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Organic counterions and micellar parameters: methyl-, chloro-, and phenyl-substituted acetates
Organic Gounterions and Micellar Parameters: Methyl-, Ghloro-, and PhenyI-Substituted Acetates A. L. U N D E R W O O D * AND E. W. ANACKER'~ *Department of Chemistry, Emory University, Atlanta, Georgia 30322 and tDepartment of Chemistry, Montana State University, Bozeman, Montana 59717
Received October 19, 1983; acceptedJanuary 31, 1984 Critical micelleconcentrationsand aggregationnumbers obtaifiedby light scatteringare reportedfor decyltrimethylammoniummicellesin 0.5000 m solutions of the followingcounterions as sodium salts: acetate, methylacetate (propionate), dimethylacetate (isobutyrate), trimethylacetate (pivalate), chloroacetate,dichloroacetate,trichloroacetate,phenylacetate,and diphenylacetate.Counterioneffectiveness in promotingmicelle formation is interpretable largelyin terms of hydrophobicinteractions, but other factors which are more difficultto identify are also involved. Association, probably dimer formation, has been observedin sodium diphenylacetatesolutions. INTRODUCTION The nature of the counterion is a significant factor in the formation of micelles by ionic surfactants, and studies of inorganic counterions have disclosed striking effects upon CMC values and aggregation numbers (1, 2). Thus it is strange that organic counterions have been largely neglected, at least in relation to the large number that might have been examined. Among the systems studied are dodecyl sulfate micelles with tetramethyl-, -ethyl-, and -npropylammonium counterions (3), tetradecylpyridinium with alkyl sulfate (4), decanoate and dodecanoate with tetraalkylammonium (5), perfluoronanoate with methylammonium (6), and decylammonium with dichloroacetate (7). The effects of several methylammonium counterions upon chain flexibility in a decyl sulfate lyotropic nematic phase have been studied (8), and it has been reported that salicylate and m- and p-chlorobenzoates induce viscoelasticity as counterions with hexadecyltrimethylammonium aggregates (9). These studies, interesting as each may be, are fragmented by the variety of surfactant ions and in any event involve a relatively small number of counterions. In some instances,
the nature o f the counterion is only incidental to the research goals, and possible counterion effects are difficult to discern and to correlate with other work. Thus tittle is known regarding the structural features which determine the effectiveness of a counterion in promoting micelle formation. Accordingly, we have undertaken light scattering determinations o f CMC values and aggregation numbers for decyltrimethylammonium (DTA +) micelles with a large number of organic counterions. This work began with a series of n-alkyl carboxylates, where the effect o f increasing chain length was interpreted straightforwardly in terms of hydrophobic interactions (10). In the present paper are reported results for a series of substituted acetate counterions. The incidental observation of association in aqueous solutions of sodium diphenylacetate, probably yielding a dimer, is also reported. MATERIALS AND METHODS Apparatus. The same photometer and refractometer used in the previous study (10) were employed. Free bromide concentrations were determined potentiometrically using a bromide ion selective electrode (Model 94-
128 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid andlnterface Science, Vol. 100, No. 1, July 1984
SUBSTITUTED ACETATE COUNTERIONS 35A, Orion Research Inc.) with a Radiometer PHM 26 meter readable to 0.1 mV in the expanded scale mode. Materials. Decyltrimethylammonium bromide (DTAB) was prepared as before (10). Anal. Calcd. for C13H30NBr: Br, 28.51%. Found: 28.49%. Scattering data for solutions in 0.5000 m sodium bromide fell on the curve obtained with the earlier batch. Sodium acetate was a reagent chemical. The other sodium salts were prepared by potentiometric titrations of aqueous solutions or suspensions of the acids to the equivalence points with sodium hydroxide, removal of water by lyophilization, and recrystallizations of the solid residues. The acids and the solvents for recrystallization of the sodium salts were as follows: methylacetic (propionic, reagent grade), 1:12 water-ethanol; dimethylacetic (isobutyric, Aldrich Chemical Co., 99+%), 1:1 ethanol-acetone; trimethylacetic (pivalic, Aldrich, 99+%), 1:1 ethanol-acetone; chloroacetic (reagent grade), aqueous acetone; dichloroacetic (Aldrich 97%), 1:5 ethanolacetone; trichloroacetic (reagent grade), water; phenylacetate (Aldrich, recrystallized twice from acetone), 1:40 water-acetone; diphenylacetic (Eastman Kodak Co., recrystallized twice from ethanol), ethanol. Sodium chloroacetate did not crystallize in the usual sense but was precipitated by addition of acetone to the aqueous solution; two liquid layers formed initially, but repeated stirring of the lower layer with portions of fresh acetone precipitated the salt. All of the salts were dried in vacuo at about 70°C, lightly ground, and stored over phosphorus pentoxide. Methods. Solutions were prepared by weight. Those for light scattering were filtered under nitrogen pressure through a Coming ultrafine frit directly into the cell. Measurements were performed at 25 + 1°C. The general approach for calculating micellar aggregation numbers has been described (1, 2, 10, 11), but the equations actually employed represent a modification of the earlier theory which specifically recognizes the presence of two different counterions in the solution (12).
129
Each light scattering run involved ten to twelve DTAB solutions in 0.5000 m aqueous sodium salt (NaX) of such concentrations as to define the CMC and provide sufficient data above and below it. The laboratory water was redistilled from alkaline permanganate. If it were possible to do so, it would be preferable to prepare the DTA ÷ salt of each counterion (X-) in order to examine solutions of DTAX rather than of DTAB in NaX. However, this is not feasible for screening a large number of counterions. Whereas DTAB is beautifully crystalline and easily handled, many of the DTAX salts are exceedingly hygroscopic and intractable waxy or sticky materials. In addition, preparative methods may lead to contamination with peptized silver salts or substances leached from ion exchange resins which would be highly undesirable in light scattering studies. Employing 0.5000 m NaX as the solvent not only provides a very large ratio of mx- to mBr- near the cmc but also makes more realistic the assumption in the theoretical treatment that activity coefficients remain constant. RESULTS Scattering data for DTAB-NaX (0.5000 m)-H20 solutions are shown in Figs. 1 and 2. The plotted R90 value is Rayleigh's ratio for a 90 ° scattering angle corrected for solvent scattering. The refractive index gradients used in calculating aggregation numbers are given in Table I. In Table II are listed CMC values, charge-corrected aggregation numbers (N), effective micellar charges (p), and degrees of counterion dissociation (p/N) for the various counterions (X-). The CMC values are shown in Fig. 3. Calculations were based upon the assumption that Br- and X- are bound to the micelles in the same ratios as those of their analytical concentrations in the solutions (Model III in ref. 1). The assumption that the micelles bind only X- yields virtually identical results. In no case was dissymmetry detected by measurements at scattering angles of 45 and 135 °. Triphenylacetate could not be inJournal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
UNDERWOOD
130
AND ANACKER
5-
I
3
4
2
0.5000 m sodium chloride were examined for evidence of association. The results are seen in Fig. 4, where scattering data for sodium phenylacetate are also shown. As explained below, it is probable that the phenylacetate ion is monomeric in aqueous solution while diphenylacetate forms dimers. It may be shown that the scattering behavior of an aqueous solution of NamXm where Na + and NaaXL("-q) are the only species is described by
n n '22V° m '2 7"
_(1 +m-q) m
002 004
0108 0110 oL,2
DTAB MOLALITY
FIG. 1. Scattering results for D T A B - N a X (0.5 m ) - H 2 0 solutions• Curves 1-4: X - = acetate, propionate, isobutyrate, and pivalate, respectively.
eluded in the counterion series because of precipitation with DTA + and because the sodium salt was less soluble than 0.5 m. To establish that Br- was displaced (at least near the CMC) and that only the putative counterion, X-, was in fact bound to the miceUes, concentrations of free Br- were determined potentiometrically in 0.5000 m N a X solutions with mDTAB = 5 × CMC. Bromide was completely displaced under these stringent conditions by the following ions: phenyl- and diphenylacetate, di- and trichloroacetate, and trimethylacetate. For the other ions, complete displacement was observed with mDTAB = 1.5 × CMC except for the least favorable case, acetate; here, about 20% of the bound counterions were Br-, but extrapolation from several measurements at varying levels of DTAB suggested virtually complete displacement as the CMC was approached. Because the scattering by 0.5000 m sodium diphenylacetate was appreciably greater than that from any of the other NaX solutions in the absence of DTAB, solutions covering a range of concentrations in both water and Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
[1 + (0 In 7_+/0 In m'z)T,v]
where H = 32rc3n2/3NX 4 and n = solution refractive index N = Avogadro's number X = wavelength of light measured in a vacuum V ° = volume of solution containing 1000 g of water
"°i / /
0
0.02 0.04 0.06 0.08 0.10 0.12 DTA8 MOLALITY
FIG. 2. Scattering results for D T A B - N a X (0.5 m)-H20 solutions. Curves 1-5: X - = chloroacetate, trichloroacetate, dichloroacetate, phenylacetate, and diphenylacetate, respectively; higher points are omitted for the latter three ions to save space.
131
SUBSTITUTED ACETATE COUNTERIONS TABLE I Concentration Gradients of the Refractive Index at 4358 A and 25°C (On/Om)o.5 m r~x
X-
DTAB~
NaBrb
NaX
DTAXc
CHACO0CHaCH2COO(CHa)2CHCOO(CH3)3CCOOCICH2COOC12CHCOOC13CCOOC6HsCH2COO(C6Hs)2CHCOO-
0.0374 0.0369 0.0364 0.0353 0.0372 0.0359 0.0355 0.0340 0.0300
0.0139 0.0137 0.0136 0.0133 0.0137 0.0137 0.0127 0.0131 0.0131
0.0107 0.0133 0.0153 0.0176 0.0156 0.0179 0.0266 0.0310 0.0499
0.0342 0.0365 0.0381 0.0395 0.0392 0.0401 0.0494 0.0520 0.0668
aFor mDTAa ~< 0.12. b For mNaBr ~< 0.12. c Estimated from (On/OmDvAs) + ( d n / O m N a x ) -- (On/OmNasr).
r = turbidity in excess of that of water m~ = mm2 m2 = stoichiometric molality of NamXm
aggregation n u m b e r for phenylacetate ion. However, this m a y be misleading; in fact, the figure shows a high point at m~ = 0.1 which suggests the beginning of an upward bend that might yield a m u c h larger intercept. This would reflect a rather abrupt change in the dependence of (0 In 7±10 In m'2)r,v upon m~ in a concentration region near 0.1 m. It is difficult to test this view directly because the light scattering data on dilute solutions of small ions are insufficiently precise and activity coefficients for sodium phenylacetate are unavailable. However, we gain support from the upper curve in Fig. 5 for sodium chloride. Values
n'2 = ( On/ Om'2)r,e
3'_+ = m e a n molal ionic activity coefficient of NamXm.
The intercept of a plot of ( H n ~ V ° r n ' 2 / z ) vs m~ should be (1 + m - q ) / m ; if the only ions in solution are N a ÷ and X-, then m = 1, q = 0 , a n d ( 1 + m - q ) / m = 2. The lower curve in Fig. 5 shows, however, an intercept of about 1.33. If q = 0, this intercept, at face value, yields m = 3.0 as an
TABLE II CMC Values, Charge-Corrected Aggregation Numbers, Effective Micellar Charges, and Degrees of Counterion Dissociation for Decyltrimethylammonium Micelles with Substituted Acetate Counterions Counterion(X-)
CMC (molal)
Aggregation number(N)
Micellar charge(p)
Degreeof counteiion dissociation(p/N)
CH3COO-
0.041
35
4.7
0.13
CH3CH2COO(CH3)2CHCOO(CH3)3CCOO-
0.036 0.029 0.016
37 32 32
9.6 7.9 8.7
0.26 0.25 0.27
C1CH2COOCI2CHCOOCI3CCOO-
0.026 0.009 0.015
38 63 18
8.5 16.4 4.4
0.22 0.26 0.24
C6HsCH2COO(C6Hs)2CHCOO-
0.009 0.0014
37 32
11.1 9.0
0.30 0.28
Journal of Colloid and Interface Science, Vol. 100,No. 1, July 1984
132
UNDERWOOD AND ANACKER
2.0
m--040~
y ~ 1.8
~3o ..a
I
.J 0
e
o
•
.-"
E ~ 1.6
N~
o
\
~E
\
I
1.4 1.2
--C6
O
I j
I
2
NUMBEROFSUBSTITUENTSONACETATE FIG.3. CMCvaluesfor decyltrimethylammonium mice|leswith substitutedacetate countefions.Arrowsat the left show corresponding values for alkyl chains of varying length in n-alkylcarboxylates (10).
of (Hn'22V°m'z/r)were calculated from the right-hand side of the above equation with m -- 1, q = 0, and mean molal ionic activity coefficients from the literature (1 3). It is seen that extrapolation from higher points gives an incorrect intercept near 1.7 but that a value of 2.0 is reasonable when all of the points are considered. Thus we suppose that phenylacetate ion, like chloride, is monomeric in
6 5
%4 x
g3
tY
2 I
0
0.2 0.4 0.6 0.8 1.0 MOLALITY
FIG. 4. Scattering results for sodium phenyl- and diphenylacetates. Top curve: sodium diphenylacetate in 0.5 m NaC1; middle curve: sodium diphenylacetate in water; bottom curve: sodium phenylacetate in water. Journal of Colloid and Interface Science, Vol. 100, N o . l, J u l y 1984
o,
o'8
/ m 2
,12
,16
FIG. 5. Scattering plots for sodium phenylacetate and sodium chloride solutions (see text). Solid circles: calculated points for sodium chloride solutions. Open circles: experimental points for sodium phenylacetate solutions.
aqueous solution. The larger slopes seen in Fig. 4 at higher concentrations of sodium diphenylacetate clearly suggest the formation of small aggregates, probably dimers, a process which is facilitated by the addition of sodium chloride. DISCUSSION
It is seen in Table II that aggregation numbers, except for di- and trichloroacetate counterions, are approximately the same, with a range of only 32 to 38. In the earlier study with n-alkyl carboxylates (10), similar aggregation numbers were obtained, ranging from 31 to 48, with only one of the seven values above 38. If we assume that the micellar core is a sphere whose properties are those of a liquid hydrocarbon droplet, an aggregation number can be calculated which agrees reasonably well with the experimental values. That length of extended hydrocarbon chain believed to be embedded in the solvent-free core becomes the radius for estimating the core volume, and the number of chains that will provide a weight of hydrocarbon corresponding to the tabulated density is calculated. The density of decane (0.7300 g/cm 3) and the
SUBSTITUTED ACETATE COUNTERIONS
133
length of a decyt chain (12.82 A) yield an aggregation number of 27. Equations 6-4 and I 2 3 6-5 ofref. 14 give 40 if the number of carbon I . 5 ~ atoms in the core is 10; if the first carbon of the decyl chain is considered to be outside the O~ hydrophobic core, the calculated aggregation _~-z° 0,5 80 9 number becomes 33. Although there is evidence suggesting that several methylene groups may be exposed to water (15, 16), the exact -2.5 extent of water penetration is uncertain; a recent study employing small angle neutron scattering suggests that at most only one I I I 12f I 2 methylene group experiences appreciable walr .3 ter contact (17). FIG. 6. Log CMC (molal) vs hydrophobicity parameter, Fromherz's surfactant block model predicts ~r. R groups in counterions R-COO- are indicated by a maximal aggregation n u m b e r of 50 for a numbers: 1, H; 2, CH3; 3, C2H5;4, (CH3)2CH;5, n-C3H7; decyl micelle; the actual number, expected to 6, (CH3)3C; 7, n-C4ng, 8, CrHsCH2; 9, CrH,I (cyclohexyl); be smaller than 50 because of head group re- 10, n-CsHH ; 11, (CrHs)2CH; 12, n-Crnt3; 13, C1CH2; 14, pulsions that generate vacancies in the block C13C; 15, C12CH. Solid circles: this study. Open circles: data from Ref. (10). Hatched circle:data from Ref. (23). lattice, cannot be calculated (18). In the earlier study (10), hydrophobicity appeared to be the major factor in the effects the other sloping downward showing the effect of n-alkyl carboxylate counterions upon the of increasing hydrophobicity of R groups CMC values for DTA ÷ micelles, and the pres- larger than ethyl. The least squares version of ent results for methyl- and phenyl-substituted the latter is the locus of the equation acetates are interpretable on the same basis. log CMC = - 0 . 3 2 - 0.611r. We wish to correlate the hydrophobicity of R In order for the intrinsic hydrophobicity of in counterion R-COO- with the CMC. For this purpose, we use the ~r parameter, defined a counterion substituent to facilitate micelle formation, that substituent (or a portion of as follows (19-21): it) must be transferred from water into an oily a'R = log PR-H -- log PH2 milieu during the assembly of the micelle. At where P is a partition coefficient for a solute the same time, the carboxylate moiety must distributed between n-octanol and water. Be- remain near head groups in the Stern layer. cause experimental values of P are not avail- Apparently a substituent must be larger than able for most of our cases, P values were cal- ethyl before its penetration of the micellar inculated by the "fragment method" (22), and terior allows appreciable contact with surfacthe corresponding values of ~r must be viewed tant chains. The di- and trimethylacetates may also be as approximate. Figure 6 shows the CMC data in relation compared with their n-alkyl counterparts to 7r parameters for the present case as well bearing the same number of carbon atoms by as the earlier study; also shown is a point rep- reference to the arrows along the ordinate o f resenting cyclohexane carboxylate, where the Fig. 3 showing CMC values from the earlier CMC was obtained by the surface tension study (10). The CMC for CH3CHECH2COOmethod (23). Although a smooth curve is of 0.027 m is only slightly lower than that drawn in the figure, the data could be rep- for (CH3)ECHCOO- (0.029 m); the same is resented equally well by two straight lines, one true for CHa(CH2)3COO- (0.014 m) and horizontal based upon points 1 through 3 and (CH3)3CCOO- (0.016 m). Measurements with Journal of Colloid and Interface Science. Vol. 100!No.
1, July 1984
134
UNDERWOOD AND ANACKER
CPK models show that an extended C 4 n-alkyl chain is about twice as long as (CH3)3C-, while the diameter o f the latter group is the larger by nearly 50%. The fact that the shape o f the substituent is relatively unimportant suggests that the micellar surface is a fluid region which can easily accommodate a variety of insertions. It should be recalled that the micelle does not form in the absence of counterions; we are not depicting interactions of ions with a preformed surface. Rather, one supposes that the nature of the surface reflects, at its formation, the character of the counterion. Hydrophobic patches that interact favorably with, say, (CH3)3C- are at least partly induced by the participation of that group in the formation process. Menger has proposed analogous induction of complementarity to serve the solvation needs of guest molecules solubilized in micelles (24), and we may picture a large organic ion as playing simultaneously the roles of both solubilized guest and counterion. The phenylacetates cannot be compared directly with the n-alkyl carboxylates because, under our experimental conditions, counterions bearing alkyl chains with the requisite numbers of carbon atoms could not be studied (10). Roughly, though, we see that, carbon for carbon, phenyl groups are less effective than alkyl chains by comparing C6HsCH2COO(CMC = 0.009 m) with CH3(CH2)sCOO(CMC = 0.001 m); indeed, the two phenyl groups of (C6Hs)2CHCOO- (CMC -- 0.0014 m) are no more effective than a hexyl chain (CMC -- 0.0010 m). This accords with the evidence from solubility studies that aromatic hydrocarbons are considerably less hydrophobic than their aliphatic counterparts (25), and it is seen in Fig. 6 that the effects of both aromatic and aliphatic substituents correlate reasonably well with their 7r parameters. The phenyl substituent may be somewhat more effective in lowering the CMC than its hydrophobicity alone suggests (see point 8 in Fig. 6), although the point may reflect merely an error in a- calculated by the fragment method. That there may be a favorable electrostatic interaction between the cationic head Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
group and the 7r-electron system of the aromatic ring is suggested by N M R studies of quaternary a m m o n i u m salts in certain aromatic solvents (26, 27). On the other hand, the CMC for diphenylacetate is concordant with its hydrophobicity (point 11 in Fig. 6). The results obtained with the chloroacetates are recorded for their intrinsic value, but a convincing interpretation cannot be offered at this time. It is seen in Fig. 6 that the enhanced effectiveness of these ions as compared with acetate is unrelated to hydrophobicity. The electronegative chlorine substituents presumably experience favorable interactions within the polar region at the micellar surface. Despite the apparent pliability of the micellar surface in accommodating a variety of sizes and shapes of counterion substituents and the overall good correlation between CMC and hydrophobicity, there remain, as in many micelle studies, unexplained differences in detail. For example, the CMC with benzoate (C6HsCOO-) is 0.0035 m (23), while that for the more hydrophobic phenylacetate (C6HsCH2COO-) is somewhat higher (0.009 m). ACKNOWLEDGMENTS Discussions with F. M. Mengerand C. G. Trowbridge and the financialsupportofthe EmoryUniversityResearch Fund are gratefullyacknowledged;Mengerkindlyprovided the bromide-selectiveelectrode. REFERENCES 1. Anacker,E. W., and Ghose, H. M., J. Phys. Chem. 67, 1713 (1963). 2. Anacker,E. W., and Ghose, H. M., J. Amer. Chem. Soc. 90, 3161 (1968). 3. Mukerjee,P., Mysels,K. J., and Kapauan, P., J. Phys. Chem. 71, 4166 (1967). 4, Hoffman,H., Niisslein,H., and Ulbricht, W., in "Micellization, Solubilization, and Microemulsions" (K. L. Mittal, Ed.), Vol. l, p. 263. Plenum Press, New York, 1977. 5, Kale, K. M., and Zana, R., J. Colloid Interface Sci. 61, 312 (1977). 6. Hoffmann, H., Ulbricht, W., and Tagesson, B., Z. Phys. Chem. (Wiesbaden) 113, 17 (1978). 7. Stilbs,P., and Lindman, B., J. Phys. Chem. 85, 2587 (1981).
SUBSTITUTED ACETATE COUNTERIONS
8. Reeves, L. W., and Tracey, A. S., J. Amer. Chem. Soc. 97, 5729 (1975). 9. Gravsholt, S., J. Colloid Interface Sci. 57, 575 (1976). 10. Anacker, E. W., and Underwood, A. L., J. Phys. Chem. 85, 2463 (1981). 11. Geer, R. D., Eylar, E. H., and Anacker, E. W., J. Phys. Chem. 75, 369 (1971). 12. Anacker, E. W., and Underwood, A. L., "Three-component Light Scattering Theory for Solutions ContainingTwo Different Counterions," Paper No. 169, 36th Northwest Regional Meeting, American Chemical Society, Bozeman, Montana, June 19, 1981; manuscript in preparation. 13. Robinson, R. A., and Harned, H. S., Chem. Rev. 28, 419 (1941). 14. Tanford, C., "The Hydrophobic Effect: Formation of Micelles and Biological Membranes," 2rid Ed., Ch. 6. John Wiley, New York, 1980. 15. Clifford, J., Trans. Faraday Soc. 61, 1276 (1965).
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16. CorkiU, J. M., Goodman, J. F., and Walker, T., Trans. Faraday Soc. 63, 768 (1967). 17. Bendedouch, D., Chen, S.-H., and Koehler, W. C., J. Phys. Chem. 87, 153 (1983). 18. Fromherz, P., Chem. Phys. Lett. 77, 460 (1980). 19. Fujita, T., Iwasa, J., and Hansch, C., J. Amer. Chem, Soc. 86, 5175 (1964). 20. Leo, A., Hansch, C., and Elkins, D., Chem. Rev. 71, 525 (1971). 21. Hansch, C., and Leo, A., "Substituent Constants for Correlation Analysis in Chemistry and Biology." John Wiley, New York, 1979. 22. Ref. 21, Ch. IV. 23. Underwood, A. L., unpublished observation. 24. Meuger, F. M., Acct. Chem. Res. 12, 111 (1979). 25. Ref. 14, p, 10. 26. Taylor, R. P., and Kuntz, I. D., Jr., J. Amer. Chem. Soc. 92, 4813 (1970). 27. Abraham, R. J., Lewtas, K., and Thomas, W. A., J. Chem. Soc., Perkin Trans. H 1964 (1977).
Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
Received October 19, 1983; acceptedJanuary 31, 1984 Critical micelleconcentrationsand aggregationnumbers obtaifiedby light scatteringare reportedfor decyltrimethylammoniummicellesin 0.5000 m solutions of the followingcounterions as sodium salts: acetate, methylacetate (propionate), dimethylacetate (isobutyrate), trimethylacetate (pivalate), chloroacetate,dichloroacetate,trichloroacetate,phenylacetate,and diphenylacetate.Counterioneffectiveness in promotingmicelle formation is interpretable largelyin terms of hydrophobicinteractions, but other factors which are more difficultto identify are also involved. Association, probably dimer formation, has been observedin sodium diphenylacetatesolutions. INTRODUCTION The nature of the counterion is a significant factor in the formation of micelles by ionic surfactants, and studies of inorganic counterions have disclosed striking effects upon CMC values and aggregation numbers (1, 2). Thus it is strange that organic counterions have been largely neglected, at least in relation to the large number that might have been examined. Among the systems studied are dodecyl sulfate micelles with tetramethyl-, -ethyl-, and -npropylammonium counterions (3), tetradecylpyridinium with alkyl sulfate (4), decanoate and dodecanoate with tetraalkylammonium (5), perfluoronanoate with methylammonium (6), and decylammonium with dichloroacetate (7). The effects of several methylammonium counterions upon chain flexibility in a decyl sulfate lyotropic nematic phase have been studied (8), and it has been reported that salicylate and m- and p-chlorobenzoates induce viscoelasticity as counterions with hexadecyltrimethylammonium aggregates (9). These studies, interesting as each may be, are fragmented by the variety of surfactant ions and in any event involve a relatively small number of counterions. In some instances,
the nature o f the counterion is only incidental to the research goals, and possible counterion effects are difficult to discern and to correlate with other work. Thus tittle is known regarding the structural features which determine the effectiveness of a counterion in promoting micelle formation. Accordingly, we have undertaken light scattering determinations o f CMC values and aggregation numbers for decyltrimethylammonium (DTA +) micelles with a large number of organic counterions. This work began with a series of n-alkyl carboxylates, where the effect o f increasing chain length was interpreted straightforwardly in terms of hydrophobic interactions (10). In the present paper are reported results for a series of substituted acetate counterions. The incidental observation of association in aqueous solutions of sodium diphenylacetate, probably yielding a dimer, is also reported. MATERIALS AND METHODS Apparatus. The same photometer and refractometer used in the previous study (10) were employed. Free bromide concentrations were determined potentiometrically using a bromide ion selective electrode (Model 94-
128 0021-9797/84 $3.00 Copyright © 1984 by Academic Press, Inc. All rights of reproduction in any form reserved.
Journal of Colloid andlnterface Science, Vol. 100, No. 1, July 1984
SUBSTITUTED ACETATE COUNTERIONS 35A, Orion Research Inc.) with a Radiometer PHM 26 meter readable to 0.1 mV in the expanded scale mode. Materials. Decyltrimethylammonium bromide (DTAB) was prepared as before (10). Anal. Calcd. for C13H30NBr: Br, 28.51%. Found: 28.49%. Scattering data for solutions in 0.5000 m sodium bromide fell on the curve obtained with the earlier batch. Sodium acetate was a reagent chemical. The other sodium salts were prepared by potentiometric titrations of aqueous solutions or suspensions of the acids to the equivalence points with sodium hydroxide, removal of water by lyophilization, and recrystallizations of the solid residues. The acids and the solvents for recrystallization of the sodium salts were as follows: methylacetic (propionic, reagent grade), 1:12 water-ethanol; dimethylacetic (isobutyric, Aldrich Chemical Co., 99+%), 1:1 ethanol-acetone; trimethylacetic (pivalic, Aldrich, 99+%), 1:1 ethanol-acetone; chloroacetic (reagent grade), aqueous acetone; dichloroacetic (Aldrich 97%), 1:5 ethanolacetone; trichloroacetic (reagent grade), water; phenylacetate (Aldrich, recrystallized twice from acetone), 1:40 water-acetone; diphenylacetic (Eastman Kodak Co., recrystallized twice from ethanol), ethanol. Sodium chloroacetate did not crystallize in the usual sense but was precipitated by addition of acetone to the aqueous solution; two liquid layers formed initially, but repeated stirring of the lower layer with portions of fresh acetone precipitated the salt. All of the salts were dried in vacuo at about 70°C, lightly ground, and stored over phosphorus pentoxide. Methods. Solutions were prepared by weight. Those for light scattering were filtered under nitrogen pressure through a Coming ultrafine frit directly into the cell. Measurements were performed at 25 + 1°C. The general approach for calculating micellar aggregation numbers has been described (1, 2, 10, 11), but the equations actually employed represent a modification of the earlier theory which specifically recognizes the presence of two different counterions in the solution (12).
129
Each light scattering run involved ten to twelve DTAB solutions in 0.5000 m aqueous sodium salt (NaX) of such concentrations as to define the CMC and provide sufficient data above and below it. The laboratory water was redistilled from alkaline permanganate. If it were possible to do so, it would be preferable to prepare the DTA ÷ salt of each counterion (X-) in order to examine solutions of DTAX rather than of DTAB in NaX. However, this is not feasible for screening a large number of counterions. Whereas DTAB is beautifully crystalline and easily handled, many of the DTAX salts are exceedingly hygroscopic and intractable waxy or sticky materials. In addition, preparative methods may lead to contamination with peptized silver salts or substances leached from ion exchange resins which would be highly undesirable in light scattering studies. Employing 0.5000 m NaX as the solvent not only provides a very large ratio of mx- to mBr- near the cmc but also makes more realistic the assumption in the theoretical treatment that activity coefficients remain constant. RESULTS Scattering data for DTAB-NaX (0.5000 m)-H20 solutions are shown in Figs. 1 and 2. The plotted R90 value is Rayleigh's ratio for a 90 ° scattering angle corrected for solvent scattering. The refractive index gradients used in calculating aggregation numbers are given in Table I. In Table II are listed CMC values, charge-corrected aggregation numbers (N), effective micellar charges (p), and degrees of counterion dissociation (p/N) for the various counterions (X-). The CMC values are shown in Fig. 3. Calculations were based upon the assumption that Br- and X- are bound to the micelles in the same ratios as those of their analytical concentrations in the solutions (Model III in ref. 1). The assumption that the micelles bind only X- yields virtually identical results. In no case was dissymmetry detected by measurements at scattering angles of 45 and 135 °. Triphenylacetate could not be inJournal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
UNDERWOOD
130
AND ANACKER
5-
I
3
4
2
0.5000 m sodium chloride were examined for evidence of association. The results are seen in Fig. 4, where scattering data for sodium phenylacetate are also shown. As explained below, it is probable that the phenylacetate ion is monomeric in aqueous solution while diphenylacetate forms dimers. It may be shown that the scattering behavior of an aqueous solution of NamXm where Na + and NaaXL("-q) are the only species is described by
n n '22V° m '2 7"
_(1 +m-q) m
002 004
0108 0110 oL,2
DTAB MOLALITY
FIG. 1. Scattering results for D T A B - N a X (0.5 m ) - H 2 0 solutions• Curves 1-4: X - = acetate, propionate, isobutyrate, and pivalate, respectively.
eluded in the counterion series because of precipitation with DTA + and because the sodium salt was less soluble than 0.5 m. To establish that Br- was displaced (at least near the CMC) and that only the putative counterion, X-, was in fact bound to the miceUes, concentrations of free Br- were determined potentiometrically in 0.5000 m N a X solutions with mDTAB = 5 × CMC. Bromide was completely displaced under these stringent conditions by the following ions: phenyl- and diphenylacetate, di- and trichloroacetate, and trimethylacetate. For the other ions, complete displacement was observed with mDTAB = 1.5 × CMC except for the least favorable case, acetate; here, about 20% of the bound counterions were Br-, but extrapolation from several measurements at varying levels of DTAB suggested virtually complete displacement as the CMC was approached. Because the scattering by 0.5000 m sodium diphenylacetate was appreciably greater than that from any of the other NaX solutions in the absence of DTAB, solutions covering a range of concentrations in both water and Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
[1 + (0 In 7_+/0 In m'z)T,v]
where H = 32rc3n2/3NX 4 and n = solution refractive index N = Avogadro's number X = wavelength of light measured in a vacuum V ° = volume of solution containing 1000 g of water
"°i / /
0
0.02 0.04 0.06 0.08 0.10 0.12 DTA8 MOLALITY
FIG. 2. Scattering results for D T A B - N a X (0.5 m)-H20 solutions. Curves 1-5: X - = chloroacetate, trichloroacetate, dichloroacetate, phenylacetate, and diphenylacetate, respectively; higher points are omitted for the latter three ions to save space.
131
SUBSTITUTED ACETATE COUNTERIONS TABLE I Concentration Gradients of the Refractive Index at 4358 A and 25°C (On/Om)o.5 m r~x
X-
DTAB~
NaBrb
NaX
DTAXc
CHACO0CHaCH2COO(CHa)2CHCOO(CH3)3CCOOCICH2COOC12CHCOOC13CCOOC6HsCH2COO(C6Hs)2CHCOO-
0.0374 0.0369 0.0364 0.0353 0.0372 0.0359 0.0355 0.0340 0.0300
0.0139 0.0137 0.0136 0.0133 0.0137 0.0137 0.0127 0.0131 0.0131
0.0107 0.0133 0.0153 0.0176 0.0156 0.0179 0.0266 0.0310 0.0499
0.0342 0.0365 0.0381 0.0395 0.0392 0.0401 0.0494 0.0520 0.0668
aFor mDTAa ~< 0.12. b For mNaBr ~< 0.12. c Estimated from (On/OmDvAs) + ( d n / O m N a x ) -- (On/OmNasr).
r = turbidity in excess of that of water m~ = mm2 m2 = stoichiometric molality of NamXm
aggregation n u m b e r for phenylacetate ion. However, this m a y be misleading; in fact, the figure shows a high point at m~ = 0.1 which suggests the beginning of an upward bend that might yield a m u c h larger intercept. This would reflect a rather abrupt change in the dependence of (0 In 7±10 In m'2)r,v upon m~ in a concentration region near 0.1 m. It is difficult to test this view directly because the light scattering data on dilute solutions of small ions are insufficiently precise and activity coefficients for sodium phenylacetate are unavailable. However, we gain support from the upper curve in Fig. 5 for sodium chloride. Values
n'2 = ( On/ Om'2)r,e
3'_+ = m e a n molal ionic activity coefficient of NamXm.
The intercept of a plot of ( H n ~ V ° r n ' 2 / z ) vs m~ should be (1 + m - q ) / m ; if the only ions in solution are N a ÷ and X-, then m = 1, q = 0 , a n d ( 1 + m - q ) / m = 2. The lower curve in Fig. 5 shows, however, an intercept of about 1.33. If q = 0, this intercept, at face value, yields m = 3.0 as an
TABLE II CMC Values, Charge-Corrected Aggregation Numbers, Effective Micellar Charges, and Degrees of Counterion Dissociation for Decyltrimethylammonium Micelles with Substituted Acetate Counterions Counterion(X-)
CMC (molal)
Aggregation number(N)
Micellar charge(p)
Degreeof counteiion dissociation(p/N)
CH3COO-
0.041
35
4.7
0.13
CH3CH2COO(CH3)2CHCOO(CH3)3CCOO-
0.036 0.029 0.016
37 32 32
9.6 7.9 8.7
0.26 0.25 0.27
C1CH2COOCI2CHCOOCI3CCOO-
0.026 0.009 0.015
38 63 18
8.5 16.4 4.4
0.22 0.26 0.24
C6HsCH2COO(C6Hs)2CHCOO-
0.009 0.0014
37 32
11.1 9.0
0.30 0.28
Journal of Colloid and Interface Science, Vol. 100,No. 1, July 1984
132
UNDERWOOD AND ANACKER
2.0
m--040~
y ~ 1.8
~3o ..a
I
.J 0
e
o
•
.-"
E ~ 1.6
N~
o
\
~E
\
I
1.4 1.2
--C6
O
I j
I
2
NUMBEROFSUBSTITUENTSONACETATE FIG.3. CMCvaluesfor decyltrimethylammonium mice|leswith substitutedacetate countefions.Arrowsat the left show corresponding values for alkyl chains of varying length in n-alkylcarboxylates (10).
of (Hn'22V°m'z/r)were calculated from the right-hand side of the above equation with m -- 1, q = 0, and mean molal ionic activity coefficients from the literature (1 3). It is seen that extrapolation from higher points gives an incorrect intercept near 1.7 but that a value of 2.0 is reasonable when all of the points are considered. Thus we suppose that phenylacetate ion, like chloride, is monomeric in
6 5
%4 x
g3
tY
2 I
0
0.2 0.4 0.6 0.8 1.0 MOLALITY
FIG. 4. Scattering results for sodium phenyl- and diphenylacetates. Top curve: sodium diphenylacetate in 0.5 m NaC1; middle curve: sodium diphenylacetate in water; bottom curve: sodium phenylacetate in water. Journal of Colloid and Interface Science, Vol. 100, N o . l, J u l y 1984
o,
o'8
/ m 2
,12
,16
FIG. 5. Scattering plots for sodium phenylacetate and sodium chloride solutions (see text). Solid circles: calculated points for sodium chloride solutions. Open circles: experimental points for sodium phenylacetate solutions.
aqueous solution. The larger slopes seen in Fig. 4 at higher concentrations of sodium diphenylacetate clearly suggest the formation of small aggregates, probably dimers, a process which is facilitated by the addition of sodium chloride. DISCUSSION
It is seen in Table II that aggregation numbers, except for di- and trichloroacetate counterions, are approximately the same, with a range of only 32 to 38. In the earlier study with n-alkyl carboxylates (10), similar aggregation numbers were obtained, ranging from 31 to 48, with only one of the seven values above 38. If we assume that the micellar core is a sphere whose properties are those of a liquid hydrocarbon droplet, an aggregation number can be calculated which agrees reasonably well with the experimental values. That length of extended hydrocarbon chain believed to be embedded in the solvent-free core becomes the radius for estimating the core volume, and the number of chains that will provide a weight of hydrocarbon corresponding to the tabulated density is calculated. The density of decane (0.7300 g/cm 3) and the
SUBSTITUTED ACETATE COUNTERIONS
133
length of a decyt chain (12.82 A) yield an aggregation number of 27. Equations 6-4 and I 2 3 6-5 ofref. 14 give 40 if the number of carbon I . 5 ~ atoms in the core is 10; if the first carbon of the decyl chain is considered to be outside the O~ hydrophobic core, the calculated aggregation _~-z° 0,5 80 9 number becomes 33. Although there is evidence suggesting that several methylene groups may be exposed to water (15, 16), the exact -2.5 extent of water penetration is uncertain; a recent study employing small angle neutron scattering suggests that at most only one I I I 12f I 2 methylene group experiences appreciable walr .3 ter contact (17). FIG. 6. Log CMC (molal) vs hydrophobicity parameter, Fromherz's surfactant block model predicts ~r. R groups in counterions R-COO- are indicated by a maximal aggregation n u m b e r of 50 for a numbers: 1, H; 2, CH3; 3, C2H5;4, (CH3)2CH;5, n-C3H7; decyl micelle; the actual number, expected to 6, (CH3)3C; 7, n-C4ng, 8, CrHsCH2; 9, CrH,I (cyclohexyl); be smaller than 50 because of head group re- 10, n-CsHH ; 11, (CrHs)2CH; 12, n-Crnt3; 13, C1CH2; 14, pulsions that generate vacancies in the block C13C; 15, C12CH. Solid circles: this study. Open circles: data from Ref. (10). Hatched circle:data from Ref. (23). lattice, cannot be calculated (18). In the earlier study (10), hydrophobicity appeared to be the major factor in the effects the other sloping downward showing the effect of n-alkyl carboxylate counterions upon the of increasing hydrophobicity of R groups CMC values for DTA ÷ micelles, and the pres- larger than ethyl. The least squares version of ent results for methyl- and phenyl-substituted the latter is the locus of the equation acetates are interpretable on the same basis. log CMC = - 0 . 3 2 - 0.611r. We wish to correlate the hydrophobicity of R In order for the intrinsic hydrophobicity of in counterion R-COO- with the CMC. For this purpose, we use the ~r parameter, defined a counterion substituent to facilitate micelle formation, that substituent (or a portion of as follows (19-21): it) must be transferred from water into an oily a'R = log PR-H -- log PH2 milieu during the assembly of the micelle. At where P is a partition coefficient for a solute the same time, the carboxylate moiety must distributed between n-octanol and water. Be- remain near head groups in the Stern layer. cause experimental values of P are not avail- Apparently a substituent must be larger than able for most of our cases, P values were cal- ethyl before its penetration of the micellar inculated by the "fragment method" (22), and terior allows appreciable contact with surfacthe corresponding values of ~r must be viewed tant chains. The di- and trimethylacetates may also be as approximate. Figure 6 shows the CMC data in relation compared with their n-alkyl counterparts to 7r parameters for the present case as well bearing the same number of carbon atoms by as the earlier study; also shown is a point rep- reference to the arrows along the ordinate o f resenting cyclohexane carboxylate, where the Fig. 3 showing CMC values from the earlier CMC was obtained by the surface tension study (10). The CMC for CH3CHECH2COOmethod (23). Although a smooth curve is of 0.027 m is only slightly lower than that drawn in the figure, the data could be rep- for (CH3)ECHCOO- (0.029 m); the same is resented equally well by two straight lines, one true for CHa(CH2)3COO- (0.014 m) and horizontal based upon points 1 through 3 and (CH3)3CCOO- (0.016 m). Measurements with Journal of Colloid and Interface Science. Vol. 100!No.
1, July 1984
134
UNDERWOOD AND ANACKER
CPK models show that an extended C 4 n-alkyl chain is about twice as long as (CH3)3C-, while the diameter o f the latter group is the larger by nearly 50%. The fact that the shape o f the substituent is relatively unimportant suggests that the micellar surface is a fluid region which can easily accommodate a variety of insertions. It should be recalled that the micelle does not form in the absence of counterions; we are not depicting interactions of ions with a preformed surface. Rather, one supposes that the nature of the surface reflects, at its formation, the character of the counterion. Hydrophobic patches that interact favorably with, say, (CH3)3C- are at least partly induced by the participation of that group in the formation process. Menger has proposed analogous induction of complementarity to serve the solvation needs of guest molecules solubilized in micelles (24), and we may picture a large organic ion as playing simultaneously the roles of both solubilized guest and counterion. The phenylacetates cannot be compared directly with the n-alkyl carboxylates because, under our experimental conditions, counterions bearing alkyl chains with the requisite numbers of carbon atoms could not be studied (10). Roughly, though, we see that, carbon for carbon, phenyl groups are less effective than alkyl chains by comparing C6HsCH2COO(CMC = 0.009 m) with CH3(CH2)sCOO(CMC = 0.001 m); indeed, the two phenyl groups of (C6Hs)2CHCOO- (CMC -- 0.0014 m) are no more effective than a hexyl chain (CMC -- 0.0010 m). This accords with the evidence from solubility studies that aromatic hydrocarbons are considerably less hydrophobic than their aliphatic counterparts (25), and it is seen in Fig. 6 that the effects of both aromatic and aliphatic substituents correlate reasonably well with their 7r parameters. The phenyl substituent may be somewhat more effective in lowering the CMC than its hydrophobicity alone suggests (see point 8 in Fig. 6), although the point may reflect merely an error in a- calculated by the fragment method. That there may be a favorable electrostatic interaction between the cationic head Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984
group and the 7r-electron system of the aromatic ring is suggested by N M R studies of quaternary a m m o n i u m salts in certain aromatic solvents (26, 27). On the other hand, the CMC for diphenylacetate is concordant with its hydrophobicity (point 11 in Fig. 6). The results obtained with the chloroacetates are recorded for their intrinsic value, but a convincing interpretation cannot be offered at this time. It is seen in Fig. 6 that the enhanced effectiveness of these ions as compared with acetate is unrelated to hydrophobicity. The electronegative chlorine substituents presumably experience favorable interactions within the polar region at the micellar surface. Despite the apparent pliability of the micellar surface in accommodating a variety of sizes and shapes of counterion substituents and the overall good correlation between CMC and hydrophobicity, there remain, as in many micelle studies, unexplained differences in detail. For example, the CMC with benzoate (C6HsCOO-) is 0.0035 m (23), while that for the more hydrophobic phenylacetate (C6HsCH2COO-) is somewhat higher (0.009 m). ACKNOWLEDGMENTS Discussions with F. M. Mengerand C. G. Trowbridge and the financialsupportofthe EmoryUniversityResearch Fund are gratefullyacknowledged;Mengerkindlyprovided the bromide-selectiveelectrode. REFERENCES 1. Anacker,E. W., and Ghose, H. M., J. Phys. Chem. 67, 1713 (1963). 2. Anacker,E. W., and Ghose, H. M., J. Amer. Chem. Soc. 90, 3161 (1968). 3. Mukerjee,P., Mysels,K. J., and Kapauan, P., J. Phys. Chem. 71, 4166 (1967). 4, Hoffman,H., Niisslein,H., and Ulbricht, W., in "Micellization, Solubilization, and Microemulsions" (K. L. Mittal, Ed.), Vol. l, p. 263. Plenum Press, New York, 1977. 5, Kale, K. M., and Zana, R., J. Colloid Interface Sci. 61, 312 (1977). 6. Hoffmann, H., Ulbricht, W., and Tagesson, B., Z. Phys. Chem. (Wiesbaden) 113, 17 (1978). 7. Stilbs,P., and Lindman, B., J. Phys. Chem. 85, 2587 (1981).
SUBSTITUTED ACETATE COUNTERIONS
8. Reeves, L. W., and Tracey, A. S., J. Amer. Chem. Soc. 97, 5729 (1975). 9. Gravsholt, S., J. Colloid Interface Sci. 57, 575 (1976). 10. Anacker, E. W., and Underwood, A. L., J. Phys. Chem. 85, 2463 (1981). 11. Geer, R. D., Eylar, E. H., and Anacker, E. W., J. Phys. Chem. 75, 369 (1971). 12. Anacker, E. W., and Underwood, A. L., "Three-component Light Scattering Theory for Solutions ContainingTwo Different Counterions," Paper No. 169, 36th Northwest Regional Meeting, American Chemical Society, Bozeman, Montana, June 19, 1981; manuscript in preparation. 13. Robinson, R. A., and Harned, H. S., Chem. Rev. 28, 419 (1941). 14. Tanford, C., "The Hydrophobic Effect: Formation of Micelles and Biological Membranes," 2rid Ed., Ch. 6. John Wiley, New York, 1980. 15. Clifford, J., Trans. Faraday Soc. 61, 1276 (1965).
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16. CorkiU, J. M., Goodman, J. F., and Walker, T., Trans. Faraday Soc. 63, 768 (1967). 17. Bendedouch, D., Chen, S.-H., and Koehler, W. C., J. Phys. Chem. 87, 153 (1983). 18. Fromherz, P., Chem. Phys. Lett. 77, 460 (1980). 19. Fujita, T., Iwasa, J., and Hansch, C., J. Amer. Chem, Soc. 86, 5175 (1964). 20. Leo, A., Hansch, C., and Elkins, D., Chem. Rev. 71, 525 (1971). 21. Hansch, C., and Leo, A., "Substituent Constants for Correlation Analysis in Chemistry and Biology." John Wiley, New York, 1979. 22. Ref. 21, Ch. IV. 23. Underwood, A. L., unpublished observation. 24. Meuger, F. M., Acct. Chem. Res. 12, 111 (1979). 25. Ref. 14, p, 10. 26. Taylor, R. P., and Kuntz, I. D., Jr., J. Amer. Chem. Soc. 92, 4813 (1970). 27. Abraham, R. J., Lewtas, K., and Thomas, W. A., J. Chem. Soc., Perkin Trans. H 1964 (1977).
Journal of Colloid and Interface Science, Vol. 100, No. 1, July 1984