FOSTER DEE SNELL and CORNELIA T. SNELL Foster D. Snell, Inc., New York, Ν. Υ.
Fifty Years of Detergent Progress DETERGENCY is itself a combina tion of physical-chemical properties. Therefore the story of development of detergents—soap or syndet—wan ders into the surfactants used for special purposes. Before World W a r I, chemistry was a minor profession in this coun try. G e r m a n y was d o m i n a n t in synthetic organic chemistry, which started in the U. S. because of short ages of dyes a n d medicinals d u r i n g t h a t war. O n e measure of the prog ress of chemistry is the increase in AMERICAN C H E M I C A L S O C I E T Y m e m
bership in 50 years from 5000 to 83,000. A m o n g these m e m b e r s are the m e n w h o m a d e our 50 years of detergent progress. Soap In
1909,
w h e n the
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ISTRY first m a d e its a p p e a r a n c e , only a n occasional article on soap was published. Syndcts did not a p p e a r until decades later. Soap, a pre historic chemical product, was largely m a d e by the kettle process. T h e glycerol was often not recovered. Some cold-process soap in which glycerol and residual unsaponified fat r e m a i n was still manufactured. T h e p r i m a r y function of soap was as a detergent, but to it were attrib uted from time to time germicidal properties. Phenolic bodies were a d d e d to at least one b r a n d . I t is only within recent years that per fumed soaps—antiseptic because of a d d e d germicides such as 2,2'-dih y d r o x y - 3 , 5 , 6 , 3 ' , 5 ' , 6 ' - hexachlorodiphenyl m e t h a n e , 2,2 '-ethylidene-
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bis(4-chloro-6-nitrophenol), or other structures—have met with success. Any fat or oil will saponify to form soap; tallow, cottonseed oil, coco nut oil, peanut oil, whale oil. Selec tion has been largely on a price basis, with hydrogénation to harden the soft oils and inclusion of sufficient lauric acid oil to give good lathering. A pioneer in investigating the chemistry of soap making was Martin H. ' Ittner, who studied the bleaching of fats, and developed production of pure glycerol and analytical procedures pertaining to soap and its manufacture. By 1917 specifications for soap had appeared and a few years later standard methods of analysis were published. Soap is built or extended with less expensive materials. In 1923 Albert S. Richardson presented data on the use of sodium silicate in soap to improve its detergency and maintain the desired alkaline pH. Its prevention of corrosion of metals was stressed by James G. Vail. Detergent power was studied in 1929 by F. H. Rhodes, using standard soiled cloth and a washing cylinder, and reporting results in terms of the brightness of the washed cloth. Further developments were three types of machines washing multiple samples. Many laboratories prepare their own artificially soiled fabrics and at least three sources supply them commercially. Soiling ingredients vary from carbon black to the dust collected in air conditioners. Laundries are very cost-conscious. Early formulas consisted of 3 parts of soda ash or modified soda to 1 part of 88 to 92% chip tallow soap. In the late 20's and early 30's this changed. First proprietary products contained largely sodium metasilicate—not to be confused with the 1:3.86 N a 2 0 : S i 0 2 silicates incorporated in bar soaps. Their success over a period of years at the 1 to 1 ratio led to marketing of sodium orthosilicate at the 2 to 1 ratio.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Late in the 1930's John W. Bodman caused a change in floating bar soaps. Instead of having air beaten into it in the liquid phase, the soap was mechanically aerated during cooling. The net result was a more uniform air dispersion, a whiter bar which did not warp, and higher soap content. A revolution also occurred in household soaps. Household spray-dried granules built with silicate and phosphate had been displacing the brown bar of laundry soap, now little more than a memory. The sequestering power of a molecularly dehydrated phosphate, sodium hexametaphosphate, locks up calcium and magnesium ions so that they are no longer precipitated by soap. In the early 30's relatively inexpensive tetrasodium pyrophosphate was used as a séquestrant in granules. A later development was sodium tripoly phosphate. Continuous soap-making techniques have always intrigued the imagination. Reaction of soda ash and anhydrous fat in a vacuum to distill glycerol in a stream of carbon dioxide was successful but not commercialized. A later development was the wet-way saponification of the fat in a centrifugal operation—the Sharpies process. Others have been modified kettle processes. Some kettles in use today were in use before the start of the half century we are describing. Soap always contains a trace of unsaponified fat. Tin salts prevent its becoming rancid, to the detriment of odor and color. James W. McBain pioneered in the study of the theoretical aspects of soap and soap solutions—soap phases as produced by salting out and the nature of soap solutions. For example, he reported that a dye which was not soluble in water apparently was dissolved in soap solution, a phenomenon designated as solubilization. He showed that organic liquids such as glycols could
be made miscible with hydrocarbon solvents such as carbon tetrachloride by the presence of soap. McBain advanced the idea that there are several different forms of soap micelle. The smallest he pictured as spheri cal, with the polar ends of the chains sticking out toward the surrounding water, the largest, as flat plates, again with the polar ends toward the water. The spherical micelles he believed to be highly ionic, the plate lets only slightly ionic. The present picture agrees more or less with this, in that it is believed that many forms of micelles exist. William D. Harkins made x-raymeasurements of micelle size. He pictured soap micelles as double layers of molecules, arranged with the cations at the outside in a water layer, and the fatty parts pointing toward one another. The double layers of molecules were visualized as separated from one another by a thin water layer. Probably the exact form of the micelle is not so important as the fact that such men as McBain and Harkins gave us a better understanding of the behavior of soap in solution and of changes in behavior in terms of con centration. Ralph H. Ferguson, Robert D. Void, and others have also added to our knowledge of the physical forms of solid soap. Phase diagram studies show a number of crystalline forms. Syndets
Syndets are relatively new. Shortly after World War I sulfo nated naphthalene derivatives had some importance as textile wetting agents. In the 1920's, in Germany, hydrogenolysis of fatty esters to long-chain alcohols was followed by fractional distillation of the alcohols. The desired distillate was esterified with sulfuric acid to form the sulfate, which in turn was neutralized with caustic soda. Schrauth used a chrome-copper catalyst and 200 atm. of hydrogen at 320° to effect hydrogenolysis, the only really difficult step. Soap, although an excellent deter gent in soft water, decomposes in acid solution, forming insoluble fatty acids; it is salted out at high concen trations of electrolytes; and it forms gummy precipitates with the cal cium and magnesium compounds in hard water. The large cities on
Syndets Grow While Soap Sales Drop Source: Association of American Soap and Glycerin Producers
Sales (hundred millions of pounds)
Population (millions)
- T o t a l Soap a n d S y n d e t Sales
30
1910
1920
1930
the Ohio and Mississippi Rivers and bordering the Great Lakes are accustomed to water of 7 to 8 grains of hardness—that of New York is under 1 grain. Many areas, particularly those relatively remote from the sea coasts, have water of 15 to 20 grains of hardness. The new alcohol sulfates formed soluble calcium and magnesium soaps which were themselves good detergents. They were the answer to household and industrial needs for cleansers in hard-water areas. In washing textiles before acid dye ing, extreme care had been neces sary in order to remove all of the soap, before the advent of syndets. The multiple patents were pooled and Du Pont produced the industrial grade, with Procter & Gamble producing the household product. Dreft, the first household syndet, was born 25 years ago this summer. It contained roughly one third so dium lauryl sulfate and two thirds sodium sulfate. Other new surfactants were developed by the IG. Igcpons A and Τ patented by Fritz Gruenther in 1933 were condensation products of isethionic acid and a fatty acid. While expensive, they filled a need in the textile industry, particularly because they were stable in alkaline solutions. The cost has been brought
1940
1950
1960
down to a level where Igcpon Τ is present in a commercial household syndet; These products were a challenge to American chemists. Lawrence H. Flett is credited with reacting chlorinated kerosine with benzene to give kerylbenzene. Sulfonated, this was Nacconol NR, available in 1933. Later developments substi tuted propylene tetramer for the chain from kerosine to give a more uniform product now known as dodecylbenzene sulfonate. So man ufactured, the branches of the side chain arc no greater than methyl groups. Alphonse Ε. Yaeger of American Cyanamid patented the sodium sulfosuccinates in 1938. Aerosol O T , the dioctyl ester, is an extraordi narily powerful wetting agent. In 1939 a list of brands of syn thetic surfactants available included 160 items. Two years later, 350 were listed. In the fourth revision by John W. McCutcheon, just out, over 2000 were named. Many new agents were quickly adopted by the textile industry for wetting, leveling, penetrating, or detergent properties. Gradually it was realized that lowering of interfacial tension could be of advantage in many processes—in electroplating, pigment manufacture, paper makVOL.
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ing, fruit washing, as a spreading and wetting agent in insecticides, for rug washing by use of high foamers, as an emulsifier in cosmetics, and for general cleaning. In some uses, the agents supplanted soap, in others they were effective where soap was unsatisfactory. Surfactant Evaluation
New products or possible new prod ucts were usually tested by simple and rapid physical determinations— foaming value, surface tension, and interfacial tension. If a product was poor, it was rejected at this point. Further tests were made by experimental washing of artificially soiled swatches in the Launderometer. Thomas H. Vaughn com pared values obtained in a Launderometer and in a commercial washer, and concluded that a single wash in a Launderometer gave satisfactory relative results. The use of this instrument, as well as much other valuable information, is given by Jay C. Harris ("Detergency Evaluation and Testing"). In 1934, one of the authors stated, "Rarely is a patent issued in the field that docs not permit of hun dreds of variations." One patent alone gave methods for making 42 surfactants. A definite degree of balance must be maintained be tween the water-attracting and waterrepelling groups, although a certain amount of leeway is permitted. The more important water-attracting or polar groups used commercially were — S 0 2 O N a , —OS02ONa, —SO2OH, and — O S 0 2 O H — sulfonates and sulfates. Other polar groups include ·—SH, — Ο — , —OH, = C O , — CHO, — N 0 2 , —NH 2 , —NHR, —NR* —CN, — CNS, —COOR, —OP03H2, —OP02H2, — O S 2 0 2 H , - CI, —Br, and — I . In syndets, only one sulfonate or sulfate group could be used, attached to a hydrocarbon chain to give a molec ular weight of about 300. Two polar groups would be too many for proper balance. In the alkyl sulfates, a straightchain hydrocarbon gives a better detergent than a branched chain, because the straight-chain molecule orients better at the interface and permits closer packing of detergent molecules. Branched-chain struc tures are more water-soluble and tend to increase wetting power and decrease detergent power. Good 50 A
detergent results have been obtained with straight hydrocarbon chains of Ci 2 to Cie attached to the sodium sulfate group. For many years coco nut oil was the chief source of the hydrocarbon portion of the alkyl sulfates. More recently hydrocar bons from other sources have become available for the alkyl group—from petroleum fractions and from tallow alcohols. Tallow is now a readily available and relatively inexpensive domestic source of fatty material because of the drop in soap produc tion. Coconut oil is more expensive and more subject to variation in price. The original fatty alcohol produc tion by Du Pont was by hydrogenolysis at high pressure. Later, for Dreft manufacture, Procter & Gamble installed sodium reduction. Both types have been multiplied as patents expired. In a continuous method, Procter & Gamble now converts fats to the methyl esters, distills, hydrogenates, and reduces them to the alcohols with excess hydrogen at high pressure, in the presence of a chrome-copper catalyst. Of the surfactants used in syndets, sodium dodecylbenzene sulfonate is in largest production. The 2.9 bil lion pounds of syndets sold in 1957 would have contained 750,000,000 to 1 billion pounds of surfactant, the balance being various builders. Nature of Detergency
Progress is made, though slowly, in understanding detergency. In the laundry and household cleaning, most soiled materials bear a film of oil, more or less heavy, according to the nature of the substrate. The first contact of the detergent with the soil is generally at an oil-water interface. The detergent solution must first wet the oily part, then emulsify it, and so remove it from the article being cleaned. It must wet solid particles of soil and then disperse them in the cleaning solu tion. Permanent dispersion is impor tant; otherwise part of the soil can be redeposited. Snell and Reich have shown that the detergent is ordinarily sorbed from solution onto the substrate or material being cleaned. This sorptivc effect must be important enough so that the substrate will sorb the detergent in preference to the soil. They found that mechanical action in laundry
INDUSTRIAL AND ENGINEERING CHEMISTRY
work could be responsible for more than 5 0 % of the cleaning action. Osborne C. Bacon also reported the importance of mechanical action to loosen and remove the soil. Nonionics
The earliest surfactant and syndet was anionic or anion-activc, having a large anion attached to a small cation such as sodium, in sodium dodecylsulfate. Other cations were soon used, such as ammonium and triethanolamine, to give more soluble compounds useful in shampoos. A somewhat different type was the nonionic, in which no strictly ionic group was present, but a combina tion of several water-attracting atoms. As an example, ethylene oxide is added to a fatty acid, fatty alcohol, or substituted phenol to give a chain of —CH2O groups terminating in hydroxyl. More complex are the Spans, fatty acid esters of a polyalcohol such as sorbitol. For greater water solubility ethylene oxide is added to give the Tweens. Both are brands of the Atlas Powder Co. Nonionics are necessarily expen sive, as compared with anionics, partly in terms of raw materials and partly because of operational costs. In spite of cost, many combinations have been developed. Thus in an investigation of detergents for use on shipboard in sea water, Thomas Vaughn found four satisfactory: the sodium salt of a fatty acid sulfo nated amide, a polyalkyl ether con densate of fatty acids, an alkyl aryl polyether alcohol, and the polyeth ylene oxide derivative of sorbitan mono-oleate. The last three are nonionic, the water-attracting por tion being supplied by oxygen or hydroxyl groups. Nonionics have found many uses— in scouring raw wool, in combination with antiseptic cationics to give germicidal cleaners for dairies, brew eries, and restaurants, in dry-clean ing, and in pigment dispersion. Washing of Cotton
Soap can be used for washing in hard water, but part of the soap softens the water before the rest becomes effective for cleansing. Enough soap has to be used to dis perse the calcium and magnesium soap precipitates, so that they will not deposit on the fabrics being washed. Even so, during rinsing,
some precipitate is often deposited; white goods become grayish in time. Even though the early syndets did not precipitate in hard water, they did not wash cotton goods white. In 1944 a patent was filed by David Byerly on the building of syndets with massive amounts of tripolyphôsphate, more than double the amount of the surfactant present. This proved to be the answer and the principle is applied to all heavyduty syndets. The patent was issued in 1949. During the war phosphorus was needed for other uses and new plants were out of the question. Immediately following the war the race was on to build enough phosphorus capacity to make the molecularly dehydrated phosphates to build the syndets the public was demanding. Building of Syndets
Unlike soaps, the syndets were not successfully built by substantial amounts of such alkaline salts as orthophosphate, soda ash, and metasilicate. Rather, the heavy-duty types have developed as a carefully tailored composition in which each ingredient has its specific function. The early syndets were of the lightduty type. They usually contained more than 50% of sodium sulfate, ^present because of caustic soda added to neutralize the excess sulfuric acid needed to sulfate or sulfonate the product. This brought the price per pound down, comparable with soap. For many purposes such "light-duty" products are satisfactory. Much research on builders resulted in the discovery of a number of additives, such as sodium tripolyphosphate, sodium silicate, sodium sulfate, carboxymethylcellulose, fatty amide, and a fluorescent dye. The first three have been mentioned as builders. The big developments in builders and hence in heavyduty syndet production took place only after World War II. A largescale polyphosphate industry could be developed only after the war, during which benzene and sulfuric acid were also short. However, the sodium sulfate proved experimentally to be a desirable additive for light-duty washing; the product gave better detergency with sodium sulfate than when the corresponding amount of surfactant was used alone. The sodium sulfate appears to pro-
mote micelle formation by the surfactant, just as alkaline salts do with soap. Such light-duty products are appropriate for dishwashing and for laundering silks and nylons. The family wash requires heavy-duty types. A typical product will contain 18 to 2 5 % of one of the anionic surfactants, upward of 40% of sodium tripolyphosphate, and a few per cent of residual sodium sulfate. A fraction of 1% of carboxymethylcellulose prevents soil redeposition. Syndets compare unfavorably with soap in foaming power. Some 3 to 7 % of condensed fatty amide modifies the foam to resemble that of soap. Precipitation of alkaline-earth soaps gives an adherent protective coating to metals, but syndets do not provide that. So around 5 % of sodium silicate is added as a corrosion preventive. A small fraction of 1 % of a material, usually a urea derivative, is added to prevent darkening of silverware. Fluorescent dyes are added to both soaps and syndets to whiten by giving off a blue fluorescence. This eliminates blueing in washing white goods. Sorption of Syndets
In studies with radioactive calcium, Joseph M. Lambert in 1950 showed the close relation of detergency to the affinity of anionic detergents for sorption on cotton goods. More recently it has been shown that nonionics are sorbed to a lesser extent than anionics. Arthur L. Meader, Jr., showed that a detergent is not only sorbed by fabric, but also by soil. He reported sorption to be physical on cotton, but both physical and chemical on wool. Thus, in part, the manner in which detergents work has been revealed by such studies. In 1956 it was shown by Lloyd Osipow that fatty-acid monoesters of sucrose are good emulsifiers and detergents; they can be used successfully for washing cotton. The monoesters are not yet available, but the dicsters have become commercial. They serve as effective, low-cost, nontoxic emulsifiers in margarin. Of the nonionics, these are so far the lowest in cost in terms of raw materials. Low Foamers
With the large-scale distribution of
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automatic washing machines, lowfoaming syndets became desirable, particularly in rotary-drum machines. Nonionics were well suited for this in terms of their low-foaming character. Tall oil and alkyl phenol condensates of ethylene oxide chains were effective, as well as alkanolamide derivatives of fatty acids. The Future
This is the department which needs James Bryant Conant's plastic crystal ball. The growth of the syndet market by displacement of soap is largely over, except in toilet bars. Three brands of synthetic bars are in national distribution and at least two others in test markets. Their only serious flaw is cost, which is in line with the best milled soap and noncompetitive with floating bar soap. Ten years hence they will have the bulk of the toilet soap market. The remaining toilet bars will contain some synthetic surfactant to disperse alkaline-earth soaps. The public will have largely forgotten that a bar used to feel slippery and even slimy; the feel of the synthetic on the skin is different, because it is free rinsing. Progress of syndets in the power laundry field has been slow. It will continue to be, for reasons of cost. What about uses where it does not replace ' soap? Some present uses will expand—flooding of oil sands, additions to animal feeds, use in fertilizers. Some new ones will come along—treatment of land to decrease run-off of water, treatment of large water areas to decrease evaporation, even addition to human food to promote digestion. The rate of increase of volume will slow down but not stop, partially because of population increases. Plausible future figures are:
1958 1963 1968
Millions of Pounds Soap Syndets 1200 3000 1000 3S00 750 4000
But substantial reductions of cost of manufacture could easily upset those figures. New large-scale uses are particularly dependent on price. VOL. 50, NO. 8
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