Binding of Hydroxide Ion to Positive Micelles
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
1851
Ion Exchange in Micellar Solutions. 2.’ Binding of Hydroxide Ion to Positive Micelles H. Chaimovich,” J. B. S. Bonllha, M. J. Polltl, and F. H. Quina* Group for Interfacial Studies (GIST), Instituto de Qdmlca, Universidade de SZo, Paulo, Caixa Postal 20.780, SZo, Paulo, S.P., Erasil (Received August 2, 1978; Revised Manuscript Received February 2, 1979)
A kinetic study of the alkaline hydrolysis of a substrate (N-methyl-4-cyanopyridinium ion, MCP) which resides exclusively in the intermicellar aqueous phase provides direct evidence for the binding of -OHion to positive micelles. The observed rate behavior as a function of total detergent concentration can be adequately described by the ion-exchange model in micellar solutions. This treatment provides selectivity coefficients = 0.08 f 0.02 for the binding of -OH to hexadecyltrimethylammonium bromide micelles and KoH/Cl = 0.14 f 0.02 for the binding of -OH to tetradecyltrimethylammonium chloride micelles, This work represents the first quantitative description of the interaction of -OH with positive micelles.
Introduction Reactions involving alkaline hydrolysis in the presence of cationic micelles have played a formative role in the development of fundamental concepts of micellar catalysis.2 Given the importance of such reactions, it is noteworthy that no definitive description of the binding of -OH to cationic micelles (D’Y-)has been forth~oming.~ The failure to arrive at such a description is not attributable to a dearth of relevant experimental investigation, but rather t o intervening experimental difficultie~.~ In the present work, we present kinetic results for the alkaline hydrolysis of the N-methyl-4-cyanopyridinium ion (MCP) in the intermicellar aqueous phase of N-hexadecyl-N,N,N-trimethylammonium bromide (CTAB) and of N-tetradecyl-N,N,N-trimethylammonium chloride (TTAC1). Analysis of these results from the point of view of ion exchange1 leads to the first quantitative description of the binding of the -OH ion to positive micelles. Model We have shown in the preceding article that the selectivity constant for binding of a reactive ionic species such as the hydroxide ion -OH to positive detergents (such as CTAB or TTAC1) in the absence of buffer should be described by the following equations: KOH/Y = [OHblYf/([OHf]Yb) (1) where Y = Br- (CTAB) or C1- (TTAC1) and Yf = ~ C +D CmC + [OH,] (2) Yb = (1 - Cl!)c~ - [OH,] (3) (4) [OHTI = [OH,] + [OH,] In these equations, [OH,] is the total analytical concentration of added OH- and [OHb], Yb, [OH,], and Yf refer to the analytical concentrations of bound and free ions, respectively; CD is the concentration of micellized detergent (equal to CT - cmc, where CT is the total detergent concentration), crnc is the critical micelle concentration, and a is the degree of ionization of the mice1le.l From eq 1-4,it is apparent that, given reasonable values for a,measurement of the cmc and either [OH,] or [OH,] would permit the evaluation of KOHIY.Moreover, at high detergent concentration, the expression for KOHIYapproaches the limit
This limit should provide an indirect confirmation of the Contribution No. 3.
value of that is insensitive to the precise value of the cmc. From the brief analysis presented up to this point, it is patent that the problem of experimental design reduces to the determination of either [OH,] or [OH,] as a function of CD. Although it should, in principle, be feasible to determine [OHf] indirectly via pH or conductivity measurements, attempts to measure [OH,] in this manner have proven to be less than sati~factory.~ An alternative approach would be the indirect determination of either [OH,] or [OHf] from kinetic data for a reaction involving -OH in which the substrate is completely localized in either the micellar or the intermicellar aqueous phases. Taking into consideration the factors involved in any such determination of [OH,], principally the uncertanity in the “effective local concentration” in the micellar phase,l one finds that kinetic measurements of [OH,] are clearly the more preferable alternative. Given, then, a reactive substrate which resides exclusively in the intermicellar aqueous phase, the following rate expression can be formulated for pseudo-first-order conditions:
k, = kz’[OHf]
(6)
Thus, by using appropriate values of the second-order rate constant, k2/,measured in the absence of detergent (where [OHf] = [OH,]), one can analyze the behavior of the apparent first-order rate constant k, as a function of CT in terms of [OH,].
Experimental Section Apparatus. Absorption spectra and kinetic data were obtained with a Beckman M-25 kinetic system thermostated at 30 “C. Measurements of pH were carried out at 30 “C with a Metrohm Herisau E388 compensator equipped with semimicro combination electrode referenced against standard buffer solutions (Beckman Inc.). Conductivity measurements were performed at 30 “C with a dipping cell and a Beckman RC 18A conductivity bridge. Surface tension measurements were carried out with an A. Kruss Model 8551 De Nouy tensiometer equipped with a Pt ring. Vapor phase chromatographic (GLPC) analyses were performed on a Hewlett-Packard Model 5750 research chromatograph with flame ionization detectors. Materials. Deionized water, doubly distilled in glass, was freshly boiled and saturated with argon in all experiments. Sodium hydroxide (Merck, Titrisol) was used as received. Tetramethylammonium bromide (TMAB) was prepared by careful neutralization of tetramethylammonium hydroxide (Merck) with HBr (Merck) followed
0022-3654/79/2083-1851$01.00/00 1979 American Chemical Society
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The Journal of Physical Chemistty, Vol. 83, No. 14, 1979
Chaimovich et al.
80, by lyophilization. Tetramethylammonium chloride wap obtained from Merck. CTAB (Merck p.a.) was extracted several times with ether and recrystallized three times from acetone-ethanol (85:15). TTACl was prepared from lot no. B6X (Eastman Kodak) of commercial hexadecyltrimethylammonium chloride (CTACI). This lot was extracted (Soxhlet) with refluxing ether (over LiAlHJ for 12 h, recrystallized three times from acetone:ethanol(85:15), and dried in vacuo over PZOb The resultant TTACl had a cmc of 5.35 X M (conductimetric and surface tension). Hoffman degradation5 of TTACl showed that the resultant alkene was 99.5% tetradecene (GLPC on a 6 f t X 1/8 in. stainless steel column of 10% UCW-98 on 80-100 mesh Chromosorb AWDMCS with Aldrich 1-alkenes as reference standards). An apparent molecular weight of 288 f 1 g/mol (calculated lo0 2 8 for TTAC1, 291.5 g/mol) was obtained by titration of chloride.6 The parent peak in the mass spectrum (70 eV, CTMAYI x IO2 ( M I direct insert probe at 220 "C) was m / e 241 (TTAClFlgure 1. Effect of TMAB (0)and TMACl (0)on the rate of alkaline CH3C1)with no evidence for a significant peak at m / e 269 hydrolysisof the N-methyl-kyanopyridinium ion. Curves are calculated (CTAC1-CH3C1). These data may be compared with those (seetext). [Oh] = 20 m M The soli line is calculated for k$ = 0.0985 for lot no. B7A of unpurified Eastman CTAC1, which gave s-' and B = 1.50; the upper and lower dashed curves represent k~.' a 1-tetradecene:l-hexadeceneratio of 6535 (plus a trace = 0.097 s-' and 5 = 2.00 and k = 0.10 s-' and B = 1.00, res-'; the solid line spectively. [OH,] = 1.0 mM: k$= 2.15 X of 1-dodecene) by GLPC after Hoffman degradation and is calculated for 5 = 1.50; the upper and lower dashed curves represent a mass spectral m / e 241:269 intensity ratio of 60:40. " 5 = 2.00 and 5 = 1.00, respectively. N-Methyl-4-cyanopyridiniumtetrafluoroborate (mp 119-120 "C) was prepared by treatment of the correMCP. For this purpose, we choose to employ tetrasponding iodide salt' with silver tetrafluoroborate (from methylammonium bromide (TMAB) and chloride HBF, and Ag,O) followed by recrystallization from (TMACl) as representative nondetergent analogues of methanol (Aldrich, spectroquality). The absorption CTAB and TTAC1. The data for k, as a function of the spectrum of this material ( E 4825 M-' cm-', A,, 278 nm) concentration of TMAB or TMACl (Figure 1)show that was in excellent agreement with the published data for the the rate of alkaline hydrolysis of MCP decreases slightly perchlorate salt.7 with increasing salt concentration a t both 1 and 20 mM Methods. Kinetic data were obtained as follows: an OHT. appropriate aliquot of an aqueous solution of NaOH was This salt dependence of h, in the absence of micelles added to a thermally equilibrated aqueous solution conimplies that the application of eq 6 to rate data in the taining MCP (5 X M), or MCP plus TMAB, TMAC1, presence of micelles must, of necessity, take into account CTAB, or TTAC1. The variation of the optical density salt effects on kz' in the intermicellar aqueous phase. with time was followed at either 260 or 230 nm. PseuThese rate data were represented by a conventional do-first-order rate constants were calculated in the usual Bransted-Bjerrum treatment.1° Thus, employing manner from log (OD, - OD,) vs. time plots, which were k,' = k *O 10-/(rW (7) linear over at least three half-lives. The P / A product ratios were determined from the where k,, is the pseudo-first-order rate constant at ~1 = 0, absorption spectra after complete reaction (>lo half lives), the extended Debye-HuckellO form for f ( p l / ? a method which gives ratios in excellent agreement with those calculated from direct measurements of ~ y a n i d e . ~ , ~ Critical micelle concentrations (cmc) of CTAB and TTACl in the presence and absence of OH- were determined at where 30 "C from conductivity and/or surface tension measurements under an argon atmosphere. All calculations = [OHT] + [TMAY] (9) and simulations of kinetic curves were performed on a values of B in the range of 1.5 f 0.5 give reasonable Hewlett-Packard Model 10 programable calculator. agreement with the experimental data, especially for 20 Results mM OHT. The experimental data for k for the alkaline hydrolysis The alkaline hydrolysis of N-alkyl-4-cyanopyridinium of MCP in the presence of C*AB and TTACl are presions produces N-alkylpyridone (P) plus -CN and the ented in Figures 2 and 3, respectively. The calculation of N-alkyl-4-carboxamidopyridiniumion (A) as a result of the smooth curves, based on eq 1-4 and 6 and the ionic -OH attack on the ring or on the CN group, respectively. strength dependence of k2/, will be analyzed in detail in As we have previously shown, CTAB dramatically affects the Discussion. the P/A product distribution ratio of cyanopyridinium ions The rate data obtained at 1 mM OHT in the presence that are incorporated in the micellar phase, Le., the Nof both TMAB and CTAB exhibit a more pronounced dodecyl-4-cyanopyridinium (DOCP) and N-hexadecyl-4experimental scatter. We attribute the lower quality of cyanopyridinium (HexCP) ions.8 On the other hand, the the rate data at 1mM OHT to the enhanced sensitivity of P / A ratio for MCP (0.5 at 1 mM OH and 1.22 at 20 mM k, to experimental variables at this OHT. These include OH) exhibited no significant changes upon addition of the longer reaction half-life, greater effect of adventitious either CTAB or TTAC1. acidic impurities, including COP absorption, and the Since the rates of reactions involving ionic species are marked dependence of k, on pH in this region.' Although known to be ionic strength dependent, we investigated the the rate data at 1 mM OHT are less precise than those at ionic strength effect on h i for the alkaline hydrolysis of
1
The Journal of Physical Chemistry, Vol. 83, No. 14, 1979
Binding of Hydroxide Ion to Positive Micelles 80,
n
1
9 X
3 Y
1
I
I
1
I
2
0
I
I
4
I
I
8
6
[CT] x 102(M)
Figure 2. Effect of CTAB on the rate of alkaline hydrolysis of the N-methykbcyanopyridiniumion. Curves are calculated (seetext). [ O b ] = 20 mM: k$ = 0.0985 s-’, B = 1.50, a = 0.20, cmc = 6 X 10M; the solid line is calculated for KOHIBr= 0.080; the upper and lower dashed curves represent Kmw = 0.060 and K , = 0.10, respectively. s-’, 6 = 1.50, 01 = 0.20, cmc [OH,] = 1 mM: k ~ . ’= 2.15 X = 7.5 X M. The solid line and upper and lower dashed curves are calculated for KOHmr values of 0.08, 0.06, and 0.10, respectively.
t -
70
X
3 Y
50
I
0
I
I
I
2
1
[C,l
I
6
4
I
I
I
8
x 10‘ (MI
Flgure 3. Effect TTACl on the rate of alkaline hydrolysis of the Nmethyl-4-cyanopyridinium ion. Curves are calculated (see text). [OH = 0.02 M: k f i o= 0.0985 s-’, B = 1.50, a = 0.20, cmc = 3.8 X 10 M. The solid line is calculated for KoHla= 0.14; the upper and lower dashed curves represent Kwla = 0.12 and KmIa = 0.16, respectively.
1853
kaline hydrolysis of MCP without affecting the P / A product ratio. Moreover, the addition of 0.10 M KBr wipes out the inhibition produced by CTAB (0.05 M) on the alkaline (pH 11.8) hydrolysis of MCP and the P/A product ratio is equal to that in 0.10 M KBr in the absence of CTAB.8J2 These data, taken as a whole, thus provide unequivocal evidence for the exclusive localization of the MCP ion in the intermicellar aqueous phase as required by the model. Although the alkaline hydrolysis of MCP occurs exclusively in the intermicellar space, the (small) ionic strength dependence of the rate (Figure 1) implies that application of eq 6 to the observed rate data for the alkaline hydrolysis of the MCP ion in the presence of micellar CTAB or TTACl must take into account the contribution of the detergent to the intermicellar ionic strength. As we have noted p r e v i ~ u s l y , ~the ~ Jprecise ~ manner in which to treat the effective ionic strength in the intermicellar aqueous phase is not entirely clear. Nevertheless, data for fluorescence quenching by excluded ions in the intermicellar aqueous phasel1#l3and for the incorporation of charged substrates into micelles11J2strongly suggest that the contribution of the micelles themselves to the “effective intermicellar ionic strength” is minimal. We have therefore chosen to represent the effective ionic strength by the following expression14 (10) k f ( m i c ) = CYcD + cmc + [OHTI In addition, the influence of the positive micelle on the OH- ion kinetic behavior has been explicitly taken into account by discriminating between “bound” and “free” OH- ions. Although eq 10 may be an imperfect solution to the complex problem at hand, we contend that it represents the best alternative presently available. Within the framework of the ion-exchange1 model a knowledge of the values of KOHlY, cmc, and a and of the ionic strength dependence of k2 (obtained in the absence of detergent) is sufficient to permit, a priori, prediction of kinetic behavior in the intermicellar aqueous phase. Thus, predicted curves for the dependence of k, on CT for the alkaline hydrolysis of MCP can be generated by using the following equations:
4
20 mM, they are clearly confirmatory of the value of KoH/Br determined from the data at 20 mM OHT (vide infra).
Discussion The key factor in our experimental approach is the complete exclusion of the MCP ion from the micellar phase. The initial recognition of this exclusion stemmed from the analysis of the quenching of the emission of the Ru(bpy),2+ion by MCP in micellar solutions of CTAB and N-hexadecylpyridinium chloride.ll The same conclusion can be derived independently from a comparison of the alkaline hydrolysis of the MCP ion with that of the corresponding long chain derivatives. The rates of alkaline hydrolysis of both DOCP and HexCP are accelerated by order of magnitude and the P/A ratios increase markedly in the presence of micellar CTAB.* The addition of KBr produces both an increase in the substrate binding constantl1J2 and a decrease in the micellar rate acceleration in the CTAB-modified alkaline hydrolysis of DOCP and HexCP. In sharp constrast, micellar CTAB and TTACl do not accelerate, and in fact inhibit (Figures 2 and 3), the al-
(12) [OH,] = [OHTI - [OHbl + 4(1 - KOH/Y)X [OHb] = {-A1 i[OHT]KOH/Y(~ - ~)c01~’~)/[2(1 - KoH/Y)](13) A i = ~ C Dcmc KoH/Y[OHT] (1 - a ) C & o ~ j y (14)
+
where k,’ was calculated from eq 7 and 8 with p as defined by eq 10. The value of a for CTAB has been shown to be 0.2 f 0.05 under a variety of experimental conditions.15 We have determined both the cmc and the ionic strength dependence of ki. Thus, the only other parameter necessary for our calculations is KOHpr. An initial estimate of KOHp, 0.1 was obtained from the high detergent concentration limit (eq 5) a t 1 mM OHT. Figure 2 shows the calculated curves for three values of KOHpr at 1 and 20 mM OHT. The observed rate data for 20 mM OHT are quite adequately simulated by our model with a value of KoH/Br = 0.08 f 0.02, CY = 0.2 f 0.05, and B = 1.5 f 0.5 (eq 8). Figure 2 also shows predicted curves for 1 mM OHT which, within the limitation of the kinetic data at this OHT (vide supra), confirm the magnitude of KOH/Br.
1854
The Journal of Physical Chemistry, Vol. 83, No. 14, 7979
Figure 3 shows the agreement between the model and the experimental data for a different detergent (TTAC1). Assuming an LY = 0.2 f 0.05, one finds that the experimental rate data are adequately represented by a KoHjCl of 0.14 f 0.02. The validity of any model must be examined in the light of its ability to accommodate the experimental behavior of the system under disparate conditions. The simulated curves adequately reproduce the rate data a t two quite different values of OHTover the entire concentration range of detergent studied. Moreover, the failure of previous methods to quantify the binding of OH- to micellar CTAB has led several authors to suggest that OH- binds inefficiently to the m i ~ e l l e . ~Our values for KOHIYindeed confirm that OH- competes inefficiently with both C1- and Br- for sites a t the micelle surface. These considerations provide strong support for the adequacy of the assumptions employed in the treatment of the rate data and for the ion-exchange model in micellar solutions.'
E. J. R. Sudholter and J. B. F. N. Engberts
Pesquisa do Estado de SBo Paulo (FAPESP 77/0285 to F.H.Q. and 76/0401 to H.C.) and the CNPq (7186/75 to H.C.).
References and Notes For paper 1 in this series, see F. H. Quina and H. Chaimovich, preceding paper in this issue. E. J. Fendler and J. H. Fendler, "Catalysis In Micellar and Macromolecular Systems", Academic Press, New York, 1975. In this regard, see ref 4 and references clted therein. C. A. Bunton, K. Ohmenzetter, and L. Sepilveda, J. Phys. Chem., 81, 2000 (1977). L. R. Romstead and E. H. Cordes, J . Am. Chem. SOC.,90, 4404 (1968). 0. Schales and S. S. Schales, J. Biol. Chem., 140, 879 (1941). E. M. Kosower and J. W. Patton, Tetrahedron, 22, 2081 (1966). M. Politi, I. M. Cuccovia, H. Chalmovich, M. L. C. de Almeida, J. B. S. Bonilha, and F. H. Quina, Tetrahedron Lett., 115 (1978). S. Spurlin, W. Hinze, and D. W. Amstrong. Anal. Lett., 10,997 (1977). B. Permutter-Hayman, Prog. React. Klnet., 6, 239 (1972). F. H. Quina, Tese de Livre DocBncia, Universidade de SHo Paulo, 1977. J. M. Politi, J. B. S. Bonilha, H. Chaimovlch, and F. H. Quina, unpublished results. F. H. Quina and V. G. Toscano, J. Phys. Chem., 81, 1750 (1977). Since we have assumed' that cy remains constant
Acknowledgment. F.H.Q. thanks the Conselho Nacional de Desenvolvimento Cientifico e Tecnolbgico (CNPq) and the Financiadora de Estudos e Projetos (FINEP) for fellowship support. M.J.P. is an undergraduate fellow and J.B.S.B. a graduate fellow of the CNPq. This work was supported by grants from the FundaGBo de Amparo 5
!&(rnIc)
= aCD
+ cmc + [OH,] +
Yd
where Yd represents the concentrationof Y which has been displaced upon binding of OH- to the micelle. Thus Yd = [OH,] and [OH,] Yd =! [OH,]. L. R. Romsted, Thesis, Indiana University, Bloomington, Ind., 1975.
+
Salt Effects on the Critical Micellar Concentration, Iodide Counterion Binding, and Surface Micropolarity of I-Methyl-4-dodecylpyridinium Iodide Micelles Ernst J. R. Sudholter and Jan B. F. N. Engberts" Department of Organic Chemistry, University of Groningen, Nijenborgh, 9747 AG Groningen, The Netherlands (Recelved January 22, 1979)
Electrolyte effects on the crnc and iodide counterion binding of micelles of 1-methyl-4-dodecylpyridinium iodide (1) have been investigated by conductance measurements and by UV spectroscopy. The intramolecular charge-transfer (CT) absorption band of the ionic head group of 1 was successfully employed as an intrinsic microscopic medium polarity reporter for the innermost part of the electrical double layer. This micropolarity can be expressed in terms of Kosower's 2 values which demonstrate the reduced polarity near the micellar surface. The applicability of Mukerjee's band-match method reveals that the Stern layer is quite homogeneous. The presence of electrolytes only modestly affects the micropolarity in the Stern layer. Sodium salts decrease the cmc in the order C1- < Br- < NO3- < I- < OTs- (which parallels the lyotropic series for the inorganic anions) and the effect on the cmc follows the Shinoda equation. The above order also applies for the ability of the anions to reduce the iodide counterion binding (B)as shown by the decrease in the molar extinction coefficient of the intramolecular CT band. The association constant for binding of iodide ions to the long-chain pyridinium cations within the micelles was also derived from the optical absorption data. The deviant salt effect of sodium p-toluenesulfonate is briefly discussed.
Introduction A large variety of reactions catalyzed by micelle-forming surfactants have served as valuable model processes for the study of microenvironmental factors which affect the high efficiency of chemical transformations in the biological realm. In this context, the large electrical potential in the Stern layer of micelles composed of ionic detergent molecules as well as medium effects characteristic of the micellar pseudophase have been recognized as integral parts of the catalytic advantages offered to micelle-bound substrates.14 0022-3654/79/2083-1854$01 .OO/O
For a proper understanding of micellar catalysis, knowledge of fundamental micellar properties such as size, shape, stability, counterion binding, and micropolarity in the Stern layer and micellar core is indispensible. In this paper we report a study of the aggregation process of 1-methyl-4-dodecylpyidinium iodide (1) a t 25 OC in water C H 3 ( C H 2)11 G ' - C H a
I-
1
and in several aqueous electrolyte solutions (NaC1, NaBr, 0 1979 American Chemical
Society
