J. Phys. Chem. 1983, 87, 3584-3586
3584
40 34560 cm-1
\=43
Bars
F+25
Bars
F+15
Bars
I
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3
-V
5,49
OC
Flgure 4. Fluorescence excitation spectra of pure N-methylindole in a supersonic free jet at various N-methylindole pressures.
spectra display more transitions than for alcohol. These dioxane-complex transitions are separated by spacings comparable to those observed for phenol-dioxane complexes6 and are therefore assigned to the hydrogen bond bending vibration. When water is added, a new band of the indole-water complex appears in the lower wavelength region displaced by 135 cm-' from the 0-0 free indole band (Figure 3c). The similar displacement of all the complex bands from the indole origin for molecules as different as dioxane, ~~
alcohol, and water pleads in favor of a similar kind of bonding. Similar displacements were observed for the 0-H- - -0bond of phenol Complexes? Here we suggest that the observed shift reveals the hydrogen bonding of the NH group of indole. Indole can only be a hydrogen donor to dioxane, thus, all complexes involve indole as a donor. In contrast, for the same ta* transitions, when the fluorescent molecule is an hydrogen acceptor as is the case for isoquinolein,' the hydrogen-bonded complexes are characterized by a much smaller shift. There still lies the possibility of hydrogen bonding with the a electron cloud of the benzene ring. To ascertain NH hydrogen bond formation, we have substituted NH with a N-methyl group. ( b )NMI Complexes. The excitation spectrum (Figure 2b) of pure N-methylindole expanded in helium shows the same characteristic vibrations as indole. The origin, located at 34 560 cm-l, id displaced by 680 cm-l from that of pure indole. When the helium pressure is increased, we observe, as is shown in Figure 4, the (He),, (He)2,... complexes of N-methylindole attributed through their helium pressure dependence. Moreover in the presence of dioxane or alcohol, no additional band appears except for those of the helium complexes in the same concentration range as we used for indole (0,5 torr partial pressure). Hence polar systems are not easily condensed on the indole ring and the bands on the complexes observed for indole are characteristic of the N-H hydrogen bond complexes. We have found evidence for hydrogen bond formation in supersonic jet expansion of indole. The bands assigned to the complex are characterized by shifts of ca. 150 cm-' to the red of indole transitions. These shifts are similar to those deduced by Kadiri from UV absorption spectroscopy in solution.8 Thus solution spectroscopy can provide useful information on the hydrogen-bonded complexes of various molecules.
Acknowledgment. We thank the referee for indicating to us the possible presence of water complexes. Registry No. Indole, 120-72-9;N-methylindole, 603-76-9; ethanol, 64-17-5; dioxane, 123-91-1;water, 7732-18-5.
~
(6) Hauro Abe, Nachiko Mikami, and Mitsuo Ito, J.Phys. Chem., 83, 1768-71 (1982).
(7) P. M. Felker and A. H. Zewail, Chem. Phys. Lett., 94, 448 (1983). (8) A. M. Kadiri, These docteur d'dtat, Universite de Bordeaux, 1978.
Variation of Counterion Binding in Micelles of Cetyltrimethylammonium Hydroxide Hernan Chalmovlch,' Iolanda M. Cuccovla, Deparmento de Bloquimica, Instituto de Qulmica, UniversMade de Sa0 Paulo, Sa0 Paulo, Brazil
Clifford A. Bunton,' and John R. Moffatt Department of Chemistty, University of California, Santa Barbara, California 93 106 (Received: M y 16, 1983; I n Final form: June 28, 1983)
Reaction of N-methyl-4-cyanopyridiniumfluoroborate (1) with OH- in aqueous micelles of cetyltrimethylammonium hydroxide (CTAOH) occurs wholly in the aqueous pseudophase. Comparison of the rate constants in CTAOH and NaOH allows estimation of the concentrations of free and micellar bound OH-. The values of a,the fractional ionization of the micelles, decrease with increasing concentration of OH-, in agreement with evidence from reactions of substrates bound to micelles of CTAOH. The effects of normal, nonfunctional, micelles upon reaction rates and equilibria are generally treated on the
assumption that reactants are distributed between micelles and water, treating each as a pseudophase, and with no
0022-3654/83/2087-3584$0 1.50/0 0 1983 American Chemical
Society
The Journal of Physical Chemistry, Vol. 87, No. 19, 1983 3585
Letters
reaction across the micelle-water interface. This model fits the variations of rate constant with surfactant concentrations for bimolecular reactions of relatively hydrophobic reagents.’ It has been extended to reactions of hydrophilic ions with the additional assumptions that counterions are distributed between the two pseudophases according to an ion-exchange equation and that CY and p are independent of the nature and concentration of added
ICTAOH~. M
L
I
/
n
/
ion^.^-^ This treatment is satisfactory provided that the predominant counterion is not very hydrolphilic, or in high concentration. It appears to fail if the hydrophilic counis in high concentrati~n.~~~ In these terion, e.g., OH- or P, cases the reaction is faster than predicted, suggesting that the concentration of reactive counterion in the micellar pseudophase increases with increasing total concentrat i ~ nor, that ~ ~ there is reaction across the micelle-water
I
I
I
0 02
0 04
006 [CTAOH],
[NOOH],
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Following the first hypothesis it was shown that for reactions in CTAOH, for example, the apparent variation of P, eq 1
p = [OH-,] / [Dn]
(1)
followed the mass action eq 25b (2) K’OH= [OH-,]/[OH-,I([Dn] - [OH-MI) where Dn denotes micellized surfactant and subscripts W and M denote aqueous and micellar pseudophases, re-
spectively. The rate of reaction in the micellar pseudophase is assumed to be proportional to p, and this treatment fits variations of rate and equilibrium constants with [CTAOH] reasonably ~ e l l . ~However, ~J there are questions as to the physical significance of eq 2. For example, it is written as if the limits of p are 0 and 1, which is physically impossible? In addition, the apparent variation of p may represent an increase in the effective concentration of OH-, for example, at the micellar surface due to shrinkage of the Stern layer with increasing [OH-]. Alternatively one could assume that ions in the diffuse, Gouy-Chapman layer can react with micellar-bound substrate. The earlier experiments were on reactions at the micellar surface, where the rate should depend upon and we now describe results of experiments on a substrate which is completely excluded from the micelles. The rate of such a reaction should depend upon [OH-,], Le., upon CY = 1 - 6, and therefore be essentially unaffected by shrinkage of the Stern layer or reaction across the micelle-water interface.
I
I” I
I
01 013 014
008
M
Figure 1. Variation of k + for the overall reaction of N-methyl-4cyanopyridinium ion at 25.0 OC: in NaOH, 0; in CTAOH, 0.The insert gives results in dilute CTAOH. The lines for results in CTAOH are calculated, see text.
TABLE I:
Reaction in Mixtures of CTAOH
f
NaOHa
[ N a O H ] , [CTAOH], [ O H - w ] ,
M
0.005 0.005 0.05 0.05 0.05 0.05
M
M
k h , S-’
0.014b 0.005
0.0085
0.005 0.01 0.08
0.051 0.053 0.067
0.0178 (0.022) 0.119 0.114 (0.12) 0.134 (0.12) 0.161 (0.15)
a Rate constants in parentheses are calculated f r o m By interpola. [ O H - w ] , eq 2, a n d values of k $ i n NaOH. tion.
The substrate was the N-methyl-4-cyanopyridinium ion (1) which reacts with OH- giving pyridone (2) or amide (3).4.9a,bV10
FN
b+ Me
CN-
2 CO-NH, I
1
0 Me+
3 (1) (a) Cordes, E. H. f i r e Appl. Chem. 1978,50,617. (b) Martinek,
K.; Yatsimirski, A. K.; Levashov, A. V.; Berezin, I. V. In ‘Micellization, Solubilization and Microemulsions”; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 489. (c) Bunton, C. A. Catal. Reu. Sci. Eng. 1979, 20, 1. (2) The fractional ionization of a micelle is designated as a,and 8, the fraction of micellar head groups neutralized by counterions, is given by 1- a. (3) Romsted, L. S. in ref lb, p 509. (4) Chaimovich, H.; Aleixo, R. M. V.; Cuccovia, I. M.; Zanette, D.; Quina, F. H. In ‘Solution Behavior of Surfactants”; Mittal, K. L.; Fender, E. J., Ed.;Plenum Press: New York, 1982; Vol. 2, p 949. (5) (a) Bunton, C. A.; Romsted, L. S.; Savelli, G. J. Am. Chem. Soc. 1979,101,1253. (b) Bunton, C. A,; Savelli, G. J. Phys. Chem. 1981,85, 4118. Bunton, C. A.; Romsted, L. S. in ref 4, p 975. (6) Nome, F.; Rubiera, A. F.; Franco, C.; Ionescu, L. J . Phys. Chem. 1982,86, 1881. (7) Cipicimi, A.; Savelli, G.; Bunton, C. A. J.Phys. Chem. Accepted
for publication. (8) Equation 2 can be modified by arbitrarily imposing limits on 8, and the modified equation fits the rate data, but with a different value of
Kbw
The mechanisms of these reactions have been established,l0 and the ratio of pyridone to amide in aqueous solution increases with increasing pH, and when reaction occurs in a cationic m i ~ e l l e . ~Thus J ~ the products of reaction of 1 established that reaction occurs wholly in the aqueous pseudophase, i.e., 1 does not bind to cationic micelles? but that with more hydrophobic substrates there is considerable reaction in the micellar p s e u d o p h a ~ e . ~ ~ Therefore the rate constant for conversion of 1 into 2 + 3 gives a direct way of estimating [OH,]. The surfactant (9) (a) Chaimovich, H.; Politi, M. J.; Bonilha, J. B. S.; Quina, F. H. J . Phys. Chem. 1979,83, 1851. (b) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Baumgarten, E.; Martins-Franchetti, S. M.; Chaimovich, H. Ibid. 1980, 84, 361. (c) Politi, M.; Cuccovia, I. M.; Chaimovich, H.; Almeido, M. L. C.; Bonilha, J. B. S.; Quina, F. H. Tetrahedron Lett. 1978, 115. (10)Kosower, E. M.; Patton, J. W. Tetrahedron 1966, 22, 2081.
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The Journal of Physical Chemistry, Vol. 87, No. 19, 1983
was cetyltrimethylammonium hydroxide (CTAOH, nC16H,NMe30H).5b Comparison of the product spectra in CTAOH with those for reaction of 1 in NaOH and alkaline CTABr showed that substrate was not binding to the micelles. The first-order rate constants, k,, for reactions in surfactants and in NaOH in the absence of surfactant are in Figure 1and Table I. Comparison of k, in the presence and absence of surfactant gives an estimate of [OH-,], provided that we assume that NaOH and micellized surfactants have similar kinetic electrolyte effects," at a given [OH-,], cf. ref 9. The line drawn from the data in CTAOH (Figure 1)was calculated by taking values of [OH-,] from eq 2 with K'oH = 55 M-' 5b and cmc = M,12assuming that if [OH-,] = [NaOH] the value of k, will be the same in the two solutions, i.e., neglecting specific electrolyte effects. The rate constants in parentheses in Table I were calculated on the same basis, taking cmc = M with added 0.005 M NaOH and cmc = 0 in 0.05 M NaOH. (The calculations are insensitive to small changes in the value of the cmc.) The agreement between observed and predicted values of k, is reasonable, except at the highest [CTAOH], where our assumptions regarding medium effects are highly suspect.14 Addition of organic solvent to water typically speeds reactions of OH- and other anionic nucleophiles. The 0.13 M CTAOH solution contains ca. 5% surfactant and this amount of organic material could speed attack of OH- upon substrate in the aqueous pseudophases, regardless of any micellar effect. We see no simple way of eliminating the uncertainty caused by the medium effects, but our evidence is consistent with micelles of CTAOH having an unusually high cy, i.e., low /3, as indicated by conductivity measurements and by kinetic experiments on miceller bound substrate^.^^ The question remains as to the physical significance of this apparently high ionic dissociation of micelles of CTAOH. Micelle formation depends upon a balance of opposing forces, which for most surfactants results in a fairly narrow distribution of micellar sizes, at least for (11) The surfactant solution contains monomeric surfactant cation, free OH-, and micelles which are multivalent cations, and we assume that the salt effect of the cations is the same as that of an equivalent amount of Na+. (12) The concentration of monomeric surfactant is generally assumed to be given by the critical micelle concentration, cmc.13 The cmc of CTAOH does not appear to be well but it is ca. loT3M. The uncertainty in the estimated value of p is relatively unimportant, except for the most dilute surfactant. (13) Menger, F. M.; Portnoy, C. E. J.Am. Chem. SOC. 1967,89,4968. (14) Estimation of [OH-,] (or a) from the reacticity of 1 involves assumptions about kinetic electrolyte effects and estimation of [OH-,] (and 8) from reactivitites in the micellar pseudophase involves assumptions about binding of substrates to micelles.5b
Letters
dilute surfactant. But CTAOH may contain unusually large amounts of small high charge density aggregates which bind little OH-, but bind hydrophobic solutes, giving apparent high values of cy. If these aggregates grow on addition of OH-, @ will increase toward values more typical of ionic micelles, Le., be in the range 0.7-0.8. The concept of specific ion binding in a well-defined micellar Stern layer can be usefully applied to many s y ~ t e m sbut , ~ it~ may ~ ~ ~be inapplicable to the binding of hydrophilic ions such as OHor F-.6 In a micelle of CTAOH, for example, bound substrate may react with OH- which is close to, but has not penetrated, the micellar ~ u r f a c e . ~In * ~that ~ event second-order rate constants in the micelle, relative to those in water, should decrease with increasing substrate hydrophobicity, because the more hydrophobic the substrate the less time it will spend, on the average, in the water rich region adjacent to the micellar surface. A limited amount of evidence supports this h y p o t h e s i ~ . ~ ~ The physical evidence presently available does not distinguish between the alternative descriptions of a fully formed micelle with variable @, eq 2, or a distribution of micellar sizes of CTAOH, or of reaction involving counterions not localized in a Stern layer, cf. ref 5a and 6. But a hydrophilic cationic substrate such as 1 will be excluded from a micelle, and its immediate e n v i r o n ~so , ~that it is realistic to use its reactivity to estimate the amount of OHwhich is free in the aqueous pseudophase. The description of micellar binding based on eq 2 is undoubtedly an oversimplification, but it appears to be satisfactory for the limited range of conditions used in this work.
Experimental Section Materials. N-Methyl-4-cyanopyridiniumfluoroborate (1) was prepared by standard methods.1° Cetyltrimethylammonium hydroxide (CTAOH) was prepared in solution as except that Ba(OH)2was added to solid cetyltrimethylammonium sulfate, instead of to a solution of it. This modified procedure gave a solution of CTAOH which could more easily be freed of BaS04 by centrifugation. The preparation was carried out in the absence of C02 under N2. Kinetics. The faster reactions were followed at 25.0 OC on a Durrum stopped-flow spectrophotometer, and the slower reactions were followed in a Gilford spectrophotometer, at 260 nm.9 The first-order rate constants, k,, M. are in reciprocal seconds, and [substrate] = Acknowledgment. Support of this work by the National Science Foundation (Chemical Dynamics) and the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CN Pq), Brasilia, is gratefully acknowledged. We acknowledge valuable discussions with Dr. L. S. Romsted.
