SYNTHETIC DETERGENTS AND SURFACE ACTIVITY1 CORNELIA T. SNELL, Foster D. Snell, Incorporated, New York
J n s ~AS SILK stockings have largely been replaced by principle on which "standard soil" for laboratory studies nylon, so has soap been replaced for a number of uses by is based. The problem, then is to remove more or modern synthetic products, also the result of intensive iess oily particles of dirt. . chemical research. Although synthetic detergents, NATURE OF CLEANING sometimes termed soapless soaps, f i s t appeared hack in In order for a solution of soap or synthetic detergent 19201s, many people did not realize their real significance until the war period when soap was difficult to to remove soil, it must first wet it. Plain water fails to get here a t home. Then special soap-synthetic com- do this because of the oiliness. That is why your binations had to be supplied in huge quantities to our windshield is not washed clean during a rainstorm; Army and Navy to do a good cleaning job in all kinds the rain water fails to wet the oily film on the glass and of water and even in sea water. Producers of syn- so runs off zigzag in streamlets. When a detergent is thetics did their best to meet the emergency needs, so present, the solution wets both glass and soil so that the that we now have a synthetic detergent industry latter can be readily loosened. The same is true in amounting to more than a half-billion pounds a year. washing dishes and in washing clothing; wetting the Before taking up surface activity in general, and soil and the surface to be cleaned is the first step, specific types of agents in particular, it would seem including penetration of the solution to the underdesirable to have an understanding of some of the prin- lying surface to which the soil sticks. The next requirement is that the soil he separated ciples involved in cleaning so as to see why some chemical compositions are better for the purpose then others. into small particles dispersed throughout the detergent solution, and prevented from coagulating into clumps NATURE OF SOIL which might settle again on the cleaned articles. In Whether on the hands, clothing, table linen, dishes, other words, the cleaning solution must have both walls, or floors, soil may be considered as much the wetting and 'dispersing power. If the soil were oil same, differing in amount rather than in kind of only, the detergent would have to remove the oil materials present. It consists of water soluble ma- and keep it dispersed in fine droplets, which means terials which can be easily rinsed away and therefore that a detergent is also an emulsifying agent. A good offer no real cleaning problem, along with insoluble detergent like soap is high in wetting power, in ability substances such as lint, dust, soet, some types of pro- to emulsify oil, and in dispersing.powel-the ability to teins and oily matter, which have to be wet, loosened, suspend solid particles and prevent their redeposition. and then removed by mechanical friction or rubbing. Alkaline salts such as sodium carbonate, phosphate, These are what make cleaning difficult, particularly the and silicate, neutralize the acid present in soil, an imoily matter, itself complex and more or less sticky and portant factor in the presence of soap, and one which adherent, causing otherwise loosely adherent solid makes them extremely useful as soap builders, but a and sooty soil to cling to fabrics and to smooth surfaces. less important factor in the presence of synthetic deterThe oily part can be considered as a mixture of grease gents. However, synthetic -detergents can be built, of the lubricant type, of food oils and fats-that is, and we will go into that later. Soil also commonly saponifiable oils-and of traces of free fatty acids. contains saponifiable fats and fatty acids. Alkaline The latter are formed when saponifiable oil< turn salts react with the latter to form soap. Even though rancid or decompose. Soil is almost always found the amount may be only a trace, this soap is extremely to be moderately acid, so that acids other than those effective in cleaning because it is formed in the dirt itself. But this is straying a little from the subject. Synformed from the decomposition of fat are no doubt present. The proportions of these different ingredients thetic detergents were originally developed not because vary with the type of article or surface being' cleaned, of a scarcity of soap but to overcome its disadvantages. While soap is the best possible general cleaning agent in but all are important. As a matter of fact, scientists, who have worked on soft water, it is not so good-in fact it is pretty baddetergent problems, consider that soil consists of solid in hard water. The calcium and magnesium salts which uarticles lightlv coated with an oil film. This is the make the water hard give soft gummy precipitates hy reaction with soap, and these stick to the article being 'The annual Marie Curie lecture, given a t State College, Pennsylvania, May 5, 1947, under the auspices of the Palladium cleaned and are as difficult to wash off with excess soap chapter of Iota Sigma Pi. as the original soil itself. Consequently, in hard water
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the first part of the soap added is consumed in softening the water, and only after the water is softened will the excess do the cleaning for which it was intended. This inconvenient property of soap involves not only additional labor in cleaning hut increased cost. It has been estimated that if the amount of soap required per family per year in water of zero hardness costs $1.65, the cost with water of 10-grain hardness or 170 p. p. m. calculated as calcium carbonate, would be $8.99, and with water of 35-grain hardness or 600 p. p. m. $228.42.' This last value for hardness is extreme. Municipally softened water may contain 3-5 grains of hardness per gallon. For example, St.. Louis softens its water to about 5 grains. Some cities use hard water-like Champlain-Urbana with 17.5 grains, Dayton with 21 grains, and Canton, Ohio, with 30 grains of hardness. Probably an average figure more or less representative of the hard water of the country would be 10 grains per gallon or 170 p. p. m. It is therefore desirable for general household cleaners to be effective in water of a t least this degree of hardness.
JOURNAL OF CHEMICAL EDUCATION
detergent in hot water, say a t 140°F, which is frequently used in commercial laundries; in cold water its solubility is too low for it to have the same degree of efficiency. In fact, sodium stearate is a major ingredient in commercial laundry soap. The balance achieved in soap between the polar group a t the end, and the straight hydrocarbon chain to which it is attached, appears to be ideal for detergent ability. The reactivity of this particular polar group,-COONa, constitutes its greatest disadvantage. The polar groups most frequently used in surfaceactive agents to produce water solubility include sulfate, --OSOaNa; sulfonate,SOaNa; amine -NH2; amide, -CONHz; other substituted nitrogen groups; and oxygen-containing groups such as ester,-COOR; hydroxy, --OH; and ether, ROR. The presence of unsaturated bonds in a hydrocarbon radical also 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 are also neutralized, mostly with hydrochloric acid, thus giving organic salts in both NATURE OF SURFACE-ACTIVE AGENTS cases. The relative balance between the two oppositesIn comparison with soap all synthetic detergents are resistant to hard water and also to mild acidity, which polar and nonpolar groups-determines the degree of decomposes soap with the formation of insoluble fatty surface activity of the compound. The general pheacids. Synthetics naturally differ among themselves nomenon of surface activity is related to the extent of with respect to these properties because of wide dif- concentration of the molecules of the substance a t the ferences in chemical structure. Some compositions are surface of a solution and to the degree of orientation of these concentrated surface molecules. In aqueous better suited to one application, some to another. Many types of compounds have been developed solution soap molecules are pictured as concentrating having surface-active properties, and thousands of and lining up in the surface layer inmore or less parallel patents have been issued to cover such compounds. rows, with the --GOONa group m the solution and the Some are merely wetting agents to give improved hydrocarbon group sticking out toward the air. At a wetting power on different types of surfaces and in water-oil interface, the -COON8 group in the surface different media, while others are detergents having molecules would be dissolved in the water, the hydroefficient cleansing action. Actually wetting agents carbon group in the oil. merge into detergents, so that a strict line of demarDetergents are in addition distinguished by the fact cation cannot always be drawn. that within the solution groups of ions or molecules In addition to original use as textile %gents,many becomeassociated to form micelles. With anion-active applications have been found in widely divergent in- detergents such as soap these micelles are negatively dustries. Because of very different uses many com- charged. Because of their ability to associate in this pounds have been synthesized or '%ailored" to meet way they are termed colloidal electrolytes. particular needs. Wherever wettability of a surface is As the chain length of the nonpolar portion is ina factor special surface-active agents become useful, as creased, the solubility of the chmpound in water is in spraying vegetation with an emulsion that will decreased. A relatively high molecular-weight nonactually wet the leaves, in cleaning dairy equipment polar or hydrocarbon portion is desirable for agents to and preventing the formation of "milkstone," in dis- be used as detergents, while a relatively low molecularpersing pigment in both oil and water paints, in washing weight nonpolar portion usually gives greater effectiveand sterilizing bottles, in cleaning metals prior to plating ness as a wetting agent. Lathering properties are rein emulsifying oils and waxes in cosmetic products, etc. lated to solubility and to surface orientation. If an All surface-active agents, including soap, combine agent is too soluble or too insoluble it will not produce polar and nonpolar groups in the molecule; this is a copious lather. The detergent types of surfaceabout all that the many types have in common. For active agents have been found to be better foam proexample, take a soap such as sodium stearate, which ducers than the strictly wetting-agent types of products. Branched-chain hydrocarbon groups give marked has the formula CHs(CHz)&OONa, or RCOONa, where R, the nonpolar group is a C1?alkyl chain and wetting power; the more closely packed straight-chain COONa is the polar group. This would be an excellent hydrocarbon groups give good detergency. Placing the water-solubilizing or polar groups in the middle of a FLAYMORE,D., Soap and Sanzlary Chemicals, 22, 48-9, 89 a branched chain increases wetting power; placing 1946).
907
OCTOBER. 1947
the polar group a t the end of a long straight-chain hydrocarbon group increases the group of properties which make for detergency. Other effects of. chemical structure can perhaps be brought out best by a discussion of the pore important types under which surface-active agents may be classified. Types of Surface-active Agents Classification is. difficult because many compounds produced in small amounts do not fit into any of the general classes which can be selected to cover fairly closely related types of compounds The following' have been chosen for discussion purpoSes. With each class are given one or more examples of the better known commercial products designated by trade name: (1) Sodium salt of sulfonated alkyl aryl compounds: Nacconol NR, Santomerse 1, Oronite, Swerl. (2) Sodium alkyl sulfates and acyl glyceryl monosulfates: Duponol WA, Gardinol WA, Arctic Syntex M, Igepon AP, Dreft, Vel. (3) Sodium alkyl d o n a t e s : MP189. (4) Sodium dialkyl monosulfosuccinates and alkyl sulfoacetates: Aerosol OT, Nacconol LAL. (5) Sodium salt of sulfated and sulfonated amides: Igepon T. (6) Sulfated and sulfonated oils and their.sodium salts: Turkey red oil. (7) Cation-active compounds including alkjl pyredinium chlorides and quaternary ammonium salts such as benzyl trialkonium chlorides: Ceepryn, Roccal, Zephiran. (8) Noniopic compounds such as complex fatty-acid amides, complex ethers, and partial esters of polyhydric alcohols: Santomerse DT, Triton NE, Spans, Tweens, Pergal 0.
.
with benzene in the presence of aluminum chloride as c a t a l y s t t h e Friedel and Crafts reaction. The reaction product is then fractionally distilled under vacuum and sulfonated. The sulfo group becomes attached to the benzene ring, probably some in the para position and some in the orlho position, although it is customary to describe it as the para compound. The process of sulfonation is intended to give a mouosulfonate, although some disulfonate is probably formed. The sulfo group is neutralized, ordinarily with sodium hydroxide, although small amounts of such other agents as ammonia and amines are used to give special products. Excess sulfuric acid from the sulfonation process is neutralized a t the same time, accounting for part of the sodium sulfate normally present in the finished product. Manufacturers find it convenient to get a product of 60 per cent active agent and 40 per cent sodium sulfate in this way, which is then usually further built to 40 per cent active agent, or even a little less. The role played by the inorganic salt is an important one, which will be discussed later. For maximum effectiveness as a detergent the total chain length of a straight-chain hydrocarbon should be about C,, to Ca. When the benzene group is introduced it has been found to be approximately equivalent to a C straight chain. This explains the desired 'Clzalkyl group in the alkyl aryl sulfonates. However, a practical point arises in that we are not too sure how many straight-chain groups are present. In other words, during chlorination the chlorine atom is likely t o attach itself somewhere within the chain rather than $0 a terminal..carbon atom. This results in a mixture of isomers, a fact necessarily ignored when drawing a graphic formula-which represents a probable average composition. A similar situation arises when an unsaturated hydrocarbon is condensed with benzene to sive substantiallv the same end ~roduct. Instead of benzene, other products contain naphthalene as the aromatic group in the compound. In view of the foregoing discussion it will be seen that the alkyl group will need to be shorter than that added to benzene, in order to give a total hydrocarbon radical which is not too large to balance the small polar sulfonate radical.
-
The nature of the com~oundsin each class can be brought out better by a discussibn of each. (1) Sodium Salt of Sulfonated All@ Aryl Compounds. The alkyl aryl sulfonates are the most important commercially of the synthetic detergents, as represented by some of the Nacconol and Santomerse products. More than twice as much of this type is produced as of any other class. Because of their greater importance they will be discussed more fully than the other types. The general formula is RASOaNa, where R represents au alkyl group, and A an aryl group. Because the sulfonate or water-solubilizing group is on the aryl radical, the former maintains its desirable polar properties and at the same time ensures a chemically stable benzene sodium Nonyl naphthalene sodium compound. A solution of the detergent gives a copious Dodecyl sulfonate sulfonate lather. A method of preparation which is much used is first to The alkyl aryl sulfonates are used mostly as general chlorinate kerosene or a similar close-cut petroleum detergents. As already indicated, they are particularly fraction. Research has shown that the chain length of useful in hard-water areas to replace soap, and in acid the alkyl group is desirable from Cn to Clp when this solution in certain industrial processes. They are comis attached to the benzene ring. This is sometimes pounded in varying proportions with alkaline salts to termed a "keryl" group because it corresponds more or give numerous branded household cleaners. The alkyl less to kerosene. The chlorinated hydrocarbon reacts aryl sulfonates are particularly good for wahing wool.
508
They are not as effective on cotton, their action beiug very slow. (8) Sodium Alkyl Sulfates and Acyl Glycevl Monosulfates. Next inimportance to the alkyl aryl sulfonates are the alkyl sulfates, which were the first synthetic detergents to become commercially successful on a large scale, typified by the Duponols and Gardinols. The general formula is ROSOsNa, ia which R is a hydrocarbon radical of chain length Cs to Cis, usually a straight chain although it may be branched. The structure is closely related to that of soap; the waterattracting group in soap, -COONa, has been replaced by -OSOaNa, while the hydrocarbon radical is essentially the same in both. In fact, the raw materials are t,he same as for soap, namely, saponifiable oils. Coconut oil is the one most used, as it gives a mixture with a desirable average chain length, often referred to as lauryl or dodecyl. The fatty acids present in the oil as glycerides are catalytically reduced with hydrogen to the corresponding alcohols. The alcohols are then esterified with sulfuric acid and excess acid is neutralized, usually with sodium hydroxide. As with the alkyl aryl sdfonates, excess sulfuric acid remains from one step in the manufacture and is neutralized at the same time as the combined sulfate group. This results in a final mixture of the detergent and the inorganic salt, sodium sulfate. Although the starting' material is the same as that for soap the processing is more involved, so the final cost per,pound of organic agent is necessarily greater than that of soap. Alkyl sulfates are excellent hard-water detergents, even better than the alkyl aryl sulfonates. Since the compound is an ester, the alkyl sulfate is less stable in strongly acid solutions than the alkyl aryl sulfonate. The alkyl sulfate is also less stable in hot water, although not sufficiently so for this to be a detrimental factor in laundering. Alkyl sulfates are more soluble than alkyl aryl sulfonates. They lather copiously, even in cold water and in hard water. T h e commercial shampoo, Drene, whoseact.iveingredient isalkylsulfate, illustrates a special case of the strong foaming and strong emulsifying power of this class of agent?. The alkyl sulfates make'excellent hard-water detergents and in the textile industry are especially good for scouring raw wool. Related to the alkyl sulfates are acyl glyceryl monosulfates, with the general formula, RCOOCH*CH(OH)CHLEOaNa, in which R represents a chain of length of about CII, as in Arctic Syntex M, the active ingredient in the powdered detergent Vel, and in the shampoo, Halo. To make the compound, glycerol is esterified with one molecule of a fatty acid andwith one molecule of sulfnricacid to give a mixed di-ester, which is then neutralized with caustic soda. Such products are relatively new on the market as detergents although old in some more restricted fields. The raw materials make them rather costly. Acyl glyceryl monosulfates are quite similar in properties to alkyl sulfates. They are very soluble, ex-
JOURNAL OF CHEMICAL EDUCATION
cellent emulsifiers, and high in lathering ability. (3) Sodium Alkyl Sulfonates. Also rather new, alkyl sulfonates may be represented by the formula RSOJNa, in which R is a chain length varying from 5 t,o 14 carbon atoms, with a preponderance of the longer ones. As in all sulfo and snlfonate compounds, the hydrocarbon radical is connected directly to the sulfur atom of the polar group, while in sulfates the hydrocarbon radical is connected to an oxygen atom in t,he polar group. These compounds are made from a petroleum fraction which i s sulfouated with sulfur dioxide mixed wit,h 'chlorine, rather than with sulfuric acid. Sodium sulfate makes a deiirable additive, as with the other classes already mentioned. Alkyl sulfonates resemble aryl alkyl sulfonates in properties but are not quite as high in detergent value. However, they are finding a place as inexpensive hardwater detergents. They are also stable in strong acid and strong alkali aud foam reasonably well, although the volume of foam is not as great as that with the alkyl aryl sulfonates. The importance of this agent is such that a new plant is being constructed by du Pont-the only manufacturer-to more than double the capacity. Earlier attempts by other companies to produce alkyl sulfonates were not successful because of lack of control of the hydrocarbon ch'ain length, resulting in nonstandard products which never achieved commercial success. (4) Sodium Diakyl Monosuljosuecinates and AlkyI Suljoacetates. Diakyl monosulfosnccinates were developed within the last 10 years.. The general formula is: Na0.S-CHCOOR LCOOR
The hydrocarbon ra(lica1, R, is varied in length to give a series of compounds, the best known being Aerosol OT, in which R is CsHlr giving the dioctyl sulfosuccinate. The starting material is not succinic acid but fnmaric or maleic arid, CHCOOH. By reaction
II
CHCOOH with the proper alcohol, au ester is formed containing the desired alkyl group. TKe ester then reacts with sodium bisulfite, which adds on the double bond. Because expensive raw materials are used the cost of this agent is necessarily high in comparison with other types. The branched-chain compound with a high proportion of water-attracting groups gives high wetting power but little detergent value. These products are of chemical interest because t,he effect on their properties by varying the chain length of the alkyl groups has been studied in some detail. For instance when R is CloH2,, the solubility in wat,er is only 0.15 per cent. As the length of chain is reduced, solubility increases, until a t C4Hgit is 32 per cent. Thr C, ester is too soluble to have strongly surface-active properties, but if dissolved in a 20 per cent solution of electrolyte such as sodium sulfate, it is nearly as effec-
OCTOBER, 1941
509
tive as the Cs ester in plain water. Those of you prevents general use as detergents except when their who have had colloid chemistry and remember the characteristic properties .are of special significance. effect of electrolytes 'in coagulating colloids will see Since they are extremely powerful antiseptics, they are that the same principles apply to colloidal surface- generally used where bactericidal action is needed. active agents. Although several times as expensive per pound as coalThe sulfosuccinates find applications in dyeing; in tar disinfectants their potency as bactericides has fact, the claim is made that more than 50 per cent of all created a place for them in sanitation. In addition package dyeing is done with these agents. to having a very high phenol coefficientthey are non(6) Sodium Salt of Sulfated and Sulfonated Amides. irritating to the skin, noncorrosive, and nontoxic. Among the more important surface-active compounds They can therefore be used much more freely than developed a t an early stage was Igepon T, produced cresols and phenol. first in Germany by the I. G. Farbenindustrie, and For example, important applications are as detergents later in this country by the General Aniline Company. for use in mechanical diswashing, for sanitizing eating This is a specific amide with the formula ClrHa3CON- utensils and glassware in restaurants and bars, and as (CH3CHBS03Na. detergents and sterilizing agents in many food-procThe compound is fairly expensive because of use of essing industries, including fisheries, bakeries, etc. rather costly starting materials and several processing They cannot he mixed with soap or other anion-active steps. Oleyl chloride is made and condensed with the detergent, since the two large radicals of opposite sodium salt of n-methyl taurine. The final compound charge precipitate each other, so that the activity of possesses fair detergent properties and is stable in both types of agents is lost. During the war the cationmildly acid and alkaline solutions. Its principal use active agents proved extremely useful for cold steriliis as a textile agent. zation of surgical instruments, and to surgeons in the (6) Sulfated and Sulfonated Oils and Their Sodium field for prewashing or scrubbing up before operating. Salts. Although usually referred to as sulfonated, (8) Nonionic Compounds. Another class, the nonactually fatty oils treated with sulfuric acid are sulfates, ionic compounds, cannot he illustrated by a general and may he represented by the formula R(OS03Na)R1, formula because of their varied nature. Instead of in which the sulfate is not an end group. The- most having one, two, or three polar groups in the compound common sulfated oil is Turkey red oil or sulfated castor usually several oxygen or nitrogen atoms, or some of oil, familiar for many years to the textile industry. Be- both, are distributed throughout the compound. For cause the principal acid radicalisricinoleic, having both example, 10 to 16 mols of ethylene oxide have been a hydroxy group and an'unsaturated bond, it is possible added to an alcohol or phenol in Triton NE. t,o have two sulfate groups in the same hydrocarbon These products are usually. liquids. Recently a radical after reaction with sulfuric acid, one esterified nonionic liquid detergent was produced by the Monwith the hydroxy group, and one added to the double santo Chemical Company under the name of Santobond. As a result the balance between polar and non- merse DT. This is dodecyl diethylene triamine. It is polar radicals is such that the product has wetting a good detergent but does not lather, which may seem power but little detergent value. Several other oils a defect in the eyes of most people who are accustomed are sulfated industrially but in much smaller quantity . to associating detergency and lathering power. Since these agents are nonionic, they ¬ be built effect,han castor 011. '? (7) Cation-active Compounds Such, as Quaternary tively with inorganic salts and differ in this respect from Ammonium Salts. Cation-active agents are the op- both anion-active and cation-active agents. Nonionics posites of the more common anion-active agents, to may be combined with soap or with anion-active synwhich class all of those discussed up to this point belong. thetic detergents. A useful combination has been In the cation-active agents, the effective radical in found of a nonionic with an alkyl aryl sulfonate; the terms of surface activity is positively charged; in the former increases the solubility' of the latter 2 to 3 times anion-active agents such as soap and the synthetics so that higher concentrations can he prepared for special already described the effective radical is negatively purposes, such as shampoo liquids. charged. The general formula may be written as (R1R2R3&)- SPECIAL APPLICATIONS XX, in which R stands for similar or dissimilar alkyl I might mention a few specific applications of surfacegroups with at least one and preferably only one over active agents. During the war millions of man-hours 8 carbon atoms in length, or a cyclic group may be were saved by application of surface activity. To present. The latter is typified by dodecyl dimethyl illustrate with an example from the Delco plant of I~enzylammonium chloride, the active ingredient in General Motors: They were making undercarriage Hoccal and Zephiran. X is a halide, usually chloride parts for bombers. Originally, the parts were m e but sometimes bromide; dodecyl represents the average chined, spray-cleaned with an alkaline cleaner, dried, from the mixture obtained from the coconut oil. coated lightlywith oi1,andshipped. At theotherendthey Some cation-active agents are high in detergent prop- were unpacked, cleaned, dried, and put on the assembly rrties. In general they possess marked foaming, dis- line. Here is what surface activity did: The original persing, and emulsifying ability. Their high cost cleaner was changed to an emulsion cleaner from which '
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the metal parts sorbed a protective layer only a few molecules thick by preferential wetting. The cleaned parts were simply packed with no further coating or drying, no cleaning a t the otherend. They would not mst in less than 30 days. The rayon industry owes a great debt to surface activity. The platinum spinnerettes through which the plastic filaments emerge accumulate two types of deposits, one in the orifice, the other as craters around the orifice. Cation-active agents or nonionic agents sorbed on the platinum will delay the accumulation so that only about one tenth as many spinnerettes have to be removed and cleaned per day. A great mass of contradictory patent literature can be found on this subject. METHODS OF EVALUATION
In developing new surface-active compounds, in studying different structures, and in comparing the effects of different builders, it is necessary to have some method of evaluation. Rapid and simple preliminary studies which prove useful in studying surface activity are determinations of surface tension and of interfacial tension. For this we use the du Nuoy tensiometer which gives absolute rather than empirical values. A low surface tension means that the solution will displace air better than plain water will. A low interfacial tension against a surface means that the solution will wet that surface better than plain water will. Effective surface-active agents reduce surface tension from about 72 dynes per cm. for plain water to a range of 25 to 35 dynes per cm. for a solution of a surfaceactive agent. They will reduce interfacial tension against benzene containing Ck1 per cent of oleic acidthe nonaqueous phase standard in our laboratoriesfrom about 29 dynes per cm. for plain water to 5 dynes per cm. or less. Another method for preliminary studies of surfaceactive agents is measurement of dispersing power. A method that we developed which has prov8d very useful is to determine the ability of the sample solution to disperse h e particles of oiled umber, which is iron silicate. Two hundred mesh umber is coated with 4 per cent by weight of a mixture containing one per cent of oleic acid and the balance a 1:l mixture of cottonseed and light mineral oils. Shce most solid soil is lightly coated with oil, we feel that the slightly acid oiled umber can he considered fairly representative of soil particles. A definite weight of oiled umber is added to 100 ml. of the solution of agent under investigation and the Nessler tube containing this mixture is inverted 20 times, then allowed to stand for 2 hours. At the end of this time 50 ml. of the quspension are removed from the middle of the column and poured into a modified Jackson electric turbidimeter. The height in centimeters a t which the light disappears is read. Dispersing power is expressed as cm.-' X 100, so that the higher the value the better the dispersing power. Sometimes foaming ability of a new product is also
studied, although foaming power is not directly related to detergency. Further studies made with promising agents may be carried out in the Launderometer, or what we consider even better, in a typical household washing machine. So many factors enter into detergency that final comparative results are best made by by actual washing tests in which variables are controlled as far as possible. This means that the temperature and time of washing, concentration of agent, hardness of water, rat.io of load to volume of wash liquid, and type of soiled material are all fixed. Reflectance of the gray standard soiled cotton cloth is read before and after washing, in a Pfaltz and Bauer glossometer, in order to determine the amount of soil removed or per cent of brightness regained. Comparative values can be obtained showing the relative effectiveness of different agents when used to wash standard fabrics under identical conditions. LABORATORY RESULTS
Anion-active agents can be very effectively built with sodium sulfate, some of which is normally present from the manufacturing process, and more of which is added to decrease the cost of the final product. Commercial aryl alkyl sulfates are frequently about one-third organic detergent and two-thirds inorganic builder such as sodium sulfate. Commercial alkyl sulfates may even be one-quarter organic detergent and threequarters sodium sulfate. The surprising thing is that such a large amount of inorganic material possessing no surface activity of itself can be added without detracting seriously from the surface-activity .. of the organic material. To illustrate this, interfacial-tension and dispersing power values have been determined as follows:
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Aaenl ~ v o r i soap flakes Alkyl sulfona?' Alkyl sulfate
InterSurface facial Tension Tension Dynes per om. at 0.2% . .Conc.
Dispersing Power 0.025%
agent
0.175%
Na?S04
0.05% 0.2%
agent
0.15%
aplent NsnS01 -
25.7
4.7
...
91
31.9 32.7
2.6 2.9
2.1 2.2
9 7
5 7
A few figures may he of-interest to show results of actual .washing tests under identical conditions with standard soiled cotton cloth in 15-grain or 255-p. p. m. hard water. ,
Ivory 8oap flakes Ivory soap flakes Nacconol NR
0.3 0.2 0.2
."
53.7 23.5 33.0
With Ivory soap and Nacconol NR a t the same ooncentration of 0.2 per cent, the synthetic does a better cleaning job. But with sufficient excess of soap to soften the water and serve as detergent, that is, a t a concentration of 0.3 per cent, the soap is far superior. I might add that an experimental agent tested in this same series gave negative detergency. In other words,
OCTOBER, 1947
mechanical cleaning alonethe1blank- removed more soil than when the agent was added. Possibly the agent itself was sorbed by the soil, so that the effect was to add solids instead of removing them. Our experience shows that in distilled water soap is by far the most surface-active of any of the agents which we have studied. A great.dea1 of research continues in many laboratories throughout the country
in the effort to modify the existing surface-active agents, either in structure or by blending materials, so as to approach more closely in hard water the performance of soap in soft water. We still need to learn more about the underlying theory of these complex compounds. What is the real secret of detergent ability? Why is soap superior as a detergent under proper conditions? Many improvements are still to come in this field.
