SOLGTIONS O F SODIV’JI ALCOHOL SCLFATES
57
REFERESCES (1) LEWIS,G. K . : J. Franklin Inst. 226, 293 (1938). (2j LEVIS, G. S.,M.IGEL,T . T . , ASD LIPKIS, D . : J. Am. Cliem. Soc. 64, 11774 (1942), especially footnote i . (3) LEWIS,G. S., ASD SEABORG, G . T . : J. Am. Chem. Soc. 61, IS94 il939,. (4) LCDER,W. F.: Chem. Rev. 27, 547 (1940), especially p. 571. (5) PAULING, L.: The S a t u r e o j t h e Chemical Bond, 2nd Edition, p . 28-1. Cornell University Press, Ithaca, S e v T o r k (1940). (6) WALDEN, P.: Salts, i l c i d s a n d Bases; E l e c t r o l y t e s , Stereochaniistr!/. l ~ c G r x w - l I i l Book l Company, Inc., X e v York (1929). (7) Reference 6, page 48. (8) Reference 6, page 297. (9) Reference 6 , page 299.
XIISIi\lA I S SURFACE T E S S I O S - C O S C E S T R ~ ~ r I O S(‘CRT’LX SOLUTIOSS OF SODIUM ALCOHOL sur,F,yrI;S GILBERT D. 11ILES
ASD
OF
LEO SHEDLOVSKY
Colgafe-Palmolive-PeetCompaiiy, J e r s e y c i t y , S e u ; Jcrseij Received S e p t e m b e r 28, 1943
The observation of‘ minima in the surface tension-concentration c ~ i r v cior ~ aqueous solutions of various surface-active materials has been the CWUSP of considerable conjecture. Powney and Addison (12), in particular, found welldefined minima in the surface tension of solutions of sodium >altb of primary alcohol sulfates containing from tnelve to eighteen carbon atom>, The minima are as much as 5 dynes per centimeter l o w r than the value reached at higher concentrations. These “anomalous” results have presented apparent disxgreement Tvith Gibbs’ adsorption equation. The hypotheses p r o p o d t o reconcile such incongruities have been frequently discussed (1 t o 12). Usually the explanations proposed fall into two groups: First, it ha? been claimed that there are experimental difficulties which result in errors in the methods used for measuring surface tension (3, 3 , 7). On the other hand, the possible existence of abrupt changes in colligative properties of iolutioni of certain surface-active substances has been emphasized (12) and this could lead to minima in the surface tension-concentration curves. The data which we present here deal with solutions and procedures not covered by either of thc abovc e ~ p l n nations. PROCGDURC
The sulfated alcohols used in this work were of a particularly high dcgree of purity. The details of the niethods used in their preparation arc given i n a report entitled “Pome Properties Involving Surface &Ictivityof Sodiuni S d t s (ti Primary and Secondary -4lcohol Sulfates” ( 5 ) . Particular care 1~~ a- twrcitcd in
58
GILBERT D. MILES AND LEO SHEDLOVSKY
preparing the solutions and measuring the surface tension, in order to avoid accidental contamination. From the point of view of speed and ease of maintaining clean surfaces, the best procedure of those used is as follows: -4D u Noiiy tensiometer was placed upon a flat plate placed on top of a screw-jack. This permitted the uniform elevation of the instrument with respect to the surface of the solution. The material to be tested was lveighed and placed in a 1000-ml. Erlenmeyer flask which had been carefully cleaned, rinsed with distilled mater, and paraffined inside and out around the neck. The solutions were diluted by stepwise addition of measured volumes of distilled water from an automatic buret of 100-ml. capacity to the flask. During the measurement, the flask rested on the bench top next to the screwjack. An extension of glass rod approximately 1 mm. in diameter connected the platinum-iridium ring with the torque arm of the tensiometer so that the ring rested upon the surface of the solutions inside the flask. The neck of the flask was covered with a slotted sheet of suitable material, and the measurement 71-as made by gradually pulling the ring away from the surface of the solutions. By the addition of more water, the surface-tension curve was obtained for a concentration range of ten to one before it was necessary to weigh a fresh sample. Alllthe measurements reported here were made at room temperatures (25-30°C.). calibration curve n-as employed in n-hich dial readings n-ere plotted against the surface tension of pure liquids given in International Critical Tables. This method gave a precision of about fO.15 dyne per centimet er. RESULTS
The result5 of the measurements are given in figures 1, 2, and 3. In figure 1, curve h for pure sodium dodecyl sulfate n-hich had been evtracted with ethyl ether for 3G hr. in a Sohslet apparatus does not show any minimum. Curve D shows a marked minimum for solutions of the same material before the final extraction x i t h ether. Curves B and C 3how progressiyely more pronounced minima as larger amounts of dodecanol are added. If the material extracted with ether ~ m dodecanol, s the sample before eytraction contained between 0.1 and 0.5 per cent dodecanol. In figure 2, curve A for pure ether-extracted sodium tridecane-2-sulfate (13-2) (a typical sodium salt of a secondary alcohol sulfate), no minimum is apparent. In curves B and C: successive amounts of the homolog containing tu-o more carbon atoms were added. When 10 per cent of sodium pentadecane-2-sulfate (15-2) is added (curve C), a perceptible minimum appears in the curve. In order to demonstrate the more pronounced minimum obtained by the addition of a higher member of the homologous serie., 5 per cent of sodium hepta decane-2-sulfate (17-2) n-as added t o (15-2), (curve D). Only the portion of the curve at concentrations above 0.002 niolal are shon-n in figure 2, curre D. Relon- this concentration, the curve rises steeply. In figure 3 the effect of the addition of a higher homolog of the sodium salt of a primary alcohol sulfate (16-1) t o sodium dodecyl sulfate is shown in curves -1and B. n-ith the sodium secondary alcohol sulfates (figure 2), so for the sodium
59
SOLUTIOSS O F S O D I r h l ALCOHOL SULFATES
44
42
2
k!
2
K
Y
34
32
30
28
FIG.1. Effect of dodecanol upon the surface tension of solutions of sodium dodecyl sulfate (12-1). Curve -4, pure sodium dodecyl sulfate (12-1); curve B, pure (12-1) 0.1 per cent dodecanol of the amount of (12-1) ; curve C , pure (12-1) 0.5 per cent dodecanol of the amount of (12-1); curve D, (12-1) before final 36-hr. Sohslet extraction with ethyl ether.
+
+
FIG.2. Effectof sodium pentadecane-2-sulfate (15.2) and sodium heptadecane-2-sulfate (17-2) on the surface tension of solutions of sodium tridecane-2-sulfate (13-2). Curve A , pure sodium tridecane-2-sulfate (13-2); curve €3, pure (13-2) 5 per cent (15-2) of the amount of (13-2); curve C, pure (13-2) 10 per cent (15-2) of the amount of (13-2); curve D, pure (13-2) f 5 per cent (17-2) of the amount of (13-2).
+
+
GO
GILBCKT D. lII1,F.S A S D L E O SHEDLOVSRY
primary d c i ~ h o 1.ulfatc addition of the higher !io:nolog ilG-1) give5 a cur^^ I\-ith a mcnsurcnhle minimuni. Curve C in figure 1 substitiitcs sodium chloride for the (12-1) ~ ~ ~ i p ( ~ini curr i n de B.
M O L A L S IO00
Fic;. 9 . X l e r t OC rodiurii 11esnclecyl sulfate (16.1'1 :\nd of soilium chloride> upon t h e surface tci!iaion o f sodiuiii do~lecylsulfate (12-1). Curve .I. pure sotliuiii ti(~decylsulfate (12-1) ; curve 13. (12-!, i 1 1x.r cc.iit s o d i u ~ nIiexndecyl su1f:itc (16-1 of t h e anlourit of (12-1); curve C, sotliiirii clilriri~le 3 pcr cent ( 1 6 - l j of tlic, a m o u n t of sodiuiii chloride.
+
I
DISCI 3qIOY \UD C O Y C L L SI01
The wkult .. inclicak t n o \I nys in ivhich minima can he produced in surface t~n~ion-coiicr~iti ation curves. Figure 1 illu-trateq thc effect olitainecl when tn-o burface-actii e material.;, one of Tvhich is only slightly soluble in u ater, are present in the solution. From other ~ o r involving k wrface 1 iscosity TI hich has been done in these lahoratories, 11 e liave reason t o believe th3t dodecanol is adsorbed at higher concentiations than coiresponds t o that for the minimum in surface tenqion of iolution. >hewn in figure 1,curves B and C, but we have no explan at ion ' f~ the iise in the surface-tension curves of these solutions. Soap solutions are analogous to the. above mi.;ture in so far as they alio contain a water-soluble surtscc-acti\ P material and a qlightly soluble surface-active material which is strongly adsoiherl. In the caie of soap solutions, n-c conqider that the minima in \urfare tenbion ieported in the literature are probably clue to selective adsorption of the ielati\-elv insoluble fatty acid or acid soap. Owing t o hydrolysis, soap solutions m e not i ~o-compoiient \ 5y.tenis (9, 11) and therefore Gibbs' theorem c ~ a i ~ i lie o t applied in the usual way. Figure. 2 and 3 ewmplify a secoiicl means oi obtaining niinimn, 11 hich may be olxaincd bv mixing tv o anionic surface-active electrolytes. This effect should be most prmounced \:-hen the material present in the smaller proportion is a homolog of higher molecular weight, or another more qurface-active analog of the principal jngicdicnt. From the data shown in figure 3, ive may consider these
phenomena as special cncci of I he v l t effect- comparable t o tli( beell ohqcived lvith acltlctl inorganic salts (5). -11 cancentrntion* IX!OI\ O.OOi'5 molal, piire sodium tlodccyl sulfate begin- t o lose much ai it- \iiifa('(' :ici i\ i t y . On tlic other hand, -odimii hcx'idccvl sulfate at one-huntlrr(lth of thi. concwit1'itioii is still liighlg :idioihed in the presence of a strong uni-univa!cn+ clert1olvtc such a'; qcdium chloiidc i\-ho..c concentration i- 0.0075 molal; I\ lierra- in t1i.t illcd water tlic wrfnce tension of qiich n solution o d d be al)o:-cx 10 c!prid>- pc'r ccntimeter. I t has been shon-n elsen-here ( 3 ) ;hat the salt effect- tor anionic. suri'accactive mnteriali: such as the sodium alcohol wlfates are a function of thc. x,:ilrnce of thc cation. Thc salt effect of sodium dodecpl sulfate on -odium hexiclrcy: sulfate (figure 3, curve B) v e m ? t o be a function of the sodium-ion wnccnirntion up t o concentrations of 0.0075 niolal for the mixture. However, above 0.008 molal, the observed surface tension is approximately G dpnei per centimeter above the curve for sodium hexadecyl sulfate plus sodium chloride (figure 3, curve C). This mag be explained if we take into account the dynamic nature of the rquilibria lvhich maintain the surface properties of such solutions. At the air-liquid interface there is B constant exchange of ions between the interface and the interior of the solution. I n the mixture which we have studied, the roncentration of sodium dodecyl sulfate is one hundred times that of sodium hexadecyl sulfatt.. Therefore, the chances are roughly one hundred t o one that a heuadecyl sulfate ion leaving the interface will be replaced by a dodecyl sulfate ion. For this reason, the surface concentration of hesadecyl sulfate ions \rill be lon-er in siicli solutions a t concentrations where sodium dodecpl sulfate is highly 'ut facc active, than it nould be in solution. where sodium chloride was uqed inyteatl of the >odiuni dodecyl sulfate. This displacement of hexadecyl sulfate ion- from the surface should result in a n increase in surface tension compared t o the hiuiface tension of solutions of sodium hexadecyl sulfate plus sodium chloride. Our n o r k on this subject has been limited t o the data presented here, h i t we suspect t h a t minima may be expected t o appear in the surface tension-concentration curves of cationic surface-active materials if the proper materials nnd concentrations are chosen. Adam (1) notes t h a t where the surface tension passes through a minimum as the concentration increases it is possible that such cases will be explained by there being more than one capillary-active component in the system, or by the presence of small amounts of a n impurity vliich greatly increases the adsorption of the solute at certain concentrations. I t has been seen t h a t our data suggest that minima in surface tension-concentration curres are the direct result of the preyence in the solutions of more than one type of surface-active matwial. This agrees with part of the view given by Adam ( l ) ,but his suggestion that the presence of small amounts of a n impuritj- might greatly increaqe the adsorption of the solute a t certain concentrations does not concur with our concepts. -4 reexamination of the purity of materials for nhich minima have been reported is indicated before the neceqsity arises of reconciling the data with Gibhs' adsorption theorem.
62
PIERRE V.\X
RYSSELBERGHE
REFERESCES
(1) ADAM,il.K.: The Physics and Chemistry of Surfaces, 2nd Edition, pp. 113,115. Oxford Gniversity Press, London (1938). (2) ALEXANDER, A. E.: ilature 148, 752 (1941); Trans. Faraday Soc. 38,54 (1942). (3) CASSEL,1%.M.: 105th Meeting of the American Chemical Society, held in Detroit, Michigan, April, 1943. (4) CLAYTON, W.: The Theory of Emulsions and their Technical Treatment, 4th Edition, pp. 6-12, 53-55. The Blaliiston Co., Philadelphia (1943). (5) DREGER,E. E., KEIM,G. I., MILES,G. D., SHEDLOVSKY, L., ASD ROSS,J.: To be published. (6) KRAEYER, E. 0.: Advances i n Colloid Science, pp. 111, 395, 402-5. Interscience Publishers, Inc., Yew York (1942). (7) LOXG, F. A., KUTTISG,G. c . , A N D HARKISP, IT.D.: J. -4m. Chern. Soc. 69, 2197 (1937); 62, 1496 (1940). (8) LOTTERhTOSER, A , , A N D STOLL, F.: Kolloid-Z. 63, 49 (1933). (9) MCBAIN,J. W., . ~ S D DAVIES,G . P . : J. Ani. Cheni. Soc. 49, 2230 (1927). (10) MCBAIN,J. W., ASD MILLS,G. F.: Report on Progress in Physics 6,30 (1939). (11) MCBAIN,J. W., A N D WILSOX, D . A . : J. Am. Chem. Sac. 68, 379 (1936). (12) POWNEY, J., AND ADDISON,c. c.:Trans. Faraday soc. 33, 1243 (1937).
DISCUSSIOX h S D I N T E R P R E T A T I O S O F T H E MIGRATION DATA OF LAURYLSULFONC ACID IT\’ AQUEOUS SOLUTION PIERRE VAX RYSSELBERGHE Department of Chemistry, University of Oregon, Eugene, Oregon Received September 16, 1945
A few years ago, on the basis of the then available data, we gave (4) an interpretation of the osmotic coefficients, the conductivities, and the diffusion coeficients of the typical colloidal electrolyte laurylsulfonic acid in aqueous solution. An average negatively charged micelle H,L,, changing with concentration, was calculated. The values of z and z and the concentrations (H,L,) and (H+) ivere obtained b y solving, for each concentration, a system of four simultaneous equations : namely, two stoichiometric conditions, the freezing-point equation in which the experimental van’t Hoff i coefficients were introduced, and a conductivity equation based on the use of Stokes’ law according to the suggestions of J. W. McBain (3). The latter equation (equation 11 of our previous paper) is 350(H+)
(x - zy (H,L,) + 22 ___ 2113
=
AC
The values of x and x so obtained were then introduced into the Sernst formula for the diffusion coefficient, written for an unsymmetrical electrolyte whose molecule dissociateq into (z - z ) / x hydrogen ions and l/z micelles H,L,. The calculated diffusion roefficientq were found to be in satisfactory agreement with the experimental data of E. L. McBain ( I ) . I n particular, the interesting minimum in the diffusion vurw \\-a3 reproduced by the calculations haTed upon the average
