272
J. Phys. Chem. 1980, 84, 272-275
(6) C. A. Angeii, "Supercoded Water" in "Treatise on Water", F. Franks, Ed., Vol. 7, to be published. (7) J . 4 . Bacri and R. Rajaonarison, J . Phys. Lett., 40, L-403 (1979). (8) C. A. Angeli and E. J. Sare, J . Chem. Phys., 52, 1058 (1970). (9) D. H. Rasmussen and A. P. MacKenzie, J . Phys. Chem., 75, 967 (1971). (10) J. A. McMiiian and S. C. Los, Nature (London), 206, 806 (1965). (1 1) M. Sugisaki, H. Suga, and S. Seki, Bull. Chem. SOC.Jpn., 41, 2591 (1968). (12) (a) C. G. Venkatesh, S. A. Rice, and A. H. Narten, Science, 186, 927 (1975); (b) J. Wenzel, C. Linderstromlang, and S. A. Rice, ibM., 187, 428 (1975); (c) C. G. Venkatesh, S. A. Rice, and J. B. Bates, ibid., 68, 1065 (1975); (d) S. A. Rice, Curr. Top. Chem. (1976). (13) C. A. Angeil, J. Shuppert, and J. C. Tucker, J. Phys. Chem., 77, 3092 (1973). (14) G. P. Johari, Phi/. Mag., 35, 1077 (1977). (15) H. Kanno and C. A. Angeli, J . Chem. Phys., 65, 851 (1976). (16) These two chlorides show a sudden flattening out at 139 K in the To vs. composition plot like that observed over a wider range of composition by LiCl solutions. There is some evidence that this may be associated with a separation of the solution into immiscibleaqueous and salt-rich Dhases (C. A. Anaeil and E. J. Sare. J. Chem. Phvs.. 49, 4713 (1968); S. Y . Hsich,k. W. Gammon, P. B. Macedo, land C. J. Montrose, ibid., 56, 1663 (1972)). (17) M. Sugisaki, H. Suga, and S. Seki, Bull. Chem. SOC.Jpn., 41, 2586
(1968). (18) K. K. Kelley, J. Am. Chem. SOC.,57, 180 (1929).
( 19) J. A. Faucher and J. V. Koieske, phys. Chem. G/asses, 7, 202 (1 966). (20) S. M. Wolpert, A. WeRz, and 8. Wunderlich, J. Polym. Sci. A 2 , 9 , 1887 (1971). (21) M. A. DeBoit, A. J. Easteai, P. B. Macedo, and C. T. Moynihan, J. Am. Ceram. SOC.,59 [l-21, 16-21 (1976). (22) E.g., 2-methyipentane glass in the study of D. R. Dousiin and J. A. Huffman, J . Phys. Chem., 68, 1704 (1946). (23) C. A. Angeil, J. Chem. Educ., 47, 583 (1970). (24) D. H. Rasmussen, A. P. Mackenzie, J. C. Tucker, and C. A. Angeil, Science, 181, 4079 (1973). (25) M. Oguni and C. A. Angeil, to be published. (26) Heat capacity always changes more gaduaily near Toduring a cooling half cycle than during a heating haif cycle (e.g., as in Figure 1). The
differences are quantitatively accounted for by nonlinear relaxation theory (see ref 21). (27) Glass transitions during cooling which occur at much higher temperatures than those detected during reheating are routine in the splat-quenched metal alloy glasses on whlch so much attention Is currently being focussed. The frozen-in structure in such glasses is characteristlcaily a rather "loose" one in which a considerable amount of iocailzed relaxational motion is permitted, and in which C, is somewhat greater than in the corresponding crystalline solid near To. (28) B. J. Luyet, J. Phys. Chem., 43, 881 (1939); also footnote 16 in ref 13. (29) S. A. Rice, M. S. Bergren, and L. Swingie, Chem. Phys. Lett., 59,
14 (1978).
Association of Anions to Cationic Micelles Daniel Bartet, Consuelo Gamboa, and LUISSepQlveda" Depatfment of Chemistry, Faculty of Sciences, University of Chile, Las Paimeras 3425, Santiago, Chile (Received July 5, 1979) Publication cost assisted Servicio de Desarrolio Cientifico, Atfktico y de Cooperaci6n Internaclonai de la Universidad de Chile
The relative association degrees of different anions to micelles of hexadecyltrimethylammonium bromide (CTA) were measured by spectrophotometrically determining the amounb of p-toluenesulfonate (TOS) or benzenesulfonate (BS) anions desorbed from the micelles by addition of increasing amounts of NaN03, NaBr, NaC1, NaOH, NaF, NaOAc, Na2S04,Na2HP04,Na2C03,and Na2B407.A simple ion-exchangemodel was used for interpreting the results in a quantitative way and values for the ion-exchange constants were calculated. These values are discussed in terms of the hard and soft acid-base principle.
Introduction It is well known that micellar catalysis can be inhibited or practically suppressed by the addition of salts to the reaction m e d i ~ m . l -Bunton ~ et al.lr3have demonstrated that this inhibition can be satisfactorily explained in terms of a competition between salts and substrates for binding to micelles. Larsen and Magid4 measured the binding of Br-, NO,, TOS-, and OH- to CTA micelles but it was later recognized that their measurements for competitive counterion binding were incorrect in the case of OH-.5 The association of protons and hydroxyl anions to micelles is of great importance in the interpretation of reaction rates occurring in the presence of micelles where H+or OH- can act as reagents or when acid-base equilibria of the reactive species are involved.6 In addition, most measurements of the rate of reactions in the presence of micelles are carried out in buffered solutions and buffer ions are considered inert species only acting as bulk pH controllers. There is no reason to believe that buffer ions do not interact with the micellar surface. Their competitive binding to micelles could affect the amount of bound ionic substrate as well as the pH at the Stern layer due to the different distribution of acid and base buffer species between water and the micellar phase. 0022-3654/80/2084-0272$0 1.OO/O
The binding of protons to anionic micelles of sodium dodecyl sulfate has been satisfactorily measured7 but attempts to measure the binding of OH- to CTA micelles have failed.5*6 We report here a simple method which allows the quantitative determination of relative binding strengths of some anions to cationic micelles including OH- and buffer anions.
Method Our method is based on the observation that the UV spectra of aqueous solutions of some aromatic anions (S) experience a shift in the presence of CTA micelles.8 By increasing the CTA concentration the absorbance of S at a given wavelength changes gradually until it reaches a constant value. The interpretation given to this phenomenon is that a constant absorbance occurs when the CTA concentration is large enough to bind all existing S.8 At lower concentrations of CTA, S exists bound to micelles as well as free in the water phase. The change in absorbance of such a solution upon addition of a salt whose anion C competes with S for its place in the micelle is a measure of the amount of S displaced. The total amount of bound C can be obtained with the assumptions that, for 0 1980 American Chemlcal Society
The Journal of Physical Chemistty, Vol. 84, No. 3, 1980 1273
Association of Anions to Cationic Micelles TABLE 1 : Absorbtivities of TOS and BS in Water (E,) and When Completely Bound t o CTA Micelles at the Given Wavelengths ~
_
_
_
_
_
_
Ea
~
Em
~-~ 322 224 TOS 225 BS 39 5
I
A, nm
/
262 262
each molecule of S that is displaced, one molecule of C is sorbed, ,and that the degree of micellar ionization remain constant, The method requires S to have different absorbtivities in the bound and free states at the wavelength used, and also its binding to micelles to be weak so as to be removed from them by weakly binding counterions.
Experilmental Section Several series of solutions containing constant concenM for M for TOS and 4 X trations of CTA (3 X BS) and TOS or BS (1 X M) and variable concentrations of the corresponding salt (up to 0.1 M) were prepared and their absorbances recorded at a wavelength at which1 a maximum difference in absorbtivities was found between bound and free S. When buffer salts were used the pH was adjusted 2.5 pH units higher than the pK of the buffer system and the concentration of the dianions was taken as twice their molarities. The cmc in the presenoe of the salts for calculating the micellar CTA concentration was estimated from ref 8 and was 7 X 10" M. NaTOS, from Erba, and NaN03, NaBr, NaC1, NaOH, NaF, N,aOAc, Na2S04,Na2C03,Na2HP04,and Na2B407 (p.a.), from Merck, were used without further purification. CTA from MCB was recrystallized twice from ethanolether solutions. Results, and Discussion Sorption of TOS or BS by micelles of CTA is accompanied by a shift in their UV spectra to shorter wavelengths" which in turn produces a decrease in their absorbtivities in some regions of the spectrum. The concentration of CTA at which this effect becomes noticeable was 7 X lou5RA and would correspond to the cmc of CTA in the presence of either BS or TOSO8Table I shows the absorbtivities of both BS and TOS in water (E,) and when completely bound to micelles (E,), and the wavelengths at which measurements were performed. Addition of salts to solutions of CTA and BS or TOS changes their absorbances ,as expected if BS or TOS are displaced from micelles by the added anions. The results are shown in Figures 1 and 2 in which some examples of typical behavior are presented. Structural changes in the micelles could contribute to produce a similar effect but it would not account for the large differences found in going from one anion to another. TOS and BS anions interact with micelles of CTA both electrostatically and hydrophobically and a simple anion would not displace 'TOS or BS on account of only electrostatic interactions. The displacement would take place because micelles would behave as strong electrolytes in the sense that they preserve their electrical charge and if a counterion penetrates the Stern layer, another counterion goes out from it thus resulting in a constancy of the degree of dissociation of micelles. This constancy has been reported by several workers and is extensively quoted by R o m ~ t e d . ~ Finally, J~ the one-to-one ion-exchange assumption has been used with good success for obtaining the binding of protons to anionic micelles.' The above considerations indicate that the ion-exchange model with constant degree of ionization is a good approach €or dealing with these systems and it will be used
[SA L T ] x IO3
20
GO
40
80
IO0
M) and CTA (4 Absorbances of solutlons of BS (1 X M) at dlfferent concentrations of some anions.
Flgure 1. X
GO
40
80
IO0
Flgure 2. Absorbances of solutions of TOS (1 X M) and CTA (3 X M) at different concentrations of some anions.
here according to the following treatment. The different ionic exchanges between the hydrophobic anion S, the anions C coming from an added salt, and the bromide anion of the detergent itself can be written as c, + s, s, + c, (1) C, + Brm- e Br, + C, (2)
*
Br,
+ S,
F!
Sa + Br,-
(3)
274
The Journal of Physical Chemistry, Vol. 84, No. 3,
1980
Bartet, Gamboa, and Sepulveda
TABLE 11: Exchange Constants of Anions Relative t o BS and TOS, the Corresponding Correlation Coefficients, and the Ion-Exchange Constants Ratios Relative t o NO3-
NO,-
10.10
Br- "
9.20 5.72 1.80 1.41 1.43 0.90 0.85 0.44
so42-
c1-
HP0,'B40,2AcO'
c0,zOHF-
0.40
5.64 5.20 0.85
0.21 0.24
0.9942 0.9959 0.9980 0.9963 0.9958 0.9967 0.9947 0.9988 0.9918 0.9280
0.9698 0.9932 0.9789
0.9917 0.9923
1.00
1.00 1.10 1.77 5.61 7.16 7.06 11.2 11.9 23.0 25.3
1.08 6.64
26.9 23.5
If the activity coefficients are omitted,12the corresponding equilibrium constants are given by
KCIBI-
Br;Cm = ___ CaBrm-
p/Br-
= ___
SaBrmBr[Sm
4 t
where subscripts m and a refer to the micellar and aqueous phases, respectively, and the concentrations are taken in moles per liter of total solution. The concentration of all species associated to micelles is related to the degree of micellar dissociation (a)through the equation where [D,] is the detergent concentration in micellar form. The total concentrations of S, C, and Br- are given by [Stl = [Sal + [Sml [GI = [CaI + [Cml [Br,] = [CTAt] = [Br;] + [Brm-]
(5)
The fraction f of associated S is defined as f = [Sml/[Stl and can be calculated from the experimental data with the equation
where A is the absorbance of the solution due to S. In deriving eq 6 it was assumed that bound and aqueous S contribute independently to A and also that E, is independent of the number of anions present per micelle.* From the above equations it is possible to get the expression
a)[Dml (7) The @IBr term occurring in eq 7 corresponds to the particular ion-exchange equilibrium 3 and can be experimentally obtained by addition of Br- ions to CTA solutions containing S. In this case eq 4 reduces to
[Sml + [Br,-l = (1- a)[Dml (8) and the total Br- ion concentration will be given by [BrJ = [CTAt] + [Brad-] = [Br;]
+ [Br,-]
(9)
Figure 3. Graphical representation of eq 10 (seetext) for BS and TOS M) in the presence of CTA (4 X loF3and 3 X M) and (1 X added NaBr.
where Brad- represents the added Br-. After rearrangement, the following linear expression is obtained:
-'
-K (' 1
f
-
a)[Dml -f[stI)= p/B'([Brad]
f[stI)+
P/B'([CTAt] - (1 - a)[D,]) (10)
so that under the experimental conditions a plot of the first member against ([Brad-]+ [St])would result in a straight line with slope equal to KJlBr. Figure 3 shows plots of eq 10 with a = 0.2479and a cmc of 7 X M. The @IBr values so calculated are 9.2 X and 5.2 X for BS and TOS, respectively. These values together with the experimental values off and [C,] can now be introduced in eq 7 for obtaining the plots shown in Figure 4 where some anions were excluded in order to avoid overlapping of points but the linear trend is observed for all values. The linearity obtained supports the validity of the assumption on the constancy of a. The @Ic ion-exchange constants calculated from the slopes of the corresponding straight lines by using the least-squares method allow the counterions to be arranged in a lyotropic series showing their relative binding abilities to CTA micelles. The sequence is presented in Table I1 and is similar to that found by Larsen and Magid4 from heats of solution in water and in 0.1 M aqueous CTA.13 As expected TOS and BS are bound more strongly than the other anions as reflected by all €WCvalues which are less than one. Some differences are found, however, for PIcobtained with TOS or BS. Because of the presence of the p-methyl group in TOS this molecule is more strongly bound to CTA than BS and is therefore less easily
The Journal of Physical Chemistry, Vol. 84, No. 3, 1980 275
Association of Anions to Cationic Micelles
TABLE HI:
Exchange Constants ( K c / c ’ )of Anions Relative to Each Other and Based o n KBSIC Exchange Constants
C’
NO,-
F-
OH-
co,2-
AcO-
HP042-
B,0,2-
C1-
0.98 0.47 0.44 0.28 0.28 0.22 0.070 0.044
0.52 0.49 0.31 0.31 0.24 0.077 0.048
0.94 0.60 0.59 0.47 0.15 0.092
0.64 0.63 0.50 0.16 0.098
0.99 0.78 0.25 0.15
0.79 0.25 0.16
0.32 0.20
Br-
NO,-
F-
OE[CO 3 2 AcO-
HP042B40,2-
c1-
SOt42-
Br-.
25 23 12 11 7.2 7.3.
5.6 1.8 1 .I.
Flgure 4. Graphical representation of eq 7 (see text) for BS and TOS (1 X lo-:’M) in the presence of CTA (4X and 3 X M) and different added salts.
displaced by a simple anion. In this sense, the results with BS are more ireliable since less salt is needed to produce a measurable effect on absorbances. This is particularly valid for weakly bound anions such as OH- or F-. Table I1 also includes ion-exchange constants relative to that of NO,. It is seen that, in spite of the differences in absolutes values for Pic, the relatives ones are similar indicating that the competition of the different anions is independent of the nature of S. The ratio between PIcrelative to either substrate for two different imions corresponds to the exchan e constant between the anions themselves. If PIcand PY5’ represent the ion-exchange constants relative to S for anions C and C’, respectively, the exchange constant relative to each other is; given by
which represents th.e equilibrium Exchange constants for all pair of anions studied are shown in Tab1.e 111. A single comparison can be made with the data reported by Larsen and Magid4who report values between 1.43 and 1.67 (called by them selectivity ratios) for the Kc/c‘ of NO,/Br-, &s compared to 1.1found by us. The agreement is surprisingly good in spite of the different approaches used and the approximations made. The exchange measured could be considered as an acid-base reaction, in
0.62
which different bases (anions) compete for an acidic center, the site on the micelle, presumably constituted by one or more tetraalkylammonium moieties. A universal basicity scale cannot be constructed since the ordering would strongly depend on the nature of the acidic molecule taken as reference. However, Pearson’s classification of acids ,and bases into hard and softl4J5could provide a less empirical reference system than the lyotropic series. A single tetraalkylammonium group is expected to behave as a moderately soft acid, on account of R3Ct being a borderline case and because these anions are found tto be completely unhydrated.13 On these grounds one is tempted to make the generalization that ionic interactions in micellar systems follow the pattern proposed by Pearson but interactions of ions with a charged micellar surface could occur with the micelle taken as a whole or with the head charged groups and the total micelle would behave even more softly than the isolated head groups.
Acknowledgment. Support of this work by the Programa Regional de Entrenamiento de Postgrado en Ciencias Bioldgicas (PNUD/UNESCQ RLA 78/!24) and the Servieio de Desarrollo Cientifico, Artistic0 y de Cooperacidn Internacional de la Universidad de Chile is gratefully acknowledged. The authors thank Dr. H. M. Niemeyer for his interesting suggestions and comments. References and Notes (1) C. A. Bunton, E. J. Fendler, L. SepOlveda, and K.-U. Yang, J. Am. Chem. Soc., 90,5512 (1968). (2)E. J. Fendler and J. H. Fendler, “Catalysis in Micellar and Macromolecular Systems”, Academic Press, New York, 1975. (3)C. A. Bunton, Prog. Solid State Chem., 8, 239 (1973). (4)J. W. Larsen and L. J. Magld, J. Am. Chem. Soc., 98, 5774 (1074). (5)J. W. Larsen, J. Am. Chem. Soc., 97,1988 (1975). (6) C. A. Bunton, L. S.Romsted, and 0.Savelii, J. Am. Chem. Soc., 101, 1253 (1979);C. A. Bunton, F. Rhrera, and L. SepGhreda, J. Org. Chem., 43, 1166 (1978). (7)C. A. Bunton, K. Ohmenretter, and L. SepOlveda, J . Phys. Chem., 81, 2000 (1977). (8)L. SepGlveda, J. Colloid Interface Sci., 46, 372 (1974). (9) L. S.Romsted, Ph.D. Thesis, Indlana University, 1975. (10) Recently, Quina and Chaimovich” have proposed an ionsxchnnge model in micellar solutions to account for the effect of mlceiies upon reactivities and apparent pKs. The model Is based on the fundamental assumptlon that a is a constant. Their ref 19 is clarifying on this matter. (11) F. H. Quina and H. Chaimovich, J. Phys. Chem., 83, 1844 (1079). (12)C. Tanford, “The Hydrophobic Effect”, Wiley-Interscience, New b r k ,
1973. (13)These authors found B r higher in the series than NO3- on the basis of the enthalpies of transfer of NaBr from water to CTA. However, ion exchange cannot be the cause of this enthalpic effect produced by the addiin of Br- ions to CTA already containing B r counterions. Effects such as a change in mlceilar structure could be responsible for it. (14) R. G. Pearson, J. Am. Chem. Soc., 85,3533 (1963). (15) R. G. Pearson, Science, 151, 172 (1966). (16) A. Ikegami, J. Polym. Sci., A 2 , 907 (1964).
