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0 SYNDETS AND SURFACTANTS

DETERGENTSre of practical importance to all of us,since we all use them several times a day. What arepopularly known as "detergents" today were firsttermed "synthetic detergents," then this was abbrevi-ated to "syndets." Syndets are cleansing agents, ap-plied 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 be-tween air and a liquid or a solid, and they lower inter-facial tension if the interface is between two liquids or aliquid and a solid. Such lowering of interfacial tell-sion promotes ease of wetting; for example, a solutionof a syndet wets soiled garments much more readilythan plain water will. This increased ease of wettinghas led to the development of a vast number of commer-cial applications.The great disadvantage of soap is precipitation of in-soluble, curdy calcium and magnesium soaps when thesoap is dissolved in hard water, or precipitation of insol-uble fat ty acids in acid solution. This explains the de-creasing popularity of soap in hard-water areas of thecountry, which are extensive, with corresponding in-crease in popularity of syndets. The latter, in general,are soluble and effective as detergents in hard water, insolutions of strong electrolytes, and in acid media.This explains why syndets and surfactants find so manyuses in industry, where soap would be completely inef-fective.The change in the sales picture of soap versus syn-dets in the United States since 1950 is shown in Table1' with the per cent loss or gain over the previous year.

TABLE 1Sales of Soaps and Syndets in the United StatesSoap(millionsof ibs.)30002480186516451450139013251189

%Loss172512124510

Syndetsmillionsof lbs.12501434153018672063231726902916

0-/oGain.-37181011148

FOSTER DEE SNELL and CORNELIA T. SNELLFoster D. Snell, Inc.,New York, N.Y.

of the molecule may consist of carboxyl, hydroxyl,ether oxygen, sulfonate, sulfate, phosphate, amino,ammonium, or other polar groups. This part of themolecule is attracted by water rather than by oilyphases or air. Innumerable combinations are possible,resulting in wetting agents and emulsifying agents,as well as syndets. When a surfactant concentrates atan interface, it orients so that each section of the mole-cule is in the phase which attracts it. This reduces in-terfacial free energy. The presence of unsaturatedbonds in a hydrocarbon radical promotes water solu-bility. Acid groups such as sulfate and sulfonate areusually neutralized, mostly with caustic soda. Basicgroups such as amine and substituted ammonium mayalso be neutralized, mostly with hydrochloric acid. Or-ganic salts are obtained in both cases, the first contain-ing a large anion, the second a large cation.Besides these two classes of anion-active and cation-active surfactants, there is a third, the nonionics. Thelatter have usually been considered as not ionizing andmostly to have an ether or ester structure. However, aconcept has been presented of an equilibrium in solu-tion in which some large cations are counterbalancedby hydroxyls. This would explain synergistic effectswith anionics which appear to be related to the forma-tion of loose complexes of th e nonionics and anionics.However, nonionics are much less reactive than eitheranionic or cationic types, and are sorbed from solutionto a lesser degree on solid surfaces. Each of the threeclasses will be discussed in some detail. Innumerablepatents have been issued on specific compositions; theemphasis here will be placed on commercially importantproducts.

In terms of tonnage production or money value, theanionics are the most important general class of deter-gents. The leader in this group is sodium dodecyl ben-zene sulfonate. This particular syndet constitutesmore than half of all the surfactants produced, typifiedby Nacconal, Santomerse, and Oronite. Its manufac-ture involves several steps: (1) production of alkylate,(2) sulfonation of alkylate, (3 ) neutralization of sulfo-nate, (4) building and drying.

GENERAL STRUCTUREA molecule of a surfactant is always highly unsyrn-metrical; one section is polar, the other nonpolar. The A/Q&latter is commonly a hydrocarbon chain. The nonpo- NaOHlar part is attracted to oily, fatty, or waxy substances,and to air, in preference to water. The polar section -"S08iia

Because of the importance of this syndet, its productionFrom the Assao. of Am. Soap & Glycerine Producers, Ine. will be given in more detail than for other syndets.

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Alkylate Production. For several years the alkylateor hydrocarbon portion of the syndet was made bychlorination of a kerosene fraction boiling at 185'-275"C., and reacting this with benzene by the Friedel-Kraft reaction to give an alkyl benzene in which the al-kyl group was a mixture of different chain lengths, butwith an average of ClrCla. Better control of the chainlengths has been achieved by development of propylenetetramer. This is made by polymerizing purified pro-pylene in the presence of a suitable catalyst, often a fluo-ride, sometimes phosphoric acid.CHaH H H H I H H H

2 C=C-CH - C=C-CC-CHH H ' H H H HThe dimer which first forms reacts with itself or withmore monomer. Fractional distillation of the mixtureof products from the polymerization gives the desireddodecene. Ample supplies of this are available as abyproduct fraction from other processes of the petro-leum industry.It has long been known that detergency is improvedby minimum branching of the side chain or alkyl por-tion. This is the reason for use of tetrapropylenerather than tributylene. With a straight side chain,maximum foaming is at a chain length of 10-12 carbonatoms, but detergency increases with a somewhat longercarbon chain.Alkylation of benzene by reaction with dodecene iscarried out in the presence of a sulfuric acid catalyst ora hydrofluoric acid catalyst. The latter catalyst isused under pressure at about 40C. With 90%-100%sulfuric acid, the temperature is kept at 0". The acidphase may he recycled. The desired alkylate cut is sep-arated by fractional distillation. Just after WorldWar I1 there was not enough benzene being produced inthis country to meet the demand for raw materials forsyndets. The petroleum industry then took over toprovide synthetic benzene from petroleum to amplifythat from the coke oven. It strikes a humorous noteto point out that now the coke-oven henzene has to berefined 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 proc-ess the alkylate into syndets themselves. They alsosell the alkylate to the large soap com pani es th e bigt h r e e t o a few large chemical companies, to manysmaller concerns. A considerable volume of alkylate isexported for processing abroad.Suljmatim of Alkylate. Two methods are in use:sulfonation with 20% oleum (fuming HBOa) and withliquid sulfur trioxide. In the former method the alkyl-ate is precooled to about 10C. in a stainless steel, or ina glass-lined reactor. Addition of 20% oleum is madeslowly with agitation and cooling to prevent localizedoverheating. About 1.25-1.3 parts of oleum are re-quired to 1 part of alkylate to give a monosulfonate.Addition of oleum is at a rate such that the tempera-ture can he kept at 25'-30C. Agitation is continuedabout 2 hours after addition of the oleum, with thetemperature still kept at about 25'. Only a minimalamount of unreacted alkylate may remain, usually un-der 2%.The "huildmg" (see later discussion) of syndets with

various chemicals which promote detergent actionshould be mentioned here, since the method of manufac-ture depends on the desired type of end product. If alight-duty detergent containing 40y0 or more of sodiumsulfate after neutralization is required, spent acid is notnecessarily separated. For a heavy-duty detergent tobe built with tripolyphosphate and other builders,maximum separation of spent acid is essential.To remove excess acid to give a sulfonated alkylatesuitable for heavy-duty building, water is added to thesulfonator with agitation, until the free sulfuric acid is7070-80%. Ice may he added instead of water, in or-der to keep the temperature at 60"-70C. Tempera-ture control is to avoid further darkening of the prod-uct, while at the same time promoting separation of thesulfonic-acid from the aqueous sulfuric-acid layer inwhich i t is insoluble.The mixture is pumped to aseparating tank, where thesulfuric acid layer is allowed to settle for 2-3 hours. Alonger settling time tends to darken the upper sulfonicacid layer. Spent sulfuric acid is drawn off, leaving themore viscous sulfonic acid containing 10%-15% of un-reacted sulfuric acid.Use of liquid sulfur trioxide is relatively new. Ad-vantages are that excess of acid is not needed to com-plete the reaction, so that the final product containslittle sodium sulfate. The difficulties of removing anddisposing of spent acid are avoided. This results in aquicker process. The product is said to avoid the"kerosene" odor of those made by use of oleum. How-ever, liquid sulfur trioxide is not easy to handle andmust be protected from contact with moisture to avoidpolymerization to the alpha form. A high heat of re-action requires efficient heat removal to keep tempera-tures from rising excessively.It is advantageous to pretreat the alkylate with 10T0of 96% sulfuric acid. Dry sulfur trioxide is vaporized,mixed with 9 parts of moisture-free compressed air, or ofdry nitrogen, and added to the alkylate. The reactionmixture circulates in contact with an efficient heat ex-changer. Sulfonation is a t 5O0+O0C. Diluent air isremoved. About 0.36 pound of sulfur trioxide is re-quired per pound of alkylate. Reaction time is keptto a minimum, which may be about 2 hours; neutraliza-tion follows imediately.

Neutralizatim. The viscous sulfonic acid is pumpedto a stainless steel neutralizer equipped with an ex-ternal heat exchanger. Commercial caustic soda solu-tion-50yo or 70%-is pumped to a measuring tankand from there run into the neutraliier where it is di-luted t o 180jo20%. The sulfonic acid is allowed to flowslowly into the caustic solution, with which it is mixedby agitation. The temperature should be kept downto 50"-55C. The last of the caustic solution is addedin accordance with the amount indicated by ti tration ofthe still slightly acid mix, in order to give a neutral sul-fonate. The final product contains about 50% of wa-ter.

The amount of sodium sulfate is 10%-15%, withsulfonate made by the oleum process, and practicallynil with sulfonate made with sulfur trioxide. Large-scale sulfonators are going more and more to continuoussulfonation with sulfur trioxide. Five continuoussulfonation processes have been described. Two plantsare manufacturing stabilized sulfur trioxide. ForeignJOURNAL OF CHEMICAL EDUCATION

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plants are now producing sulfonates by continuousmethods.Building. The term "builder" has been accepted asmeaning a compound which itself has little or no cleans-ing action, but which will greatly enhance the cleansingaction of a detergent, whether syndet or soap. Fromthe point of view of economical use, the building ofdetergents is very important. Much less syndet withbuilder can be used than would be required to give acomparable cleansing result without a builder. Deter-gency of soiled cottons can be obtained by mixtureseontaining a builder which could not be obtained withthe pure undiluted active agent. For many purposessodium sulfate and other inexpensive salts can be usedto replace a large proportion of the more expensiveactive detergent.The builders added to give heavy-duty products con-sist in large part of molecularly dehydrated phosphatessuch as tetrasodium pyrophosphate and sodium tri-polyphosphate. The largest selling syndet cleanser onthe American market has been built to the extent ofover 50% tripolyphosphate. Another additive whichhas come into general use is sodium carboxymethyl-cellulose (CMC). This material serves to promote de-tergency by preventing redeposition of soil suspendedin the wash solution. This property of CMC is partic-ularly noticeable with the syndets, the soil-suspensioncharacteristics of which are relatively poor comparedwith those of soap, particularly when used in washingcotton textiles.Syndet products have had a corrosive action on theparts of washing machines. This has been greatly re-duced by incorporation of sodium silicate in the deter-gent formula. Soaps are not especially corrosive be-cause a film of calcium soap tends to deposit on themetal parts, by reaction of hard-water salts--calciumand 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 non-crystalline silica which is gelatinous and difficult to seewhen wet. Such a deposit will form on aluminum,copper, iron, steel, brass, and bronze. This is one reasonfor including sodium silicate in heavy-duty formulas.Sodium dodecyl benzene sulfonate does not give asvoluminous nor as long-lasting a foam as soap does.The presence of soap suds has long been a criterion fordetermining that enough soap is present in the washsolution to do the work of cleansing. It became amark of a good product; lots of suds meant satisfactorywashing. The foaming of dodecyl benzene sulfonatehas been enhanced and made much more stable byinclusion of a fa tty amide in the formula. One of the

newer compounds for this purpose is N-oleyl-N-methyl taurate.The presence of a substantive fluorescent dye im-proves the whiteness of washed linens. This dye,

although itself colorless, gives off a blue fluorescencewhich counteracts the yellow which white materialstend to develop after repeated washing. The dyemakes the material look bluer, or actually in commonterms, "whiter." Such fluorescent dyes are now usedin all soap and detergent products for the laundry.They have replaced the earlier process of a blueing stepin the home laundry. A typical formula of a heavy-duty and hence a built syndet i~ given in Table 2.VOLUME 35, NO. 6, IUNE, 958

TABLE 2TypicalHeavy-duty SyndetIngredients % by weight

Dodecyl benzene sodium sulfonete 20Sodium tripolyphos phate 45Sodium silicate (1 3.25) 5Sodium earboxymethylcellulose 0 .5Fatt y amide 2. 5Sodium sulfate 22Moisture 5Fluorescent dye traceIn a fair-sized plant, the slurry may be run into asecond tank for addition of builders, with the twotanks arranged so that they can be used iuterchange-ably for neutralization and building. The dry buildersare added with heavy-duty, screw-type agitation in asteam-jacketed crutcher, to obtain maximum disper-sion and solution. The temperature is kept a t 55"-60C., with use of live steam if necessary. Air shouldnot be beaten in as this would cause undesired foaming.Hot water is added t o prevent t,he mix from becomingtoo viscous. If the product is to be drum-dried, thewater content should be kept to 35y07,-45%; if spray-dried, it should be 50%-60%.Drying. Drum drying is used for some industrialapplications where shipment over long distances isnecessary 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-driedproduct may range from 40 to 45 pounds per cubicfoot, spray-dried from 15 to 28 pounds.For drum-drying,slurry is pumped to a trough formedby two steamheated drums of the drier. As theserotate in opposite directions, each picks up a film ofslurry which dries as it is carried around to the doctor-blades which scrape it off. The speed of rotation issuch that when the doctor-blades are reached, themoisture content will be 1.570-270. A difficulty s thattripolyphosphates may be overheated and revert inpart to orthophosphate, an undesirable change.With spray-drying, the built slurry is pumped at82"-99C. through atomizers into a stream or preheatedair or gas. The fine spray, consisting of many smalldroplets, gives a quite uniform product. Some rever-sion of tripolyphosphate occurs but can be morereadily controlled. Danger of overheating is smallif the entering gas temperature is at 400480"C.The dried particles are carried along by the gasstream to the product-collection system, where theyare discharged to drums or to a screening and packagingunit. The loss of dry product as dust in the exhaustair can be kept below 1% and, if necessary, eliminatedby installation of a wet collector. As discharged, theparticles are generally not over 65'C., after which theyare air-cooled on a vibrating conveyor. If dried to2yo-3% moisture content, they do not cake in thecontainer but remain free-flowing. Many productsapproach 10yo of retained moisture. A particle-sizeanalysis from spray-drying should give 91y0 of beadsthrough 20 mesh and on 100 mesh. Material outsidethis range is mixed with incoming slurry.Built heavy-duty alkyl aryl sulfonates, after spray-drying, have a bulk density of 15-28 pounds per cubicfoot. Variations in bulk density can be made byadjustments in operating conditions. The larger the

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bead, the fluffier and lighter the product. Light-duty be petroleum-derived or from other sources. Th esulfonates tend to be of lower density than the heavy- reaction may be depicted as follows:duty type.A L K n SULFATES fi+cy+ + CHaCH,

Next in importance to dodecyl benzene sulfonatesamong the anionics are the alkyl sulfates, or fatty- R~ R~ y +alcohol sulfates, typified by dodecyl sodium sulfate. OCH2CH90CHsCHnOHThese were the first syndets to become commercially AI I (continued to any desired chainsuccessful on a large scale, for example, the Duponols RW length.)and Gardinols. The structure is closelv related tothat of soap with -COONa as the waier-attractinggroup in soap being replaced by -OS03Na. For manyyears coconut oil was the fat used, but more recentlytallow has also been a source for making alkyl sulfates.Inedible tallow is currently in abundant supply becauseof the great decrease in demand for it in soap making.Over the last several years soaps have fallen to about30% of the total cleanser market, while syndets havecorrespondingly climbed to 70%.To make alkyl sulfates, alcohols are fist made fromthe fats by catalytic reduction of the triglycerides withsodium, or by hydrogenolysis with a chrome catalyst.The alcohols are then esterified with sulfuric acid and

neutralized. The active agent may be built anddried in much the same way as dodecyl benzene sul-fonate.A study of tallow-alcohol sulfates showed that theproperties were a little different, depending on themethod of manufacture. Unsaturated groups suchas those present in oleic acid are unchanged by sodiumreduction; with hydrogenolysis they are changed tosaturated groups. The unsaturated product is a betterfoamer than the saturated. The saturated productappeared to be somewhat more effective than unsatu-rated for hand dishwashing, but both were in general,good detergents.Alkyl sulfates are even better detergents in hardwater than alkyl aryl sulfonates. By changing fromthe sodium to some other cation such as ammoniumor triethanolamine, products suitable for use in liquidform are obtained. When made of coconut oil, thealkyl sulfates are more expensive than the alkyl arylsulfonates. With tallow stabilized a t the relativelylow price of 7 cents a pound, raw material costs of thisare competitive with the latter. Major applicationsof the alkyl sulfates are in shampoos, cosmetics, andpharmaceuticals although substantial tonnage goes intohousehold syndets.NONIONICS

Practically all of the better-known nonionic deter-gents depend on a polyoxyethylene chain for the polarportion of their molecules. The hydrocarbon sectionis frequently an alkyl substituted phenol. Polyoxy-ethylene ethers of alkyl phenols are analogous to alkylaryl sulfonates with the polyoxyethylene chain takingthe place of the sulfonate group. Another type has astraight-chain mercaptan from petroleum sources,joined to a polyoxyethylene chain.Phenol is ordinarily synthesized from pet.roleumtoday. The polyoxyethylene group is built up fromethylene oxide molecules and the ethylene oxide inturn derived from petroleum. The alkyl side chain may

In 1957 the alkyl groups commonly used in makingsuch nonionics commercially included octyl, nonyl,dodecyl, dinonyl, and pentadecyl.The nonionics, when properly used, offer exceptionalefficiency in washing operations. Further, they arehighly flexible. Because the polar or ethoxy groupcan he built up step by step to any desired degree andbecause the alkyl group can be varied, it is possible totailor-make a large variety of nonionics for specialpurposes.The ether oxygen portion is the water-attractinggroup; increasing the proportion of ethoxy groupstherefore increases water solubility. When the ethoxyportion is minor, an oil-soluble product is obtainedsuitable for use as emulsifier in cosmetic creams, etc.When the ethoxy portion is greatly increased, the prod-uct becomes water-soluble. The great majority of thistype as produced contains 60%-70% of ethylene oxide.The ether linkage makes for chemical stability of thecompound-stable in acid, alkaline, and strong-elec-trolyte solutions.With anionics, such variability is not so easy. Therethe polar group consists of a single radical and ispresent on an all-or-nothing basis. Thus for an alkylaryl sulfonate we must have a t least one sulfonategroup, bu t two sulfonate groups are entirely too muchfor ordinary purposes. However, with a nonionic ofthe type described we can have 6 , 7 , 8 , 9 , etc., oxyethyl-ene groups in the chain, varying the solubility, wettingpower, specific detergent effects, etc.An interesting modification of this type of agent isthe sulfated nonionic, or rather, a nonionic convertedto an anionic. This is made by attaching a fairlyshort polyoxyethylene chain to an alkylated phenol,then sulfating t o attach a sulfate group via an esterlinkage to the terminal hydroxyl in the polyoxyethylenechain. Such detergents retain many of the virtuesof the nonionics while avoidinv some of their disadvan--tages. For example, they are good foamers.Another type of nonionic detergent is that made byreacting ethylene oxide with tall oil. Tall oil is amixture of fa tt y and rosin acids derived from pine woodduring sulfate pulp manufacture. It is quite low inprice and of limited value as a source of fat s or oilsfor many purposes. The use of this derivative hasbeen moderately successful in the laundry. The talloil-ethylene oxide nonionics offer the advantage oflow price. They are low in foaming power, a desirableatt ribute in some applications of syndets. For example,in certain types of automatic home washers, excessivesuds interfere with the cleaning process and decreasethe over-all efficiency. For this reason low-sudsingdetergents were developed, and competitive products

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of this nature are produced by the three major soapsyndet producers, namely, Procter and Gamble, LeverBrothers, and Colgate-Palmolive. The tall oil-ethyl-ene oxide nonionics have proved useful in makinglov-sudser detergents.Somewhat related is a type of nonionic based on thecomhiuation of ethylene oxide with alcohols. Thesemay be mixtures obtained from coconut oil, tallow, orother natural fats, or they may be polyhydric alcoholssuch as sorbitol. Again the number of ethoxy groupsin the molecule will determine the properties and permittailoring to fit the needs. Highly water-soluble mem-bers of this group have proved effective as dyeing as-sistants; they promote even dyeing hy delaying the rateof dye uptake by fabric.A nonionic which was first produced commerciallyin lg5' is One by the esterification reaction between Mxmg of lngredlenta $or Synthet~cDetergent B ~ Fs Controlled onsucrose and fat tv acids from tallow. Both raw ma- strict T,~. cheduleterials are low in cost. The first of the series wassucrose dipalmitate, a fat-soluble syndet highly satis-factory as an emulsifier in margarine. When thewater-soluble monotallowate becomes available, itmay be competitive with the firmly established dodecylbenzene snlfonate. Since the sugar esters are nontoxic,they are aimed primarily a t the food, drug, and cos-metic industries. As production costs decrease, theymay be expected to find more general use.In the past, nonionic detergents have had a ratherlimited market. However, during the last few yearsthere has been a very marked growth of interest innonionic detergents and the market appears to beexpanding very rapidly. They are combined withanionics in some detereent nroducts. I t is nossihlethat they form loose compounds with them mkch likethe addition compounds of nonionics with phenols.Nonionics are also compatible with cationics and arecombined with them in various cleaning products.CATIONICS

Cationics have a large positively charged group inbalance with a halide ion, usually the chloride. Thosebest known are the quaternary ammonium halides,such as alkyl-dimethylhenzylammonium chloride.

Another well-known compound is dodecyl pyridiniumchloride.TiN// \C1 C12H15

The quaternaries are used mostly for their powerfulgermicidal effect, for example, in cleaner-sanitizersuseful in dairies and food-processing plants, wherecleansing and sanitizing are needed simultaneously.In such products, cat,ionics are combined with a smallamount of nonionic plus alkaline salts to give combinedwetting, cleaning, and sanitizing action. The emulsify-ing action of cationics has special applications, oueexample being to emulsify pentachlorophenol in waterfor use as an herbicide and soil toxicant.VOLUME 35. NO. 6, JUNE, 1958

Alkane may be used as the starting material, fromwhich an aromatic chloride is formed. In turn this isreacted with an amine such as triethanolamine toform a quaternary ammonium chloride. Such a com-pound is relatively expensive.Among their industrial uses is one based on the ca-parity of these amines to adsorb strongly on metal.Such sorption results in a mildly protective film whichgreatly reduces the corrosion of the metal. Theseamines are also useful ingredients in rust preventives.Cationics and anionics are incompatible, since thetwo large radicals of opposite charge precipitate eachother; the activity of both type of agents is then lost.

PHYSICAL-CHEMICAL PROPERTIES OF DETERGENTSOLUTIONS

Many factors and many properties are involved indetergency. How do we know that one product maybe more effective than another? In the laboratory,comparisons have been made by measuring definitephysical properties, as well as making standardizedwashing tests. The picture as to the physical-chemicalproperties which are involved in detergency is beinggradually clarified. There may be a matter of opinionas to how important a given property is, but thatproperty can be measured accurately on an absolutestandard.Surfaee Tension. The attraction between the mole-cules of a liquid sets up a force which resists breakingof the surface. With water this reaches a rather highvalue, about 72 dynes/cm. A little concentratedsoap solution added to the water will decrease thesurfare tension to around 28 dynes/cm. Measurement

of surface tension is made with the Du Nouy tensiom-eter. Although low surface tension has some relationto high foaming power, the latter is not necessarilya property of a detergent.Interfacial T a s i a . Much of the soil which offersany problem is oily. If the particle is large enough,we may be able to remove it mechanically from asurface to leave an unimportant residual oil film on thematerial (cotton or wool or ceramic or metal). Thesame type of effect may be obtained with a detergentif that detergent ruptures oil-oil bonds so that theparticle of soil is separated from the base but does nottake all of its oil with it. There is ample evidence

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Determination 05 Surface Tension wi th DuNouy Tensiometer

that such a phenomenon can occur. To all practicalpurposes one never wets the underlying particle of solidsoil and so does not separate it from its oil coating.A third form of removal of the oily soil from the baseis by having the wetting agent effectively penetratebetween the oil and the base.The interfacial tension between the detergent solu-tion and the oil is a good measure of the relative easeof soil removal by rupture of polar-nonpolar forcesof attraction. It throws no light on the relationbetween the base of substrate and the aqueous deter-gent. Soil removal in terms of actual wetting of thesurface and displacement of oily soil may be determinedin different relative terms by measurement of contactangle. The actual value obtained is a function ofthree 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 bea relative measure of the rupture of these polar-nonpolarbonds, (1) and (3) must be constant. The procedureis to measure the contact angle of the detergent solutionon the surface under oil. Even here interfacial tensionplays a role in governing the intensity of displacementforces. One of the major problems of detergencytoday is th at of determining the relative importanceof the two mechanisms of soil removal under variousconditions.For years this laboratory has been using interfacialtension measured between the detergent solution andbenzene containing O.lyo of a highly purified oleicacid to represent acidity in soil. Work in England hasconfirmed the soundness of earlier reasoning in believingthat only the fatty acid in natural soil is significant inmodifying the effect of that interface. If the detergentwets the base surface and wets the soil so that the solu-tion is interposed between them, then the first movehas been made toward removal of the soil.This problem can be analyzed in terms of physicalchemistry. The energy applied to break the inter-face is the work of cohesion of the oil less the work ofadhesion between the oil and the aqueous solution ofdetergent. In sets of data obtained with the sameoil, the cohesion of the oil is a constant and the inter-facial tension is a relative measure of the wetting of the

oil. Likewise the contact angle of the detergent solu-tion against the surface of the solid is a relative measureof the interfacial tension between them, but in differentterms. All we need is a symbiosis of those two factorsto measure the interfacial tension of the oil versusthe solid base. It does not exist.Dispersing Power. A dispersing agent for solidparticles is believed to function by forming a stablemono-layer at the solid-liquid interface. Havingseparated a particle of soil from the fabric, no detergenteffect will be accomplished unless the soil can be keptin the suspension. The importance of the lat ter factoris shown in launderometer tests on standard soiledfabrics with certain low-quality detergents. In somecases we have found the fabric to be darker afterwashing than before. We interpret that to mean thatthe soil has been deflocculated, dispersed more effi-ciently over the surface of the fabric, but not suspended.

The method for measuring dispersing power consistsof dispersing oiled umber and reading the amountsuspended after 2 hours. While soap will give anumerical value of 50 or more at 25' in terms of presenttest for dispering power, synthetic detergents givevalues of the order of 10. At higher temperatures lesssoil is dispersed. Dispersing power is the factor indetergency in which unbuilt syndets compare leastfavorably with soap.Solubility i n Micelles. The structure of micelleshas received a great deal of attention. If one assumesthat the solution is saturated with molecular detergenta t the point where initial micelle formation occurs,some very interesting calculations can be made:(1) that with increase in concentration in the rangeused for detergency, micelles are approximately uni-form in statistical size, and (2) the statistical sizevaries over the general range of 3-7 molecules. Themolecules in micelles have their hydrocarbon chainspointed inward, dissolved in each other, so to speak,while the polar groups point outward toward thewater phase. Thus the free energy of the system isa t a minimum, since each type of group is in contactwith others like it. Using an analogy by McBain,a micelle is a group of molecules and ions bound togetherlike a bunch of eels with their tails-representing thenonpolar hydrocarbon radicals-tied together. Thesolubility of soil in micelles relates to detergency andin practical terms is probably a real factor when oildroplets are to be removed.

For many decades the occasional effect of a soapin producing a real erythema of the skin, and thefrequent dryness and harshness caused by reduction ofsebaceous matter (the condition often described inadvertising as "dish-pan hands") has been attributed tohydrolysis alkalinity. Another accepted factor is thepresence of fatty acids due to hydrolysis. Developmentof synthetic detergents having a pH value not radicallydifferent from tha t of the skin, which is normallyabout pH 5, was hailed in some quarters as solving th atproblem. Strangely enough, within the limits of ac-curacy of the methods for measurement of skin reac-tions, the harshness, if not the erythema, appears tobe a t least as great from contact with synthetic deter-gent solutions. Observations of McRain and ofHarkins that water-insoluble matter is dissolved inthe micelle, probably explains this apparent anomaly.

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The skin, which ordinarily is protected and keptsoft by the oily matter supplied by the sebaceous glands,becomes dry and rough by frequent contact with adetergent solution, regardless of whether the detergentis soap or a synthetic. If the detergent is effective inremoving oily matter from dishes, apparently it isnecessarily effective in removing it from the skin.Reducing the detergent efficiency of soap by addinglanolin or other excess fatty matter reduces the dryingeffect of the solution on the skin but does not entirelyprevent it. Theoretically it is possible to visualize apolar-nonpolar structure which would be selectivelysorbed by the skin from a detergent solution. Thereis 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 fromsolution by the soil which it removes, will ordinarilybe rather minor simply because soil is present in sucha small amount relative to the surface being cleaned.Sorption of detergent by the base or surface to becleaned will depend on its nature. Thus sorptionwill not be great on a metal or on a smooth ceramic.But the surface of a porous ceramic or a fabric is verygreat and therefore possible sorption of detergent isenormous.MEASUREMENT OF COMBINED DETERGENCY

What method should we use to measure the efficiencyof removal of soil from the base? Here again one isdealing with a problem which relates to methods to beapplied industrially. When fabrics are being washed,more than 50% of the total cleaning may be by me-chanical action. The mechanical effect is easilymeasured by comparing the results obtained (1) bywashing with plain water, and (2) with a solution of adetergent under otherwise identical, controlled condi-tions.The most generally accepted method for comparingwashing efficiency is with use of what is known as alaunderometer. Briefly, this consists of a rotatingshaft carrying multiple pint jars in a housing whichpermits temperature control.Sample detergent solutions and standard soiledswatches of cloth are placed in the jars. Mechanicaldetergency can be increased by enclosing a definitenumber of metal or rubber balls in each jar. Control-lable variables which must be standardized include thefollowing: nature of base surface to be cleansed, natureof the soil, building effect on cleansing efficiency dueto added salts, pH of the solution, temperature, degreeof agitation, time of agitation, hardness of water, con-centration of detergent, ratio of base surface to becleansed to volume of solution, ratio of soil present tovolume of solution.Qualitatively the effect of successive equal incrementsof additional soil decreases rapidly. So it takes manyhundreds of times as much soil to lower the reflectanceof a fabric from 30 to 29 as from 80 to 79. One practicaleffect is that there is a large visible difference betweenremoval of 98% and 99% of the soil. Another is tha tvery little soil need redeposit on clean sections of clothto change it from brilliant white to visibly dirty gray.This explains in part why ability to prevent any r e

deposition of soil is such an important requirement fora good detergent.What we are reallv lookinn for is a numerical value~ " -to express the efficiency of the detergent. Even forrelative evaluation some kind of measurement has tobe made. The most satisfactory procedure a t presentis to read the degree of soiling of the fabric photometri-cally in terms of light reflectance before and after thedetergency operation, and then to calculate by someform of expression the degree of brightness regained.PHYSICAL FORM OF DETERGENTSThe great majority of syndets go into household prod-ucts, for dishwashing, laundering, and general clean-ing. For many years these were granular. Aboutthe middle 1950's, liquid, light-duty syndets becamepopular for dishwashing, and the production of liquidproducts increased enormously. In 1957 liquid syndetsin the United States were one-tenth of the total tonnageand one-skth the dollar volume of syndets. Originallythese liquids were based on nonionics, which are fre-quently liquids to start with. Later, dodecyl benzenesulfonate was used in the liquid products, with alcoholor some other agent present to increase its solubility,combined with a smaller proportion of nonionic.Nonionics of the condensed amine type help solubili~esodium dodecyl benzene sulfonate.New materials for making liquid syndets are beingproduced, e.g., sodium lauryl ether sulfate. Thisforms clear solutions in water of any degrec of hardness.Sulfation of the hydroxyl group of an alkyl phenol, andof the terminal hydroxyl group of an ethylene oxidecondensate, results in products with good surfactantproperties and high solubility.Following the success of the light-duty liquids, heavy-duty liquids appeared, built with tetrapotassium pyro-phosphate and other suitable compounds. These arestill in relatively low production but may achieve intime the popularity of the light-duty liquids. Thepotassium phosphate salt is necessarily more expensivethan the sodium but the latter is not soluble enoughto be used in liquid concentrates.The popularity of liquid syndets for washing dishesis probably due to their immediate solution in the dishwater, and possibly also to a belief by many housewivestha t they are not so harsh on the hands as some of thegranular products. Liquids for special purposes have

Extruding, Under Careful Tempe~ature Control. Lsng Bars fromWhich th e Final Cakes Are Made

VOLUME 35, NO. 6, JUNE, 1958

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also been produced, particularly some recent productsfor washing woolen or cashmere sweaters, and similaritems. Some of these have been advertised as "cold-water" cleaners. Another type is a liquid detergentand sanitizer for washing painted woodwork and otherhard surfaces.Only in the last couple of years have solid syndetbars become somewhat competitive with soap bars.Price is not as much of an object for the toilet bar asfor general cleaners. Research problems in makingsyndet bars have been colossal. Each of the three bigsoap and syndet companies have worked on the problemfor years, and each has a product on the market.

The more crystalline character of syndets as comparedwith the plastic nature of soap has been a great sourceof trouble in manufacture.One of the new bars was a straight synthetic, whichis both expensive and difficult to manufacture. Otherproducers have brought out composite bars containingboth soap and syndet. Such a blend can be used be-cause in the presence of the syndet, hard-water calciumand magnesium soaps which might be formed in use,are efficiently and finely dispersed. They thereforedo not give the sticky deposits of lime soaps. Theprediction is that in another five years or so, all toiletsoap bars will contain enough syndet to give this effect.

JOURNAL OF CHEMICAL EDUCATION