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i.. ;i complex operation in \\-hich niany factors are involved. Lo\\- surface tension, lo\v interfacial tension, low contact angles, and high wetting, spreading, dispersing. ampending, and emulsifying pon-er have all been recognized as factors. So. also, is the actunl solubilization of certain soil. hy dettergents. But \dint iinifying ekment iq there in tlic \vhole picture? The iinifying eleniriit is the architecture of the detergent, molecule or ion, \rhich i s in all c:~-es clxiractwizetl both hy grc:tt length and by a dual hydrophilic-livdropliohic ncitiire. The typical pattern i.j that of soap, consisting of a long 1iydrocurl)on rhain, a t the cntl of \vIiicli i: :I iwtcr-wluble, polar group. \Then particles such :is this :;rc i n - c r t d in w i t w , \\-Liter molecules are pushed apnrt , an inteiticc~lie1 \vccn \v:itt'r :lilt1 !iytliw~:irhnchain is created, and free is t1ierefoi.r stoiw.1 u p in tlie intei.f:\rc. The f ~ ciier' w c;in l)cl retlured 1):- expvlling tlic (letergen: pnrt iclr in either o First, the particles niay lie cspcilcii t o tht. sui~faceant! concentrate tliere in an oricntecl laycr. polm gronps c!iwctct! d0~~-11v~:ircl into Tvater and non-polar hydi.ocarlxm chain- sticking iip\\xrtl into t l i r : t i l , . $;uch an ~ ~ C : i s o5.ilrface ~~l~~d h ~ wniwns * lo\\-rrccl ,sii~,iacc1cnsicin, in : ~ w ~ ~ i ~ ( lwith a n c ~the e G i b h la-i>v. Tlie stronger the hydrophilic chnractcr of the polar cntl ant1 the hyclropholiic character of the non-pol:ii, end, tile stronger \vi!l 1x1 thi. t!ic.riiiod~nnriiic comp1.iisioii t o siirf'acc nctiJ-ity. Going a step farther ive may i:se t!w ccxicepts of ;i '*cac.nterof poLiri~y," a "center of noli-polmity," nntl n "~iul:ii~-noii-pc~lnrnionicnt ." A 011
1 Prcscnted at t h e Twenty-first Xatioiiwl C ~ i ~ l l o iSyiiiiiosiuni: d n.1iic.h \vas held uiider t h c auspices of t h e Ilivision of C'ollciicl ('h(1niistry of t h c . \ m ~ r i < , a C'heii:ical n Soi.ir.ty Palo .llto, California, J une 18-20, i N i .
11 ould I)c less than 21-1. Tk fifty such ci t1loid:il particles ere to coalesce into one large micel!e. t h c h iecluction in ~ i r i n c eenergy ~ ~ o u l be r l large. Surface activity and tendency to form colloidal micelles are thus two different manifestations of the same funtiammtnl chaiacter pow--ed hy all detergents hecause of their chemical antl spatial constitution. I t is unlikely that thcb :&orbed surface layerh or the colloidal aggregates (at least in dilute sohition*) :LIC made u p of molecules in tile case of either the anionic or the cationic detergent-, 4nce w c h detergents are ionized like other strong electrolyte\. T h c ioni ale the surface-active, miccllc-forming constituents nt lo\\ concentrations, the micellci being formed only \\hen the reduction in free energy result in:: from coalescence oiit tveighq the electrostatic repulsion which ions of like charge have for one another.. In addition t o ionizing and forming colloitl, soap- liytlrolyze t o form free fatty acid or acid soap, biit \$e c m for good reason-, if perhap. hastily, dismiss these products from consideration :is artn-c iwshing constituents. On the one hand, adding acid to -oap produce. free fattv acid or acid soap but does not increase n-aqhing power, and on the other hnnci, adding alkali t o soap represses hydi*olyis without cleqtroying washing pol\ e l , Furthermore, synthetic detergents siich as the alkyl sulfi-ltes and siilfonate- cleanw ne11 and get do not hydrolyze a t all under normal Istundry condition.. The theory that soap solutions cleanse bertiiiae of their hydrolysis alkalinity \\-as discredited long ago. The role of colloidal aggregates in the washing proce;.%cannot lw treated SO cawa11y, foi it is ohvioii- that tenclency t o form colloid i, common to all good detergent.:. .All workers i n this field no\v agree that the first sharp do\vnward inflection in the eqiiiwlent contlnctivitt- curve (figure 1) marks the beginning of colloid formation on :I lnrgr - c d ~ antl , the concentration :it which this occws i4 commonly called thc ”critical concentration of micelle formation.” ‘There has been mucli controi ~ i . \ *:is t o the nature of the colloidal p:trticlry IImtley ( 5 1. for namplc, po-ttilatc- the .impkst pos-ihle micelle (figure 2) in dilute .oliitioii, roughlv y)heric.,il in ihapc. t \vo ion-lengths in dlameter, made u p of long-chain ion- ai ixngpcl \\ it11 their polar hcad- tiirned o11t\\ard, in contact ivith I\ ater, and their long chain3 cwnstittiting thc hr-drocitrbon interior, and cloiici oi gegenioni in vai inhle niinibri 3IcHain ( 9 ) , on the othrr hand, i upon the exiqtence of tivo di.;tinc.t kinds of micelle, the. (,-mall)ionic and the (laige) neutral oi lumellar type (figure 2 ) . The eaibtence of lamellar micelles ai h i g h soap coriccntratiotis ( 5 per cent or !iiglicr) i:: indicated by x - r ~ ~data,? y h i t thew i.; no cli1rc.t ryidence as to the qh:Lpe antl structure of the micelle. in dilute solution4 of laiindry stlength, i.e., of the order of n fen* tenth3 of 1 per cent concentration. 1.01 pielent purposes, h o ~-\ c~*er, the important point ii that no micellar itriictiirc has yet 1)ecn postdated which ha.: a long polar-non-polar configuration with centcrs of polarity and 11011polarity separated by great distances, and thi- conhgurntion being lacking, one 2
S - r i t y u o r h o n i u s p silutions is be( onling c\tcnsivr Pcc icfcienccs r l t e d by XcI3ain I I I O T C I w c n t \!oi1, 7Tt13airi ( i ( I ( (10’ and t)J H a T h n - . P t f f l (4
i q ) and
CORRELITIXG PRINCIPLES OF DETERGENT ACTION
87
would not, a. priori, expect the micelles t o be surface active or to contribute t o the washing process by x-irtue of surface activity. This last statement requires further comment, for the micelles do solubilize many forms of dirt by solution in their interior; furthermore, they may act as a reservoir t o restore the equilibrium when soap in tnie solution is destroyed or exhausted, as by acids or by heavy loads of dirty clothes. On these grounds probably rests whatever justification there mas for the assertion which m-as popular some years ago that soaps wash *‘becauseof their colloidal properties,” despite the fact that innumerable other substances possess colloidal properties but are valueless in washing. Figure 3 illustrates qualitatively what occurs (exclusive of the results of hydrolysis) on the basis of Hartley’s concept as the concentration of an anionic SPHERICAL
LAMELLAR
t.
.+
.+
t.
t.
FIG. 2. Tn-o proposed structures of soap micelles
detergent in water is increased (14). At first only ions are present and these increase in direct proportion to the increase in total concentration. But soon micelles begin to a ~ p e a r first , ~ in small amounts, then in rapidly increasing amounts, and as they do so the rate of increase of the anions decreases to zero. The greater the number of ions which aggregate to form each unit micelle, the narrower the concentration band would be within 11-hich aggregation would occur and the larger the fraction of the detergent which would esist in aggregated form. Since some positively charged sodium ions attach themselves t o or are constituent parts of the negatively charged micelle, the riie of the curve for sodium-ion concentration also becomes more gradual. The adherence of a considerable number of sodium ion? to the micelle \roiild necPsitate mathematically, 3 McBain (9) believes t h a t soiiic few ionic micelles exist r v r n .It g i r a t dilutions and t h a t the ciitical concentration of ~ l a i n e l l n rmicelle ~ formation is q o t sharply defined as i n dicated in t h e draning. .$)
according t o the In\\. of mays avt ion, that ( ~ ~ ~ ~ I l t l tiic i : ~ imion !~ c~irveshoiild actually fall ( 5 ) . Support for thi? conciiisiou l t a a rccentiy Iwcn found in the norli with pinacyanol, which 11:~s led IIarkin,a arid his ai:ctps ( 2 ) to sL1ggest when the critical concentration is reached, S C J I ~ ~ Co previously exist,ing talloid transforms into micellar form. The existence of n maximum in t,he curve for the long-chain ion also afiords the most' sati>fying esplanation of t.lie minimum in the surface-tension curve, as has been pointed out by both Alesander (1) and P o m e y and Addison (15), if the long-chain ion is in reality the surfaceactive species in the solution. For present purposes, t,he chief significance of figure 3 is that the concentration of long-chain ions reaches its maximum at about the same concentration as
Fro. 3. Effect of change in total concentration upon t h e concentration of ions and micelles
that at which micelle formation becomes estensive. Possible correlation of thij fact with detergency phenomena is interesting and i h possible if we consider some results of n-ashing artificall>*soiled cloth, the load of cloth being small so as not t o affect greatly the concentration of free detergent in solution. Plotting whiteness of irashed cloth against concentration of detergent, the typical curve (after an initial uncertain flat, due at least partially to adsorption on the cloth and the soil) rises to a maximum and then flattens out ; Trashing efficiency increases up to a certain point, a critical lvashing concentration, beyond which further additions of detergent have little effect. If we w r e to find experimentally that the break in the washing curve and the h a k in the curire for paraffinchain ion 11s.total concentration (figure 3) occur at the same concentration, we might postulate that the amount of washing incieases as the amount of fatty ion increases, that when the one ceases t o increase the other ceases to increase or that washing i s proportional to the concentration of .futty a n i o n . This
is of course but a short step from saying that trashing is caustcl by thc Jaatty mizoas, -that they are the active constituents of the folution with respect to wishing just as they are with recpect to surface activity and colloid formation. Figure 1 s h o w for one detergent at one temperature the 1)and knon-n as the critical concentration of micelle formation. For other detergents and at other temperatures the concentrationa reiult \re shoiild expect the critical washing concentration to fall on the steep slope of the solubility curve. Experimentally, the agreement rvith this expectation is considered good on the whole, although the critical washing concentration of sodium h i r a t e falls a little to the left of the solubility curye of figure 11, while those for the other soaps lie somen-hat to the right of their solubility curves. In the case of the sulfonates, there is better agreement between washing concentration\ und the sohtbilitv resiilts obtained both by Tartar (19) and by ourselves. If u e xork a t temperatureb above T R , with increasing soap concentration n-e soon come to the invisible Change< in the solution resulting from colloid formation, a~ previously divxssed. Beyond thi:, critical concentration of colloid formation, further additions of soap increase the amount of colloid but, not appreciablp that of the long-chain ions; hence the detergency curve should, and does, level off. The critical \\ ashing concentration and the critical concentration of colloid formation should, according to oiir theory, coincide. Within reawnable limits of esperimental error they do so, ant1 they seem to fall not far fiom an upn-ard extension of the yteep slope of the ,dubility curve. 'l'hc critical Trashing concentration iz t l i i i h independent of whether the second phase which ;.?parates from the holution 19 made up of colloidal aggregates or of mlzerowq)ic
