cationics have a large positively charged group in balance with a halide ion, usually the chloride. Those best known are the quaternary ammonium halides, such as alkyl-dimethylhenzylammonium chloride.
Another well-known compound is dodecyl pyridinium chloride.
Ti
\N/ /
C1
\ C12H15
The quaternaries are used mostly for their powerful germicidal effect, for example, in cleaner-sanitizers useful in dairies and food-processing plants, where cleansing and sanitizing are needed simultaneously. I n such products, cat,ionics are combined with a small amount of nonionic plus alkaline salts to give combined wetting, cleaning, and sanitizing action. The emulsifying action of cationics has special applications, oue example being to emulsify pentachlorophenol in water for use as an herbicide and soil toxicant. VOLUME 35. NO. 6, JUNE, 1958
M x m g of lngredlenta $or Synthet~cDetergent B ~ Is F Controlled on strict T,~.schedule
Alkane may be used as the starting material, from which an aromatic chloride is formed. I n turn this is reacted with an amine such as triethanolamine to form a quaternary ammonium chloride. Such a compound is relatively expensive. Among their industrial uses is one based on the caparity of these amines to adsorb strongly on metal. Such sorption results in a mildly protective film which greatly reduces the corrosion of the metal. These amines are also useful ingredients in rust preventives. Cationics and anionics are incompatible, since the two large radicals of opposite charge precipitate each other; the activity of both type of agents is then lost. PHYSICAL-CHEMICAL PROPERTIES OF DETERGENT SOLUTIONS
Many factors and many properties are involved in detergency. How do we know that one product may be more effective than another? In the laboratory, comparisons have been made by measuring definite physical properties, as well as making standardized washing tests. The picture as to the physical-chemical properties which are involved in detergency is being gradually clarified. There may be a matter of opinion as to how important a given property is, but that property can be measured accurately on an absolute standard. Surfaee Tension. The attraction between the molecules of a liquid sets up a force which resists breaking of the surface. With water this reaches a rather high value, about 72 dynes/cm. A little concentrated soap solution added to the water will decrease the surfare tension to around 28 dynes/cm. Measurement of surface tension is made with the Du Nouy tensiometer. Although low surface tension has some relation to high foaming power, the latter is not necessarily a property of a detergent. Interfacial T a s i a . Much of the soil which offers any problem is oily. If the particle is large enough, we may be able to remove it mechanically from a surface to leave an unimportant residual oil film on the material (cotton or wool or ceramic or metal). The same type of effect may be obtained with a detergent if that detergent ruptures oil-oil bonds so that the particle of soil is separated from the base but does not take all of its oil with it. There is ample evidence 275
Determination 05 Surface Tension with DuNouy Tensiometer
that such a phenomenon can occur. To all practical purposes one never wets the underlying particle of solid soil and so does not separate it from its oil coating. A third form of removal of the oily soil from the base is by having the wetting agent effectively penetrate between the oil and the base. The interfacial tension between the detergent solution and the oil is a good measure of the relative ease of soil removal by rupture of polar-nonpolar forces of attraction. It throws no light on the relation between the base of substrate and the aqueous detergent. Soil removal in terms of actual wetting of the surface and displacement of oily soil may be determined in different relative terms by measurement of contact angle. The actual value obtained is a function of three factors: (1) surface tension of the solid; (2) interfacial tension of the solid versus the solution; and (3) surface tension of the solution. Even to be a relative measure of the rupture of these polar-nonpolar bonds, (1) and (3) must be constant. The procedure is to measure the contact angle of the detergent solution on the surface under oil. Even here interfacial tension plays a role in governing the intensity of displacement forces. One of the major problems of detergency today is that of determining the relative importance of the two mechanisms of soil removal under various conditions. For years this laboratory has been using interfacial tension measured between the detergent solution and benzene containing O.lyo of a highly purified oleic acid to represent acidity in soil. Work in England has confirmed the soundness of earlier reasoning in believing that only the fatty acid in natural soil is significant in modifying the effect of that interface. If the detergent wets the base surface and wets the soil so that the solution is interposed between them, then the first move has been made toward removal of the soil. This problem can be analyzed in terms of physical chemistry. The energy applied to break the interface is the work of cohesion of the oil less the work of adhesion between the oil and the aqueous solution of detergent. I n sets of data obtained with the same oil, the cohesion of the oil is a constant and the interfacial tension is a relative measure of the wetting of the
oil. Likewise the contact angle of the detergent solution against the surface of the solid is a relative measure of the interfacial tension between them, but in different terms. All we need is a symbiosis of those two factors to measure the interfacial tension of the oil versus the solid base. It does not exist. Dispersing Power. A dispersing agent for solid particles is believed to function by forming a stable mono-layer a t the solid-liquid interface. Having separated a particle of soil from the fabric, no detergent effect will be accomplished unless the soil can be kept in the suspension. The importance of the latter factor is shown in launderometer tests on standard soiled fabrics with certain low-quality detergents. I n some cases we have found the fabric to be darker after washing than before. We interpret that to mean that the soil has been deflocculated, dispersed more efficiently over the surface of the fabric, but not suspended. The method for measuring dispersing power consists of dispersing oiled umber and reading the amount suspended after 2 hours. While soap will give a numerical value of 50 or more a t 25' in terms of present test for dispering power, synthetic detergents give values of the order of 10. At higher temperatures less soil is dispersed. Dispersing power is the factor in detergency in which unbuilt syndets compare least favorably with soap. Solubility i n Micelles. The structure of micelles has received a great deal of attention. If one assumes that the solution is saturated with molecular detergent a t the point where initial micelle formation occurs, some very interesting calculations can be made: (1) that with increase in concentration in the range used for detergency, micelles are approximately uniform in statistical size, and (2) the statistical size varies over the general range of 3-7 molecules. The molecules in micelles have their hydrocarbon chains pointed inward, dissolved in each other, so t o speak, while the polar groups point outward toward the water phase. Thus the free energy of the system is a t a minimum, since each type of group is in contact with others like it. Using an analogy by McBain, a micelle is a group of molecules and ions bound together like a bunch of eels with their tails-representing the nonpolar hydrocarbon radicals-tied together. The solubility of soil in micelles relates to detergency and in practical terms is probably a real factor when oil droplets are to be removed. For many decades the occasional effect of a soap in producing a real erythema of the skin, and the frequent dryness and harshness caused by reduction of sebaceous matter (the condition often described in advertising as "dish-pan hands") has been attributed to hydrolysis alkalinity. Another accepted factor is the presence of fatty acids due to hydrolysis. Development of synthetic detergents having a pH value not radically different from that of the skin, which is normally about pH 5, was hailed in some quarters as solving that problem. Strangely enough, within the limits of accuracy of the methods for measurement of skin reactions, the harshness, if not the erythema, appears to be a t least as great from contact with synthetic detergent solutions. Observations of McRain and of Harkins that water-insoluble matter is dissolved in the micelle, probably explains this apparent anomaly. JOURNAL OF CHEMICAL EDUCATION
The skin, which ordinarily is protected and kept soft by the oily matter supplied by the sebaceous glands, becomes dry and rough by frequent contact with a detergent solution, regardless of whether the detergent is soap or a synthetic. If the detergent is effective in removing oily matter from dishes, apparently it is necessarily effective in removing it from the skin. Reducing the detergent efficiency of soap by adding lanolin or other excess fatty matter reduces the drying effect of the solution on the skin but does not entirely prevent it. Theoretically it is possible to visualize a polar-nonpolar structure which would be selectively sorbed by the skin from a detergent solution. There is some evidence that lecithin shows such a property, but there is little doubt that a "tailored molecule" would be more effective. Sorption of Detergents on Surface to Be Cleaned. The extent to which a detergent will be sorbed from solution by the soil which it removes, will ordinarily be rather minor simply because soil is present in such a small amount relative to the surface being cleaned. Sorption of detergent by the base or surface to be cleaned will depend on its nature. Thus sorption will not be great on a metal or on a smooth ceramic. But the surface of a porous ceramic or a fabric is very great and therefore possible sorption of detergent is enormous. MEASUREMENT OF COMBINED DETERGENCY
What method should we use to measure the efficiency of removal of soil from the base? Here again one is dealing with a problem which relates to methods to be applied industrially. When fabrics are being washed, more than 50% of the total cleaning may be by mechanical action. The mechanical effect is easily measured by comparing the results obtained (1) by washing with plain water, and (2) with a solution of a detergent under otherwise identical, controlled conditions. The most generally accepted method for comparing washing efficiency is with use of what is known as a launderometer. Briefly, this consists of a rotating shaft carrying multiple pint jars in a housing which permits temperature control. Sample detergent solutions and standard soiled swatches of cloth are placed in the jars. Mechanical detergency can be increased by enclosing a definite number of metal or rubber balls in each jar. Controllable variables which must be standardized include the following: nature of base surface to be cleansed, nature of the soil, building effect on cleansing efficiency due to added salts, pH of the solution, temperature, degree of agitation, time of agitation, hardness of water, concentration of detergent, ratio of base surface to be cleansed to volume of solution, ratio of soil present to volume of solution. Qualitatively the effect of successive equal increments of additional soil decreases rapidly. So it takes many hundreds of times as much soil to lower the reflectance of a fabric from 30 to 29 as from 80 to 79. One practical effect is that there is a large visible difference between removal of 98% and 99% of the soil. Another is that very little soil need redeposit on clean sections of cloth to change it from brilliant white to visibly dirty gray. This explains in part why ability to prevent any r e VOLUME 35, NO. 6, JUNE, 1958
deposition of soil is such an important requirement for a good detergent. What we are reallv " lookinn for is a numerical value to express the efficiency of the detergent. Even for relative evaluation some kind of measurement has to be made. The most satisfactory procedure a t present is to read the degree of soiling of the fabric photometrically in terms of light reflectance before and after the detergency operation, and then to calculate by some form of expression the degree of brightness regained. ~
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PHYSICAL FORM OF DETERGENTS
The great majority of syndets go into household products, for dishwashing, laundering, and general cleaning. For many years these were granular. About the middle 1950's, liquid, light-duty syndets became popular for dishwashing, and the production of liquid products increased enormously. In 1957 liquid syndets in the United States were one-tenth of the total tonnage and one-skth the dollar volume of syndets. Originally these liquids were based on nonionics, which are frequently liquids to start with. Later, dodecyl benzene sulfonate was used in the liquid products, with alcohol or some other agent present to increase its solubility, combined with a smaller proportion of nonionic. Nonionics of the condensed amine type help solubili~e sodium dodecyl benzene sulfonate. New materials for making liquid syndets are being produced, e.g., sodium lauryl ether sulfate. This forms clear solutions in water of any degrec of hardness. Sulfation of the hydroxyl group of an alkyl phenol, and of the terminal hydroxyl group of an ethylene oxide condensate, results in products with good surfactant properties and high solubility. Following the success of the light-duty liquids, heavyduty liquids appeared, built with tetrapotassium pyrophosphate and other suitable compounds. These are still in relatively low production but may achieve in time the popularity of the light-duty liquids. The potassium phosphate salt is necessarily more expensive than the sodium but the latter is not soluble enough to be used in liquid concentrates. The popularity of liquid syndets for washing dishes is probably due to their immediate solution in the dish water, and possibly also to a belief by many housewives that they are not so harsh on the hands as some of the granular products. Liquids for special purposes have
Extruding, Under Careful Tempe~ature Control. Lsng Bars from Which t h e Final Cakes Are Made
also been produced, particularly some recent products for washing woolen or cashmere sweaters, and similar items. Some of these have been advertised as "coldwater" cleaners. Another type is a liquid detergent and sanitizer for washing painted woodwork and other hard surfaces. Only in the last couple of years have solid syndet bars become somewhat competitive with soap bars. Price is not as much of an object for the toilet bar as for general cleaners. Research problems in making syndet bars have been colossal. Each of the three big soap and syndet companies have worked on the problem for years, and each has a product on the market.
The more crystalline character of syndets as compared with the plastic nature of soap has been a great source of trouble in manufacture. One of the new bars was a straight synthetic, which is both expensive and difficult to manufacture. Other producers have brought out composite bars containing both soap and syndet. Such a blend can be used because in the presence of the syndet, hard-water calcium and magnesium soaps which might be formed in use, are efficiently and finely dispersed. They therefore do not give the sticky deposits of lime soaps. The prediction is that in another five years or so, all toilet soap bars will contain enough syndet to give this effect.
JOURNAL OF CHEMICAL EDUCATION
