Sept., 1956
EQUILIBRIUM DIALYSIS OF SOAP AND DETERGENT SOLUTIONS
1245
EQUILIBRIUM DIALYSIS OF SOAP AND DETERGENT SOLUTIONS’ BY II. B. K L E V E N S ~ ~AND C. W. C A R R ~ ~ Received February 84,1966
Equilibrium dialysis measurements have been made on a series of anionic and cationic soaps and detergents. Contrary to previously published reports, equilibration across cellophane membranes was observed in all cases for saltrfree and polar hydrocarbon-free detergent solutions. Thc addition of dissolved hydrocarbon changed little and long chain alcohols and amine additives increased markedly the time necessary for e uilibration. A change in the ionic strength of the solvent by the addition of KCl resulted in even greater times necessary ?or transport across the cellophane membranes and, where the solvent was between 0.5-1.0N KC1, equilibration had not been obtained in 100 days. However, in these latter cases, there was always a gradual approach t o equilibrium. These data indicate that earlier measurements and all interpretations as t o critical micelle concentrations, micelle formation, etc., baaed on these measurements must be reconsidered in the light of the new findings. These interpretations are markedly clarified by a series of experiments using graded collodion membranes of varying porosities.
Recently Yang and FosterS have shown that the ions of various buffered and unbuffered detergent preparations do not distribute themselves uniformly across a cellophane membrane except at concentrations below the critical micelle concentration (CMC). It was further proposed by these authors that these measurements would give some indication of a CMC. These results were obtained with relatively non-homogeneous commercial preparations, Santomerse No. 3, principally sodium dodecylbenzene sulfonate, and a technical alkyl dimethylbenzylammonium chloride, a major constituent of which was the dodecyl salt. Subsequent discussion4 of this paper led to the suggestion that polar and apolar impurities in the detergent preparations could explain some of these results. A further impurity, the buffer salts, as well as the low molecular weight paraffin chain salts might also account for non-equilibration. More recently, an attempt has been made to explain the findings of Yang and Foster by postulating diffusion of micelles through the cellophane membra ne^.^ With increase in ionic strength of the solvent (and a corresponding increase in micellar size), there is a supposed lack of equilibration on both sides of the membrane. I n the two reports mentioned above, 2 4 4 8 hours were allowed for equilibration. According to Yang and Foster,3 longer times produced no further changes in the system, even up to one month. To clarify further the matter concerning the diffusion or non-diffusion of micellized substances through membranes, the present work has been carried out with “pure” soaps and detergents in the presence and absence of added polar and apolar compounds and electrolytes. Measurements have been extended to 6090 days to obtain equilibration, and the porosities of the membranes have been estimated by studying the diffusion of various proteins through these membranes. Finally, a series of equilibration measurements with dodecylammonium chloride and graded. collodion membranes were completed and these latter data appear to reconcile some of
the apparent inconsistencies in the published literature. Experimental
Iowa, June 25-27,1953: T H I EJOURNAL, 57,633 (1953). (5) B. 8. Harrap and I. J. O’Donnell, ibid., 68, 1097 (1954).
(6) R. G. Paquette. E. C. Lingafelter and H. V. Tartar. J . Am. Chem. SOC,65, 686 (1943).
Materials.-Perfluorohexanoic acid and perfluoro6ctanoic acid were research samples supplied by the Minnesota Mining and Manufacturing Company. Aerosol MA, sodium dihexylsulfosuccinate, waa a highly purified research sample supplied by Dr. J . I(.Dixon of the American Cyanamid Company. Two samples of sodium dodecyl sulfate, foam fractionated and free of dodecanol and inorganic impurities, were supplied by Dr. Leo Shedlovsky of Colgate-Palmolive Peet Company and by Dr. J. Bolle of the Centre Nacional des Recherches Scientifiques in Paris. Potassium dodecanoate and potassium tetradecanoate were prepared by saponification of carefully fractionated esters of the corresponding acids followed by repeated recrystallizations. Sodium octyl- and decylbenzene sulfonates were carefully puritied samples prepared by Dr. M. Pallansch in a manner similar to that used by Paquette, Lmgafelter and Tartar6 except that the reduction of the ketone was carried out by the Wolf-Kishner method in place of the Clemmensen method in order to produce compounds essentially free of possible isomers. Dodecylammonium chloride waa prepared from a carefully fractionated dodecylamine in the usual manner, followed by repeated recrystallizations. Sodium glycocholate, a bile salt, waa a commercial preparation and was included in this study because of our current interest in these bile salts and to show the effect of undetermined impurities on equilibrium studies across membranes. The solvent used for the various detergents was distilled water. For the fatty acid soaps, the solvent waa potassium hydroxide solutions at pH 10 or higher, used in order to minimize hydrolysis. For those experiments in which the ionic strength of the solvent waa varied potsssium chloride solutions were used. Dialysis.-Visking cello hane casings, 20/32 inches in diameter, were previously goiled in either distilled water or in soap solutions at least three t i e s for eriods of from 3-12 hours to remove any residual solubye components. After thorough rinsing, no Werence in time necessary for dialysis equilibration waa noted for those membranes boiled in water or in soap solutions. Graded collodion membranes were prepared using 90-96% ethanol solutions as solvents according to a method developed recently by C. w. C. Detailed procedures will be published shortly, but it haa been determined that these membranes are reproducible, have an apparent narrow range of pore sizes and are stable over extended periods in solutions of low pH. (1) A portion of this work was done in Paris while H. B. E. Ten ml. portions were added to the liquid-tight casing waa an advanced research scholar under the Fulbright program and these were placed in a tube containing ten ml. of the (1951-52). solvent. The tubes were stored and shakgn in a constant (2) (a) Mellon Institute. University of Pittsburgh, Pittsburgh, temperature water-bath (36.7”). These casings were Pennsylvania. (b) Department of Physiologicd Chemistry. Unitightly tied to minimize the possible increase in total volume versity of Minnesota, Minneapolis, Minnesota. in the casing. All data in these series were corrected for (3) Y. T. Yang and J. F. Foster, T H I SJ O U F Z N A 57, L ,628 (1953). small volume changes. A t all times, an essentially equal (4) Discusaion by J. Th. G. Overbeek, by H. B. Klevens, by K. liquid level inside and outside the membrane was mainMysela and by J. F. Foster a t the Colloid Symposium held at Ames,
H. B. KLEVENS AND C. W. CARR tained, in order to present as large a surface aa possible for dialysis. A number of membranes were tested before and after dialysis studies for possible changes in porosity due to the high osmotic pressures within the casing. No differences in ovalbumin (molecular weight 44,000) transport were observed. Analysis.-Concentrations of soap and detergent on both sides of the membrane were determined by the spectral dye method7ss using pinacyanol chloride with the anionics and acidified phenolindophenol with the cationics. In a few cases, equilibration was followed by conductivity measurements. When the concentrations were below the CMC, titrations were made with dye solutions containing amounts of the detergent above the CMC. In addition, for amine concentrations below the CMC, we used titrations with NaOH with thymolphthalein as an indicator. By the use of suitable optical techniques as well as of a standard reference end-point (indicative of the CMC), it is quite simple to obtain CMC or concentration values which are reproducible to within 3-40/,. Equilibration across a membrane is assumed to occur when agreement within this range is found for concentrations of surfactant on both sides of the membrane. Critical micelle concentrations were determined by the spectral dye method7J and, iii a few cases, by conductivity measurements and are in all cases in agreement with published data using this or a similar method. The number, N , of molecules per micelle was determined by light scattering measurements iii the case of the detergents and by ultracentrifugation for the fatty acid salts. Comments regarding the character of the X-ray pattern for the dodecylammonium chloride and the fatty acid salts were obtained from Dr. R . W. Mattoon. Repeated efforts to obtain a pattern for the two layer array in the perfluoro acids and Aerosol MA, using techniques similar to those of Dr. Mattoon, were not successful. This could be due to the small size of these micelles and to the extremely gaseous nature of these aggregates. A t all times the solvent for the dye solutions was identical with that used for the detergent solutions. Dye solutions were about 5 X 10-6 M , were usually freshly prepared and, if not, were stored in the dark until used. Due to the low fiolubility of cyanine dye solutions a t high ionic strength, it was necessary to dissolve the dye first in small amounts of water and then to dilute them with appropriate salt solutions.
Results For all the systems reported here, two perfluoro acids, sodium salt of dihexyl sulfosuccinate (Aerosol MA) , sodium dodecyl sulfate, dodecylammonium chloride, sodium n-octyl- and n-decylbenzene sulfonate, potassium dodecanoate, potassium tetradecanoate and sodium glycocholate, in which the solvent was either water or N KOH, there was no evidence of non-uniform distribution of surfactant on both sides of a membrane. These results appear to be at variance with those reported by Yang and Foster.3 These differences are seen in Fig. 1where equilibrium dialysis results with pure detergents, sodium octyl- and sodium decylbenzene sulfonate, are compared with dialysis results3 in which a commercial preparation, Santomerse No. 3 (principally sodium dodecylbenzene sulfonate) has been used. The data with the coinmercial Santomerse show a definite break at a Concentration which is approximately equivalent to its CMC; no break or discontinuity is observed with the pure alkyl benzene sulfonates. Similarly with the other purified detergent and soap preparations, we could not observe a break iii the ciirve. Although our data in Fig. 1 involve t,lie usc of water as :t solvent, a (7) R.1. I,. Corriii, I[. 1%.I i l e v e n s i ~ ~ i W. i l 1). llarkins, .I. Chem. I'hyp., 14, 216, 480 (194f;). ( 8 ) 11. 1%.I(leveu.9, 'flrls J O U R N A L61, , 1 1 4 (1Y47).
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Fig. 1.-Equilibrium dialysis of sodium alkylbenzene sulfonates: CE, concentration outside membrane; CI, concentration inside membrane; Cs (octyl) and Clo (decyl) benzene sulfonates (this report), Santomerse No. 3 (mxed alkylbenzene sulfonate): data of Yang and Foster. Numbers in brackets are the CMC values in moles per liter.
change in solvent from water to buffer had no such marked effect as the occurrence of a break in the curve of the previous data.3 With increase in concentration, there was in all cases an increase in the time necessary for equilibration. Below the CMC, for all systems studied, equilibration was obtained in about two hours; above the CMC, for a particular membrane, this time appeared to be a function of both concentration and of type of detergent used. For a series of detergent solutions, where the initial concentrations were about 10 X CMC, it is seen from the data in Table I that there is no definite correlation between equilibration times and CMC and only a small one between these times and micellar weight. There appears to be a somewhat better correlation between these times and the diffuseness of X-ray patterns indicative of the two-layer micelles. However, as these data show, there is definite equilibration in all cases. Effect of Change in Solvent.-When the solvent was changed from water to one containing various hydrocarbon and polar hydrocarbon additives as well as by increases in ionic strength, a number of very interesting findings were observed. The addition of a solubilized hydrocarbon, such as nheptane, had very little effect 011 the time necessary for equilibration. This is as expected, since the addition of hydrocarbons has been shown to have little effect on the CMC and on the size of micelles.9 Recent experiments using benzene and naphthalene as solubilized hydrocarbons show that the hydrocarbons diffuse through the membranes. When equilibration of the surfactant was reached, spectral measurements showed that the aromatic hydrocarbons had equal concentrations on both sides of t,he cellophane casing. The addition of holubilized polar hydrocarbons to various soap and detergent solutiolis results in marked decwases i i i (9) 11. R . I
