NOTES
4038
Salt Effects on Nonionic Association Colloids
than purely nonpolar solutes. Considerable progress, however, can still be made for monomer-micelle equilibria. If the number of monomers, L, in a micelle, 11,is n, and the activity coefficients of the monomer and the micelle are f and f m , the equilibrium constant for micelle formation, K , is given by
by Pasupati Mukerjee]
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Department of Physical Chemistry, I n d i a n Association for the Cultivation of Science, Jadazpur, Calcutta-$2, I n d i a (Receive$ A p r i l 12, 1965)
The influence of added electrolytes on the critical (3) micelle concentrations (c.m.c.) of nonionic association colloids in aqueous solution has been the subject of At the c.m.c., [L] can be replaced by the c.m.c. Yegmany recent studies.2-10 The purpose of this note is lecting the term containing [AI], particularly for relative purposes,16the standard free energy per monomer, to show that the effects can be fairly adequately understood in terms of some well-known salting-out and AG, is derived to be salting-in equilibria in aqueous solutions and that other suggested explanations, mentioned in the con- AG/2.303RT = -log c.ni.c. - log f -1logf, (4) n cluding remarks, are unnecessary or inaccurate. The general problem of the salting out of nonelecEquation 2 and the experimental results on many trolytes by inorganic electrolytes has been extensively nonpolar organic solutes'2,'j indicate that salt effects, investigated and reviewed.",l2 A fairly satisfactory as represented by the k, coefficients, are additive with theory has been offered by Long and l \ l ~ D e v i t ' ~ r ' ~ respect to the parts of a nonpolar molecule to about the for the special case of nonpolar molecules. I t is genersame extent as the volumes are additive. For the asally accepted that added electrolytes cause electrostricsociation colloids, we assume that the additivity of the tion of water and increase the internal pressure of the k , coefficients extends to both the nonpolar and the so1ution.l1>l2According to Long and AIcDevit, this polar parts of a molecule. Using subscripts a and b extra pressure increases the activity of the nonpolar for the hydrophobic part and the hydrophilic head molecules, thus causing the salting out. The salt group, respectively, log f is thus expressed as log fa effects usually follow the empirical equation log fb. For a micelle, the nonpolar surface exposed to the salt solution is small compared to the total log f = k,C, (1) surface of the monomers, particularly when the head groups are bulky. Keglecting this nonpolar contrjwhere 7 is the activity coefficient of the nonelectrolyte, bution, we can write log f m = 12 log fhm, where log k , is the salt-effect constant, and C , the molar concentration of the salt. Long and McDevit'3 derived an equation fork, (1) Department of Chemistry, University of Southern California
+
+
~
k, = Vl(V,
-
Fs)/2.3RTPo
(2)
in which 7, is the partial molal volume of the nonelectrolyte, V, the true molal volume, V, the partial molal volume of the electrolyte, and Po is the compressibility of mater a t the absolute temperature 5". V , - 7,can be considered to be the extent of the electrostriction of water. l4 Recently, Den0 and Spink15 have shown that some estimates made by the present author of the electrostriction from partial molal volume data14 for a variety of inorganic electrolytes give a good correlation of the relative experimental k, values for benzene although the absolute values are lower by a factor of 0.3. The effect of salts on polar molecules is much more complicated and much less understood. l 2 As association colloids contain both polar and nonpolar groups, they are expected to show a more complicated behavior The Journal of Physical Chemistry
~
~~
~~
~
Los Bngeles, Calif. 90007. (2) L. Hsiao, H. N. Dunning, and P. B. Lorenz, J . Phys. Chem., 60, 657 (1956). (3) K. Shinoda, T. Yamaguchi, and R. Hori, Bull. Chem. SOC. Japan, 34, 237 (1961). (4) K. Kuriyama, Kolloid-Z., 181, 144 (1962) (5) P. Becher, J . Colloid Sei., 17, 325 (1962). (6) P. Becher, ibicl., 18, 196 (1963). (7) K. Tori and T. Nakagawn, Kolloid-Z., 189, 50 (1963). (8) M. J. Schick, S.AM. Atlas, and F. R. Eirich, J . Phys. Chem., 66, 1326 (1962). (9) 11,J . Schick, J . Colloid Sci., 17,801 (1962). (10) M.J. Schick, J . Phys. Chem., 67, 1796 (1963). (11) I€. S. Harned and B. B. Owen. "The Phvsical Chemistrv of Electrolytic Solutions," 3rd Ed., Reinhold Publishing Corp., New Tork, K.Y . , 1958. (12) F. A. Long and W.F. McDevit, Chem. Rev., 51, 119 (1952). (13) W. F. McDevit and F. A. Long, J . Am. Chem. SOC.,74, 1773 (1952). (14) P. Mukerjee, J . Phys. Chem., 6 5 , 740, 744 (1961). (15) N. C. Den0 and C. H.Spink, ibid., 67, 1347 (1963). (16) P. Mukerjee, ibid., 66, 1375 (1962).
NOTES
4039
corresponds to the salt effect on a single hydrophilic group on the micelle. Equation 4 can now be rewritten as
fbm
-AG/2.303RT = -log c.m.c. - log fa -
(1%
f b
- 1%
fbm)
(5)
Several consequences of eq. 5 can be tested. First, if the salt effects are expressed by eq. 1, eq. 6 follows, the subscripts for the k's having the same meaning as before.
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log c m c .
- C,(k,+ k b - k b m ) = constant - k,C,
=
constant
(6)
The predicted linear dependence of log c.1n.c. on the salt, concentration is followed by many system^.^,^ Some polyoxyethylene derivatives show deviations from linearity,'O which may be caused, in part, by the heterogeneity of the systems and variations in k b m with changes in the size and composition of the micelles as salt is added. I n eq. 6, the terms k b and k b m are expected to cancel to a great extent but not exactly. On a micelle surface the hydrophilic head groups are highly concentrated. The effective concentration depends, in part, on the bulk of the head groups and may be several moles per liter. The k, coefficients are known to depend on the concentration of the nonelectrolyte. The result of this "self-interaction" effect of the nonelectrolyte12 is usually a reduction of the absolute magnitude of the k, coefficient with the concentration of the nonelectrolyte. The cancellation of the kb and k b m terms may be expected to be less complete for larger absolute values of k b and for larger bulk of the head group.
Quantitative Calculations for NaCl Solutions Several systems are analyzed in detail to emphasize the predictive value of the above theory and some of the important features of the salt effects. The calculations are restricted to the effect of NaCl at about 26". the only conditions for which sufficient data are available, but the principles should apply generally. The k, values for long-chain hydrocarbons are not available experimentally. The data of Morrison and BilletI7 on short-chain compounds, methane to butane, in SaC1, when interpolated to 25", can be expressed as k,
0.132
+ 0.032(N - 1)
(7) JThere N is the number of carbon atoms in the aliphatic chain. This equation is used here, keeping in niind the uncertainty of long extrapolations. The simplest association colloid to treat is octyl glucoside.3 Kelly, Robinson, and Stokes's have re=
cently studied the effect of NaC1 on the activity coefficient of mannitol, which is structurally similar to the glucoside group. I n 1 M NaC1, mannitol has an activity coefficient of 0.986 at infinite dilution, indicating a slight salting in, and 1.002 a t 1 Jf mannitol concentration, indicating a small effect of self-interaction of mannitol. It is thus expected that both k b and k b m for the glucoside group are small, and k, for octyl glucoside should be due mainly to ka. The experimental k, value of 0.35, obtained from the data of ref. 3, is in good agreement with the value of 0.36 calculated for octane from eq. i. The low k, value (0.134) for octylbetaine' must then be ascribed to a k b - k b m value of about 0.2. This appears to be quite reasonable. Zwitterionic molecules like betaine have high dipole moments and are salted in very substantially by ordinary electrolytes, as predicted by the theories of Debye, Scatchard, and Kirkwood and as experimentally observed.l 9 Such dipolar molecules shorn strong self-interaction effects also, as exemplified by glycine, which, in it\ zIyitterionic form, is structurally similar to betaine. In dilute XaC1, the k, coefficient of glycine decreases froin -0.28 in dilute solutions of glycine to -0.02 in 2 31 g1y~ine.l~ Thus, a considerable negative contribution of the k b k b m term is expected for betaines although it cannot be estimated very well. On the other hand, the change in k , wiIh increasing chain length should come mainly from the contnbution of the additional CH2 group> to k,. The experimental k , values of octyl-, decyl-, and dodecylbetaine are 0.134, 0.214, and 0.294.7 The average increment per CH2 group is 0.040, comparable to the value of 0.032 for short-chain alkanes (eq. 7 ) . The available k , data for polyoxyethylene deyivaare somewhat inconsistent, Severtheless, here also the major contribution seems to be the k, term. For example, the experimental k , for some branched nonylphenol derivativesg is about 0.34 conipared to the expected k, of 0.39 for nonane. A dodecanol derivative with 30 ethylene oxide unitsl0 shows a nonlinear dependence of log c.m.c. on C,. The estimated mean k , of about 0.8 is somewhat higher than the estimated k, of about 0.5, indicating some contribution from the ki, - k b m term.
Other Salts For approximate purposes, in the absence of suitable (17) T. J. Morrison and F. Billet, t J . C h e m SOC, 3819 (1952). (18) E. J. Kelly, 6 5 , 1958 (1961).
R. A. Robinson, and R. H Stokes, J . f h y s C h e m ,
(19) E. J. Cohn and J. T. Edsall, "Proteins, Amino Acids and Peptides," Reinhold Publishing Corp., New York, 1;.Y . , 1943.
Volume 69, Sumber 11 Souemher 1965
NOTES
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4040
background information, eq. 2 may be employed to calculate relative k, values in different salt sohtions. It is interesting that for octyl glucoside, for which k, = k, in NaC1, the ratio of k, values (calculated from ref. 7 ) in NaCl and NazSOc is 1:2.7, in fair agreement with the ratio 1:3.2 predicted by eq. 2 and the ratio of 1:2.7 observed for benzene.15
gelatin. We have studied the mechanical loss and shear properties of gelatin and vitreous gelatin-water systems at low temperatures and wish to present the results to provide more information on transitions that occur in this biopolymer.
Salting Out of Ionic Association Colloids
Gelatin obtained from Atlantic Gelatin (a division of General Foods Corp.) was used without any further treatment. The properties of the gelatins studied are listed in Table I. The gelat,in-water systems were prepared from Gelatin-I and distilled water by combining the two compounds and warming them in lightly stoppered test tubes in a water bath at about 90". The desired concentrations were obtained by checking the solution weight and adding solvent to make up any weight loss occurring during the solution period. Concentrations are calculated on an oven-dry gelatin basis. Vitrification of the gelatin-water systems was accomplished by rapidly cooling in liquid nitrogen and mounting the specimens while they were in a glassy state. The unplasticized gelatin was molded into plaques a t 110' and 3000 p.s.i. These plaques were desiccated until the experiment was performed. The moisture content of the Gelatin-I plaque was 9.6y0 and that of the Gelatin-I1 plaque was 8.3%.
Although the major effects of added electrolytes on the c.m.c. of ionic association colloids are due to interionic interactions, the above analysis suggests that salting-out effects, generally neglected, can be quite substantial. The calculated k, value for a dodecyl group is about 0.5. Therefore, the activity coefficient of an amphiphatic ion containing a dodecyl group should be increased by factors of 1.12, 1.78, and 3.2 in 0.1, 0.5, and 1 M NaCl because of the salting out of the chain.
Concluding Remarks The above analysis suggests that the problem of qalt effects on nonionic association colloids can be profitably attacked in terms of some fairly wellestablished ideas developed for simple nonelectrolytes. Previous explanations, based on changes in water activity,j the presence of charged impurities,6 or the unavailability of solute molecule^,^ need not be involved. Schick, Atlas, and Eirich8 have suggested salting out of the ethylene oxide chains in some ethylene oxide condensates in terms of a dehydration mechanism. As indicated above, the salting out of the hydrophobic chains seems to be the most important factor to be considered for polyethylene oxide systems also. Acknowledgments. The revision of this paper was done at the University of Southern California and was supported in part by P.H.S. Research Grant GM 10961-01 from the Division of General Medical Services, Public Health Service.
Experimental Section
Table I: Properties of Gelatin Used in Study
Gelatin-I Gelatin-I1
Moisture content,= %
PH
Bloomb
10.8 9.0
6.5 4.35
200 200
TypeC
B A
'
a As-received basis. Bloom is a standard measure of the gel strength of gelatin. It is defined as the number of grams required to force a 0.5-in. plummet of a Bloom gelometer 4 mm. into an aqueous 6.67% solids gelatin gel that has been chilled 17 hr. at 10' (J. F. Suter, U. S. Patent 3,164,560(Jan. 5, 1965)). Type B gelatin is obtained from lime-conditioned calfskin, beef hides, or ossein. Type A gelatin is obtained from acidconditioned pigskin.
Transitions in Gelatin and Vitrified Gelatin-Water Systems
by Joseph V. Koleske and Joseph A. Faucher Research and Deaelopment Department, Unwn Carbide Corporation, Chemicals Division,South Charleston, West Virginia 95'58058 (Receited April 15, 1955)
Yannas and Tobolskyl recently reported on the viscoelastic properties of plasticized and UnplaStiCiZed The Journal o j Physical Chemistry
Mechanical loss measurements were made a t about 1-5 C.P.S. with a recording torsion pendulum similar to that described by Nielsen. * These measurements were used to calculate the real, G', and the loss, G", components of the complex shear modulus. (1) J. B. Yannas and A. V. Tobolsky,
J. Phys. Chem., 68, 3880
(19w). (2)
L. E. Nielsen, R ~ Osci. . Instr., 2 2 , 690 (1951).
