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Langmuir 2007, 23, 436-442
Premicellar Accelerated Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Ion and Its 5-Tetradecyloxy Derivative Lucia Brinchi,† Pietro Di Profio,† Raimondo Germani,† Laura Goracci,† Gianfranco Savelli,*,† Nicholas D. Gillitt,‡ and Clifford A. Bunton‡ Centro di Eccellenza Materiali InnoVatiVi Nanostrutturati (CEMIN), Dipartimento di Chimica, UniVersita` di Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106-9510 ReceiVed June 23, 2006. In Final Form: September 5, 2006 Didodecyldialkylammonium chloride and bromide (alkyl ) Me, Et, n-Pr, n-Bu) accelerate the spontaneous decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion, 1,H, and its 5-tetradecyloxy derivative, 1,OTD. With most of these surfactants, first-order rate constants, kobs, go through maxima in very dilute surfactant and then decrease and go through minima as association colloids form. These phenomena are not explicable in terms of substrate-induced micellization. However, kobs increases in the N-alkyl sequence Me < Et < n-Pr < n-Bu, as is typical of decarboxylations in association colloids of single-chain surfactants. Reaction in premicelles is accelerated by an initial increase in 1,H. The factors that control relative rates of spontaneous reactions in premicelles and in the association colloids, in particular, depletion of water at the reaction center and association of substrate and quaternary ammonium centers, are discussed with respect to the roles of substrate and surfactant hydrophobicities.
Introduction Effects of association colloids, such as micelles and vesicles, on rates and equilibria of reactions in water are typically analyzed on the assumption that micelles of single-chain surfactants, for example, provide a reaction region distinct from water.1,2 Provided that reactant transfer between water and micelles is rapid, rate data can be treated quantitatively in terms of local reactant concentrations and rate constants in each region.1-3 This pseudophase model fits a great deal of data and is descriptively useful, but alternative models are also applicable, for example, some interionic reactions can be treated on the assumption that ionic micelles behave like macroions in their electrolyte effects on rates and equilibria in water.4 Micellar rate effects can also be interpreted in terms of transition state theory, and either formalism fits extensive data.5 Quantitative pseudophase treatments of bimolecular, nonsolvolytic reactions involve definition of local concentrations of reagents in the micellar pseudophase, which is generally regarded as the interfacial region at the micellar surface.1-3,6-8 For spontaneous reactions in single-chain surfactants, only partitioning * To whom correspondence should be addressed. E-mail: savelli@ unipg.it. Tel: +39-075-585-5538. Fax: +39-075-585-5538. † Universita ` di Perugia. ‡ University of California, Santa Barbara. (1) (a) Romsted, L. S. In Surfactants in Solution; Mittal, L. K., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (b) Bunton, C. A.; Savelli, G. AdV. Phys. Org. Chem. 1986, 22, 2139. (c) Romsted, L. S.; Bunton, C. A.; Yao, J. Curr. Opin. Colloid Interface Sci. 1997, 2, 622. (d) Savelli, G.; Germani, R.; Brinchi, L. In Reactions and Synthesis in Surfactant Systems, Surfactant Science Series/100; Texter, J., Ed.; M. Dekker: New York, 2001; Chapter 8, p 175. (2) Chaimovich, H.; Cuccovia, I. M. Prog. Colloid Polym. Sci. 1997, 103, 667. (3) (a) Rodenas, E.; Vera, S. J. Phys. Chem. 1985, 89, 513. (b) Amado, S.; Garcia-Rios, L.; Leis, J. R.; Rios, A. Langmuir 1997, 13, 687. (4) Sanchez, F.; Moya, M.; Rodriguez, A.; Jimenez, R.; Gomez-Herrera, C.; Yanes, C.; Lopez-Correjo, P. Langmuir 1997, 13, 3084. (5) (a) Hall, D. G. J. Phys. Chem. 1987, 91, 4287. (b) Bunton, C. A.; Yatsimirsky, A. K. Langmuir 2000, 16, 5921. (6) (a) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S. J. Am. Chem. Soc. 1993, 115, 8351. (b) Yao, J.; Romsted, L. S. Langmuir 2000, 16, 8771. (7) Menger, F. M.; Portnoy, C. E. J. Am. Chem. Soc. 1967, 89, 4698. (8) Tascioglu, S. Tetrahedron 1996, 52, 11113.
of the substrate between water and micelles has to be considered; however, partitioning depends on the amounts of free, that is, monomeric, and micellized surfactant because at the simplest level of approximation it is assumed that monomeric surfactant does not bind reactants. The concentration of monomeric surfactant is taken as the critical micelle concentration, cmc, in the reaction conditions, and for some reactions rate constants are independent of surfactant at concentrations below the cmc and increase, or decrease, as the substrate is transferred from water into micelles.1-4,7,8 However, there are often significant rate effects at surfactant < cmc in water, especially for reactions of hydrophobic substrates or polyvalent metal ions.1,9 Therefore, solutes can induce micellization, but they may also bind to monomeric surfactant or to small clusters, such as, premicelles, and in fitting kinetic data, the cmc is often treated as a disposable parameter without explicit consideration of possible reaction in premicelles. When rate constants in dilute surfactant vary monotonically with increasing surfactant, one cannot distinguish kinetically between reactant-induced micellization and reaction involving premicelles or monomeric surfactant because, for spontaneous reactions, rate constants change as substrate binds to the micelles (or premicelles) and become constant with complete micellar binding. The situation is different for nonspontaneous bimolecular reactions where transfer of two reactants has to be considered, and for micellar-accelerated reactions, rate constants typically go through maxima with increasing surfactant1 which depend on distributions of both reactants between water and the association colloids. There are extrema in plots of rate constants against surfactant for a number of spontaneous and bimolecular reactions in dilute surfactant below, or close to the cmc, which cannot be explained in terms of reactant-induced micellization and subsequent reactant binding to micelles.10-14 (9) Drennan, C. E.; Hughes, R. J.; Reinsborough, V. C.; Soriyan, O. O. Can. J. Chem. 1998, 76, 152. (10) Brinchi, L.; Di Profio, P.; Germani, R.; Giacomini, V.; Savelli, G.; Bunton, C. A. Langmuir 2000, 16, 222. (11) Brinchi, L.; Di Profio, P.; Germani, R.; Tugliani, M.; Savelli, G.; Bunton, C. A. Langmuir 2000, 16, 10101.
10.1021/la061807t CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006
Decarboxylation of 1,H and 1,OTD
When first-order rate constants for spontaneous reactions go through maxima with increasing surfactant, or there is more than one extrema in plots of rate constants of bimolecular reactions,12 the kinetics cannot be explained in terms of reactant-promoted micellization. These rare extrema must be due to kinetically productive binding of reactant to monomeric or premicellar surfactant. Considerable evidence for premicellization or interactions with monomeric surfactant comes from evidence on thermal or photochemical reactions10-15 or observation of regioselectivity,16 which cannot be ascribed to reactant-induced micellization, and it appears that relatively hydrophobic molecules or ions can interact with monomeric surfactant or small clusters of it, as well as with micelles. To this extent, one might assume that all kinetic evidence for formation of premicelles can be ascribed to interactions with reactants. However, insofar as micellization can be fitted to a multiple equilibrium, mass action treatment,17 premicelles may exist in equilibrium with monomer and micelles, although it may not be easy to detect them without using probes which may perturb the system and promote premicellization, and physical, nonprobe evidence is consistent with the existence of premicelles in dilute surfactant, especially for gemini surfactants.18 Rate decarboxylations of nitrobenzisoxazole carboxylate ions and dianionic dephosphorylations are strongly accelerated by nonpolar organic solvents,19-21 consistent with effects of association colloids on these reactions and evidence on the relationship of ion specificity to hydration.22 There is extensive evidence on formation of vesicles of didodecyldialkylammonium and similar twin-chain amphiphiles, in moderately high concentration, induced by sonication or other methods.23,24 These observations may lead to the erroneous view that these, and similar, surfactants form only vesicles, but has little relevance to results in very dilute surfactant. Ralston et al. found that the equivalent conductance of didodecyldimethylammonium chloride (DDDACl) went through a maximum at ca. 10-4 M, which shows that monomers form small assemblies which do not associate with Cl-, and other twin-chain surfactants behave similarly.25 Subsequent conductance decreases are consistent with micellization at higher concentration. (12) (a) Cuenca, A. Langmuir 2000, 16, 72-75. (b) Bacaloglu, R.; Bunton, C. A. J. Colloid Interface Sci. 1992, 153, 140. (13) Germani, G.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1767. (14) Cerichelli, G.; Mancini, G.; Luchetti, L.; Savelli, G.; Bunton, C. A. Langmuir 1994, 10, 3982. (15) Niu, S.; Gopidas, K.; Turro, N. J.; Gabor, G. Langmuir 1992, 8, 1271. (16) Balakrishnan, V. K.; Han, X.; VanLoon, G. W.; Dust, J. M.; Toullec, J.; Buncel, E. Langmuir 2004, 20, 6586. (17) Elworthy, P. H.; Florence, A. T.; Macfarlane, C. B. Solubilization by Surface ActiVe Agents; Chapman and Hall: London, 1968; Chapter1. (18) (a) Zana, R. J. Colloid Interface Sci. 2002, 246, 182. (b) Zana, R. Langmuir 2002, 18, 7759. (c) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (19) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry; VCH: Weinheim, Germany, 1988. (20) Paul, D. S.; Kemp, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (21) (a) Kirby, A. J.; Vargoglis, A. G. J. Am. Chem. Soc. 1967, 89, 415. (b) Bunton, C. A.; Fendler, E. J.; Sepulveda, L.; Yang, K.-U. J. Am. Chem. Soc. 1968, 90, 5512. (c) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko`, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673. (22) Brady, J. E.; Evans, D. F.; Warr, G. G.; Grieser, F.; Ninham, B. W. J. Phys. Chem. 1986, 90, 1853. (23) (a) Kunitake, T.; Okahata, Y.; Tamaki, K.; Kumamuro, K.; Yakayanagi, M. Chem. Lett. 1977, 387. (b) Carmona-Ribeiro, A. M. Chem. Soc. ReV. 1992, 21, 209. (24) Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.; Hirakawa S. J. Am. Chem. Soc. 1980, 102, 7877. (25) Ralston, A. W.; Eggenburger, D. H.; DuBrow, P. L. J. Am. Chem. Soc. 1948, 70, 977.
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This evidence for the formation of premicelles is supported by observation of line broadening of 1H NMR signals of DDDACl. The N-methyl and ω-methyl signals are sharp at 2.5 × 10-5 M with the expected triplet ω-methyl signal. The signals of N-CH3 become broad, and that of the ω-methyl triplet is degraded at 10-4M, but they sharpen at ca. 2.5 × 10-4M, where micelles should be present, but all the 1H signals become broad and structureless at 12.5 × 10-3 M, where larger assemblies probably form.26 This evidence for premicellization does not depend on added solutes, and the conductance results show that the premicelles are small with no affinity for high-charge density anions.25 It is simplest to examine kinetic evidence on spontaneous reactions which are accelerated by cationic micelles, for example, decarboxylation,10 dephosphorylation,11 or cyclization.14 In the present work, we examined decarboxylations of 6-nitrobenzisoxazole-3-carboxylate ions, 1,H, and its tetradecyloxy derivative, 1,OTD. Electron donation by alkoxy groups inhibits decarboxylation,20 but in other respects, these substrates should react similarly, although the hydrophobic and amphiphilic nature of 1,OTD affects its interactions with chemically inert amphiphiles and its solubility in water. There was preliminary evidence for premicellar rate extrema in decarboxylation of 1,H in very dilute DDDACl.13 The compounds and reactions considered in this work are shown in Scheme 1: Scheme 1a
a
1,H, X ) H; 1,OMe, X ) OMe; 1,OTD, X ) n-C14H29O.
When the alkoxy substituent is hydrophobic, as in 1,OTD, firstorder rate constants increase sharply in dilute cationic singlechain surfactants, go through pronounced maxima, and then, with increasing surfactant, give values typical of reaction in normal cationic micelles.10 The surfactants are cetyltrialkylammonium bromide, n-C16H33NR3 Br; R ) Me, Et, n-Pr, n-Bu, CTABr, CTEABr, CTPABr, CTBABr, respectively. Rate constants increased with headgroup bulk which should reduce the amount of water in the interfacial reaction regions of micelles and premicelles. There is no indication of significant rate maxima for reactions of the parent substrates, 1,H and 1,OMe, in solutions of singlechain cationic surfactants,10 although rate maxima were observed in the decarboxylation of 1,H in very dilute didodecyldimethylammonium chloride (DDDACl)13 and also in a cyclization reaction in solutions of DDDACl, but not in single-chain surfactants.14 There is no indication of rate extrema in bimolecular reactions of OH- in dilute DDDACl,27a although they are seen in other systems.12 These observations, which are consistent with physical evidence, indicate that moderately hydrophobic anionic substrates can interact with preexisting premicelles and generate (26) Gillitt, N. D.; Savelli, G.; Bunton, C. A. Langmuir, in press. (27) (a) Cipiciani, A.; Germani, R.; Savelli, G.; Bunton, C. A.; Mhala, M.; Moffatt, J. R. J. Chem. Soc., Perkin Trans. 2 1987, 541. (b) Cipiciani, A.; Germani, R.; Savelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1987, 553.
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Brinchi et al.
maxima in rate-surfactant profiles for some spontaneous reactions whose rates are sensitive to properties of the reaction medium. In the earlier work with single-chain surfactants, the benzisoxazole carboxylic acids were deprotonated by either Et3N or 0.01 M NaOH.10 Premicellar rate maxima were observed with 1,OTD in both conditions, but they depended slightly upon the base, and in this work, we used only 0.01 M NaOH. In the present work, we examine decarboxylation of 1,H and 1,OTD in a series of twin-chain cationic surfactants, didodecyldialkylammonium halides (Chart 1). Chart 1
Figure 1. Plots of kobs for reaction of 1,H in DDDACl (9), DDPACl (b), and DDBACl (2). Lines are computer generated to guide the eye. In water, kobs ) 3 × 10-6 s-1.
Decarboxylation of 1,H is a very useful reaction for examining local polarity and water content in many environments, which may give rate enhancements over reaction in water by orders of magnitude.20 Some of the alkoxy benzisoxazole carboxylate ions are amphiphilic and should readily associate with association colloids.10,12,24 There is evidence on structures of assemblies of very dilute didodecylammonium amphiphiles,25,26 and as noted, sonication of suspensions of twin-chain cationic surfactants in higher concentration generates metastable vesicles.23,24 However, overall effects of the very dilute didodecyldialkylammonium surfactants on rates of bimolecular reactions with OH-27a and on acid dissociation constants27b are qualitatively similar to those of single-chain quaternary ammonium halides and can be treated by using the usual pseudophase equations for reactions in micelles with no indication of vesicle formation in dilute nonsonicated solutions. The 1H NMR spectral evidence on DDDACl with concentrations >6 × 10-3 M shows that formation of large assemblies is outside the range of our kinetic work.26
Results Decarboxylation of 1,H. Most kinetic work was carried out with 10-4 M 1,H. The first-order rate constants, kobs, for reaction in the solutions of chloride surfactants are in Figure 1 and Table 1, and those in the bromide surfactants are in Figure 2 and Table 1. Except for reaction in DDDABr (Figure 2), values of kobs increase very sharply in dilute surfactant, go through maxima, and then through shallow minima before tending to plateau values, kplat, characteristic of reaction of substrate fully bound to an association colloid. In the following discussion, we use the term “micelle” loosely to describe assemblies present in surfactant at concentrations greater than approximately millimolar. The NMR evidence on DDDACl is consistent with the existence of normal micelles between approximately 2 × 10-4 and 6 × 10-3 M19 and formation of larger assemblies at higher concentration.26 The rate constants, kmax, in very dilute surfactant increase with increasing bulk of the N-alkyl group and, except for reaction in DDDABr, are slightly higher in the bromide than in the corresponding chloride surfactant. We cannot define values of kmax precisely because we observe the maxima over a limited range of surfactant, but approximate values are shown in Figures 1 and 2 and Table 1, together with values of surfactant at the apparent rate maxima. Values of kplat are also shown in Figures 1 and 2 and Table 1, but in some conditions, solubilities are such
Figure 2. Plots of kobs for reaction of 1,H in DDDABr (9), DDPABr (b), and DDBABr (2). Lines are computer generated to guide the eye. Table 1. Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Ion (1,H) in DDEACl and DDEABra 103 surfactant, M
DDEACl
DDEABr
0.1 0.2 0.3 0.4 0.5 0.8 1.0 2.0 3.0 4.0 6.0 10 15 20
3.68 8.32 14.7 15.0 14.0 9.38 9.72 10.3
5.66 26.2 22.5 15.5 16.2 18.9
11.2 11.4 11.9 12.2 12.9
a Values of 104 k , s-1, at 25.0 °C, with 1 × 10-4 M substrate, and obs 0.01 M NaOH; in the absence of surfactant, kobs ) 3 × 10-6 s-1.
that we could not reach surfactant which would give complete substrate binding and constant kobs, and listed values of kplat are then too low (Experimental Section). However, these rate constants in higher surfactant and dilute 1,H are similar to those measured earlier for the reaction of 1,H in micelles of singlechain cationic surfactants.10
Decarboxylation of 1,H and 1,OTD
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Table 2. Effect of Substrate Concentration on Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Ion (1,H) in DDBABra
reactivity of 1,OMe in water, although there may be steric effects of the C14H29O group.
104 1,H 104 DDBABr,
M
0.5 0.75 1.0 2.0 3.0 4.0 5.0 6.0 6.5 8.0 10.0
0.5
1.0
1.5
2.0
2.5
111 (1.0) 178 (1.5) 444 (1.0) 200 (0.6) 174 (4.0) 312 (2.0) 587 (1.3) 126 (6.0) 238 (3.0) 391 (2.0) 85.1 (8.0) 213 (4.0) 249 (2.6) 197 (5.0) 189 (6.0) 82.8 (13) 267 (4.3) 86.3 (16) 149 (8.0) 132 (10)
152 (0.5) 410 (1.0) 354 (0.8) 614 (1.5) 582 (1.2) 487 (1.6) 276 (3.3) 283 (4.0)
a Values of 104 k , s-1, at 25.0 °C, and 0.01 M NaOH; values in obs parentheses are [DDBABr]/[1,H]; in the absence of surfactant, kobs ) 3 × 10-6 s-1.
Table 3. Decarboxylation of 5-Tetradecyloxy-6-nitrobenzisoxazole-3-carboxylate Ion (1,OTD) in DDPACl, DDBACl, DDPABr, and DDBABra 103 surfactant, M
DDPACl
DDBACl
DDPABr
DDBABr
0.2 0.3 0.4 0.5 0.6 0.8 0.9 1.0 1.5 2.0 4.0 6.0 10
2.18 2.81 1.84 1.55 1.33 1.03
2.37 2.52 2.58 2.31 1.71 1.49
2.09 2.23 2.09 1.85 1.61
2.50 3.17 3.23 2.94 2.86 2.57 2.01
0.992
1.24
0.947 0.996 0.991 1.00
1.25 1.22 1.30 1.34
1.46 1.14 1.13
a Values of 104 k , s-1, at 25.0 °C, with 1 × 10-4 M substrate, and obs 0.01 M NaOH; in the absence of surfactant, kobs ) 2.1 × 10-7 s-1 for reaction of the methoxy derivative.
Variation of 1,H. We examined decarboxylation in dilute DDBABr with a range of [1,H] (Table 2). Positions of the initial rate maxima move to higher [DDBABr] with increasing substrate, and values of kmax initially increase in going from 0.5 × 10-4 to 1.5 × 10-4 M 1,H and are then insensitive to further increases in concentration up to kmax at [DDBABr]/[1,H] ≈ 1.5 (Table 2). This behavior is not that characteristic of reactions in normal micelles or other association colloids where, for reactions of fully bound substrate, first-order rate constants are unaffected by modest increases in substrate concentration.1 Decarboxylation of 1,OTD. We examined decarboxylation of 1,OTD in the “big-head” surfactants DDPAX and DDBAX, and the first-order rate constants, kobs, are in Table 3. The original observation of premicellar rate maxima in solutions of singlechain surfactants was on the decarboxylation of 1,OTD,10 and qualitatively effects of DDBACl and DDBABr are similar to those seen earlier. In both sets of conditions, the rate maxima are with very dilute surfactant in modest excess over 1,OTD. Values of kmax and kplat are higher within factors of approximately 2 in solutions of the twin-chain than in the single-chain surfactants examined earlier.10 Comparisons of reactivities in water and surfactant solutions cannot be made with 1,OTD, which is almost completely water insoluble but is solubilized by dilute surfactant.10 The methoxy and tetradecyloxy derivatives, 1,OMe and 1,OTD, should have similar reactivities in water in terms of electronic effects of the alkoxy groups,20 and rate comparisons are then based on the
Discussion Observation of a rate maximum for a spontaneous reaction in very dilute surfactant and a constant value of the rate constant in more concentrated surfactant cannot be explained in the simplistic terms of the well established reactant-induced micellization which would give a monotonic increase in the rate constant, up to the value for a fully micellar-bound substrate.10-14 This generalization is independent of questions regarding structures of the association colloids. In some earlier demonstrations of rate maxima for spontaneous reactions in dilute surfactant, the substrates were amphiphilic benzisoxazole carboxylate or phosphate ions, and it was understandable that they interacted with monomeric amphiphilic quaternary ammonium ions or small clusters of them.10,11 For example, alkyl tri-n-octylammonium ions, which do not form micelles, interact with relatively hydrophobic molecules and ions and accelerate some spontaneous and bimolecular reactions.1d,24,28-30 In the conditions where we see the initial rate maxima (Figures 1 and 2), there may be a mixture of premicellar assemblies of surfactant and substrate of various compositions, but observation of rate maxima requires that reaction in the premicelles be faster than that in fully formed assemblies; therefore with increasing surfactant, primitive premicellar assemblies form and rates increase. Then as the substrate becomes micellar-bound, rate constants go through shallow minima and tend to reach values characteristic of decarboxylations in larger assemblies, such as micelles. These observations cannot be explained without invoking the existence of two discrete reaction media with different composition and involving both substrate and surfactant. The only twin-tailed surfactant that does not give an initial rate maximum in the decarboxylation of 1,H is DDDABr (Figure 1), and we suppose that 1,H and Br- promote formation of an association colloid, with the monotonic increase of the rate constant.1-3 Incorporation of a substrate in a cationic premicelle, micelle, or other association colloid accelerates decarboxylation and dephosphorylation by providing reaction environments of low polarity and water content and possible interaction of the quaternary ammonium ion with the developing negative charge in the aryloxide moiety, for example, in formation of the transition state.10,11 Nonmicellizing hydrophobic alkyl trioctylammonium ions which accelerate decarboxylation and bimolecular nucleophilic reactions and a variety of other reactions of moderately hydrophobic substrates generate active ion assemblies24,28-30 similar to those formed by interaction of 1,H or 1,OTD with very dilute cationic surfactants. Decarboxylation of 1,H in aqueous solutions of alkyl tri-noctylammonium salts is faster than that in solutions of micellized single-chain cationic surfactants,24,30 although limiting values of some rate constants cannot be estimated because of the low solubility of the amphiphiles. However, for fully bound 1,H in assemblies of ethyl tri-n-octylammonium mesylate, the limiting (28) Lang, J. J. Phys. Chem. 1982, 86, 992. (29) (a) Vitagliano, V. In Aggregation Processes in Solution; Wyn-Jones, E., Gormally, J., Eds.; Elsevier: New York, 1983. (b) Reeves, R. L.; Harkawy, S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum: New York, 1977; Vol. 2. (c) Buwalda, R. T.; Janker, J. M.; Engberts, J. B. F. N. Langmuir 1999, 15, 1083 and references therein. (30) (a) Biresaw, G.; Bunton, C. A.; Quan, C.; Yang, Z.-Y. J. Am. Chem. Soc. 1984, 106, 7178. (b) Biresaw, G.; Bunton, C. A. J. Phys. Chem. 1986, 90, 5854. (c) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262.
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value of kobs is 0.02 s-1 at 25 °C29b and is higher than that of 0.0013 s-1 for the premicellar reaction in DDDACl and similar to those of 0.019 and 0.044 s-1 in DDBACl and DDBABr, respectively (Tables 1 and 2 and Figure 1). These comparisons show that values of kobs at the rate extrema in very dilute surfactant are as expected for reactions in premicelles or other small assemblies of quaternary ammonium ions. The factors that control reactivity in premicelles are those that apply to reactions in hydrophobic ammonium ions that cannot form micelles or similar association colloids. The 1H NMR spectral evidence is consistent with the assumption that formation of premicelles requires tight association with reduction of hydrocarbon-water contact26 as compared with that in the interfacial reaction region of a normal ionic micelle.1,22 Water is not excluded from this micellar region, and tight association of 1,H and a cationic premicelle could provide a reaction environment which is “drier” than a micellar water interface. Positions of the initial rate maxima in the decarboxylations (Figures 1 and 2) generally correspond to surfactant in a slight excess over 1,H or 1,OTD, which would be consistent with formation of active ion pairs or 1:1 clusters of substrate and ammonium ion. However, this description of the active species is inadequate because the premicellar rate maxima are generally higher when the surfactant counterion is Br- rather than Cl- and the active ion clusters are therefore probably of variable composition, some apparently containing Br-. Salt effects on micellar-mediated decarboxylation do not follow the pattern typical of salt inhibition of bimolecular reactions, and there are specific anion-surfactant interactions in the reaction.30 Rate constants of reactions of fully micellar-bound substrates typically do not depend on their concentration, provided that it is sufficiently low as not to perturb micellar structures.1-3 The situation is more complicated for decarboxylation of 1,H where we observe premicellar rate maxima (Figures 1 and 2). For example, positions of maxima move to higher [DDBABr] with increasing [1,H], and [DDBABr]/[1,H] is greater than the value of 1, which would correspond to stoichiometric formation of 1:1 active species (Table 2); however, the value of kmax eventually becomes approximately constant. As noted, there are uncertainties in values of kmax and positions of the rate maxima, but not in their existence. These results indicate that structures of the active premicellar species depend on [1,H]. For reaction of 1,OTD, our assumption that the reactivity in water will be very similar to that of 1,OMe may be unreliable in being based wholly on electronic substituent effects.20 In the simplest pseudophase model of micellization, monomeric surfactant is assumed to be converted abruptly into micelles at the cmc, but the alternative mass-action model17 indicates that a series of premicelles should exist in rapid equilibria with monomeric and micellized surfactant. These equilibria would be perturbed by anionic or nonionic hydrophobic solutes which interact with premicelles and/or micelles, as observed with hydrophobic probes.18b Therefore formation and structures of kinetically active premicelles should depend on concentrations of substrate, surfactant, and probably inert solutes. Acceleration of many reactions of hydrophobic substrates or polyvalent ions by surfactant at concentrations below the cmc is well established, although monotonic changes in rate constants with increasing surfactant do not distinguish between a premicellar reaction and reactant-induced micellization,1,9,31 and this failure (31) Graiani, M.; Rodriguez, A.; Fernandez, G.; Moya, M. L. Langmuir 1997, 13, 4239.
Brinchi et al. Table 4. Summarizing Data for Decarboxylations of 1,H and 1,OTD in Twin-Chain Surfactantsa substrate 1,H
1,OTD
surfactant
104 kmax, s-1
kmax/kw
104 kplat, s-1
DDDACl DDEACl DDPACl DDBACl DDDABr DDEABr DDPABr DDBABr DDBABr DDPACl DDBACl DDPABr DDBABr
12.9 (0.6) 15.0 (0.4) 65.7 (0.3) 194 (0.3)
430 500 2200 6500
26.2 (0.3) 68.5 (0.2) 444 (0.1) 614 (0.3) 2.81 (0.3) 2.58 (0.4) 2.23 (0.3) 3.23 (0.4)
870 2300 15000 20000 94 86 74 108
>8 12.6 38.8 86.0 21.3 >16 >40
1.00 1.32 1.13
a Values in parentheses are 103 surfactant, M, at the initial rate maxima. Values of kmax/kw for reaction of 1,OTD are based on the value of kw of 1,OMe; see text and Table 3.
to observe rate maxima in very dilute surfactant does not exclude a role for premicelles in the presence or absence of reactant. In decarboxylation and dephosphorylation of hydrophobic substrates in dilute single-chain surfactants, there are rate maxima at surfactant below the cmc in water,10 but with DDDACl, they are at concentrations slightly above the reported conductrimetric cmc in water.28 In view of the physical evidence for the existence of premicelles,25,26 we cannot assume that conversion of monomer into micelles may not occur over a very limited concentration range, as with single-chain surfactants, and the meaning of a “cmc” is uncertain. The Role of Substrate Hydrophobicity. Rate maxima in decarboxylations in dilute single-chain cationic surfactants were observed only with the hydrophobic 1,OTD,10 and because they depend on hydrophobicities of both the substrate and the amphiphile, it is understandable that they are evident with this substrate in both single-10 and twin-chain surfactants (Table 3). An increase in headgroup hydrophobicity generally increases kmax, but a change of counterion from Cl- to Br- does not have simple effects (Table 3). This balance of effects indicates that Br- ions can affect reactivity by promoting formation of small premicellar clusters and decrease it by competing with anionic substrates. Anions, such as Br-, rather than Cl-, should promote micellization,32 and then reaction in premicelles may not be observed. Increased headgroup bulk should decrease hydration in the reaction region, but inorganic ions are only partially dehydrated in micellar interfacial regions with the well-characterized anion specificity,1,6,33 and the long hydrophobic group in 1,OTD may decrease hydration of the carboxylate residue in water and offset the electronic inhibition by the alkoxy group.10,20 There are differences in the extents to which reactivities of 1,H and 1,OTD are affected by amphiphiles. 1,H values of kmax increase significantly with increasing bulk of the surfactant headgroup (Table 4), but we did not see this effect for reaction of 1,OTD, probably because, if the long alkyl group reduces hydration of the carboxylate moiety, increases in surfactant headgroup bulk will have less effect on reactivity. The Role of Surfactant Structure. An increase in bulk of cationic headgroups of single- and twin-chain surfactants generally accelerates decarboxylation in both premicelles and (32) Mukerjee, P.; Mysels, K. Critical Micelle Concentrations in Aqueous Surfactant Systems; National Bureau of Standards: Washincton, DC, 1971. (33) (a) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990, 94, 5068. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (c) Morgan, J. D.; Napper, D. H.; Warr, G. G. J. Phys. Chem. 1995, 99, 9458.
Decarboxylation of 1,H and 1,OTD
Figure 3. Dependence of log k upon surfactant headgroup. Open and closed points are for reactions in premicelles and micelles, respectively. Upper plot reactions of 1,H in twin-chain surfactants (Table 1 and Figure 1), middle plot, reactions of 1,H in single-chain surfactants,10 lower plot, reactions of 1,OTD in single-chain surfactants.10
micelles.10 For decarboxylations in solutions of single-chain surfactants, plots of log kobs against the number of CH2 groups, that is, going from Me to n-Bu, are approximately linear and with similar slopes, and this linear free energy relationship applies to the reaction of 1,H (Figure 3). Decarboxylation of 1,H is strongly accelerated by catalytic antibodies engineered for this purpose.34 The maximum increase over reaction in water is by a factor of ca. 104, which is very similar to that given by premicelles of DDBACl and DDBABr (Table 4). It appears the exclusion of water from the reaction region and interaction with the quaternary ammonium ion with the developing negative charge in the heterocyclic region, rather than specific interactions, are the main factors in these rate enhancements. The headgroup structural effect is also evident in cyclization14 and spontaneous hydrolyses of dinitrophenyl phosphate dianions,11 even though in the latter reaction water is involved as a nucleophile, although not necessarily in the rate-limiting step.21 The situation is more complicated for bimolecular reactions involving basic or nucleophilic anions. Increasing bulk of a cationic headgroup decreases anion binding, which inhibits the overall reaction, but if allowance is made for the decrease in local anion concentration, the second-order rate constants in the interfacial region increase with increasing headgroup bulk.1d It is difficult to isolate all the factors that control reactivities in premicelles or similar small assemblies. A necessary, but not sufficient, condition is that association with the substrate, for example, ionic association, is important, as in reactions of anionic substrates accelerated by bulky nonmicellizing quaternary ammonium ions.24,28-30 This association for organic anions of a given charge depends on their hydrophobicity and their concentration, as for decarboxylation with varying 1,H (Table 2). There is a balance between affinities of the anionic substrate for premicelles and micelles because if the latter is very high the substrate will be extensively micellar-bound, even in dilute surfactant, as with 1,H in DDDABr and single-chain surfactants (34) (a) Tarasow, T. M.; Lewis, C.; Hilvert, D. J. Am. Chem. Soc. 1994, 116, 7959 and references cited. (b) Na, J.; Houk, K. N.; Hilvert, D. J. Am. Chem. Soc. 1996, 118, 6462. (c) Hotta, K.; Kikuchi, K.; Hilvert, D. HelV. Chim. Acta 2000, 83, 2183.
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(Table 2), but regardless of substrate distributions between premicelles and micelles, observation of initial rate maxima requires that the reaction be faster in the small premicelles than in larger assemblies.10 In many respects, reactants in the micellar interfacial region behave kinetically as if they are in a homogeneous mixed aqueous-organic solvent, and the pseudophase treatment of rates and equilibria in association colloids fits this simplifying assumption.1 Premicellar rate maxima cannot be explained solely in terms of simple ion association because counterions should bind more strongly to ionic micelles, with their high surface charge density, than to monomers or premicelles.1-4,6 Micelles have loose structures with local mobility of the surfactant depending on the interplay between packing of the hydrophobic groups, and for ionic surfactants, headgroup repulsions, and interactions with counterions,1-3,35 there is waterhydrocarbon contact at the interfacial reaction region,6,35,36 and the cited increasing reactivities with increasing headgroup bulk can be ascribed to decreases in the water content of this reaction region. Decarboxylation and dephosphorylation are very much faster in anhydrous media, such as DMSO,20,21 than in aqueous micelles with their only partially dehydrated interfaces and have polarities similar to those of mixed aqueous-organic solvents.36 For an anionic hydrophobic substrate interacting with a cationic premicelle, the interaction is similar to that in a tight contact ion pair with water excluded between the partners. In decarboxylation, dephosphorylation, and deacylation, reactants and the premicellar ammonium ion(s) can be in such close contact that water is squeezed out of the reaction region. The 1H NMR spectral line broadening in very dilute DDDACl is consistent with close association of the monomers and some exclusion of water from the cationic premicelles.26 This exclusion of solvent is analogous to that observed in intimate ion pairs. In the transition states for the spontaneous reactions discussed here, charge moves away from a hydrophilic residue, for example, ionic carboxylate of phosphate, into the organic region of the substrate and interacts with a quaternary ammonium ion.20,21 Local interactions of phosphorothioate esters with a cationic surfactant appear to be stronger in premicelles than in fully formed micelles,16 and the higher reactivities than those in micelles appear to be related to the higher water content in micellar interfaces than in contact ion pairs or small clusters.1d,24,29 In decarboxylations, there are population distributions between free, premicellar, and micellar-bound substrate,10 each with different reactivities, and the balance of affinities and reactivities controls premicellar rate maxima. Premicellar complexes of substrate and surfactant may exist in solutions of 1,H and dilute single-chain surfactants, but if the population of these complexes is low, there will be no kinetic distinction between premicelle formation and reactant-induced micellization. Formation of kinetically effective populations of premicelles in decarboxylation involves a balance between hydrophobicities of both substrate and surfactant, with rate maxima in single-chain surfactants only with 1,OTD,10 but with the more hydrophobic didodecyl surfactants, they are observed in reactions of both 1,H and 1,OTD. In all conditions, formation of micelles or other assemblies at higher surfactant effectively “dissolves” the (35) (a) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980. (b) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: London, 1985. (36) (a) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957. (b) Novaki, L. P.; El Seoud, O. A. Langmuir 2000, 16, 35. (c) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (d) Yao, J.; Romsted, L. S. Langmuir 2000, 16, 8771. (e) Geng, Y.; Romsted, L. S.; Froenher, S.; Zanette, D.; Magid, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2005, 21, 562.
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premicelles, and rate constants then relax to values characteristic of reaction in the association colloids. The dependence of rate constants upon concentrations of substrate and surfactant (Tables 1 and 2) indicates that it is too simplistic to explain the rate maxima wholly in terms of formation of 1:1 contact ion pairs, although they are well characterized in some systems.15,29 There is physical evidence for the existence of premicelles of DDDACl,25,26 but interactions with benzisoxazole carboxylate ions and other relatively hydrophobic ions should promote equilibrium formation of premicelles of variable composition in dilute surfactant.37 Kinetic evidence indicates that hydrophobic residues in either the substrate or the surfactant promote formation of premicelles, and there is extensive physical evidence for premicelle formation with cationic gemini surfactants with their two alkyl groups, although in some cases, it is induced by interaction with hydrophobic probe molecules.18b It is difficult to separate the roles of hydrophobic groups in the substrate and surfactant in promoting kinetically effective premicelles with larger rate enhancements than those in larger association colloids. Comparison of the reactivities of 1,H and 1,OTD is complicated by reliance on the assumption that the similar electronic effects of the alkoxy groups would lead to the (hypothetical) rate constant of the latter in water being similar to that of 1,OMe and lower than that of 1,H by a factor of ca. 15.10 However, if the long alkyl group decreases hydration, we underestimate the (hypothetical) reactivity of I,OTD in water. Close contact between the partners in a kinetically effective premicelle reduces hydration of the carboxylate moiety, and the long alkyl group in 1,OTD should affect conformations in these assemblies to different extents in single- and twin-chain surfactants. The geometry of premicelles with 1,H will be governed by the Coulombic interaction and that between the quaternary ammonium ion and the aromatic system. These interactions will be present in premicelles with 1,OTD, but here interactions of the long alkyl groups will be important. For the reaction of 1,OTD in single-chain premicelles and micelles, a change of the headgroup from Pr3N+ to Bu3N+ approximately doubles the rate constant10 but has less effect on reaction in the twin-chain system (Table 4). There is transfer of charge from a strongly hydrated carboxylate ion to an aryloxide moiety, and conformational changes in transfers from water to a cationic premicelle or association colloid may play different roles in reactions of 1,H and 1,OTD. Decarboxylation is strongly catalyzed by antibodies,34 and conditions for their efficacy in these and similar reactions are similar to those that apply to reactions in synthetic premicelles and association colloids, such as effective substrate binding, reduced hydration, and assistance to movement of charge in the transition state. As shown earlier, rate enhancements by antibodies are similar in magnitude to those with premicelles of synthetic amphiphiles (Table 4 and ref 34).
Conclusions Didodecyldialkylammonium chloride and bromide accelerate decarboxylations of benzisoxazole carboxylate ions by forming premicellar assemblies. Reactions in these conditions are faster (37) Kinetic evidence for the existence of premicelles in mixtures of surfactant and reactive solutes does not demonstrate their existence in the absence of added solutes, although for DDDACl, it is consistent with conductometric and NMR spectral evidence,25,26 and there is strong evidence for formation of gemini premicelles.25
Brinchi et al.
than those in micelles of single-chain surfactants, but reactivities in both micelles and premicelles increase with increasing bulk of the N-alkyl headgroups, which decreases the availability of water in the reaction region. However, the kinetic data do not allow separation of the role of association of the anionic substrates with the cationic amphiphiles from that of reactivity in the premicellar and larger assemblies, and there is a similar problem with rate enhancement by hydrophobic quaternary ammonium ions.24,29,30 The results indicate how very dilute amphiphiles can affect reactivities, especially of moderately hydrophobic substrates. This behavior of premicelles may be very common in aquatic environments which contain natural and synthetic amphiphiles in very low concentrations. Higher reactivity in premicelles than in micelles is counterintuitive, but these decarboxylations are sensitive to the water present in micellar interfacial regions but not in contact ion pairs or clusters. If there is a Coulombic or dispersive driving force for association of the anionic substrate with surfactant cations, or small clusters of them, reaction may be faster in premicelles than in micelles. Kinetic evidence for premicellar induced reactivity, as indicated by rate maxima in spontaneous reactions, does not depend on rationalization of rate differences or on the structure(s) of the association colloids present in more concentrated surfactant above that examined here. Experimental Section Materials. Substrate preparations have been described.10 The surfactants were prepared and purified by the following general procedure. Dodecyl halide (0.1 mol) and N,N-dialkylamine (0.1 mol), prepared by standard procedures, in dry CH3CN (200 mL) were heated at reflux for 48 h for the methyl derivatives and 120 h for the other derivatives. The resulting pale yellow solution was concentrated in a rotary evaporator to give a semisolid crude product which was treated with Et2O, and the resulting white solid was removed by filtration. It was washed with Et2O at 0 °C, crystallized twice from Et2O, and dried over P2O5 at 50 mTorr overnight. The 1H NMR spectra of the surfactants in CDCl at 200 MHz and referred 3 to TMS follow; they are independent of the counterion. DDDAX: 1H NMR δ 0.87 (t, 6H, 2CH ), 1.13-1.40 (m, 36H, 18CH ), 1.523 2 1.75 (m, 4H, 2CH2), 3.40 (s, 6H, 2CH3), 3.45-3.55 (m, 4H, 2CH2). DDEAX: 1H NMR δ 0.88 (t, 6H, 2CH3), 1.14-1.51 (m + t, 36H + 6H, 18CH2 + 2CH2), 1.55-1.82 (m, 4H, 2CH2), 3.20-3.38 (m, 4H, 2CH2), 3.45-3.56 (m, 4H, 2CH2). DDPAX: 1H NMR δ 0.87 (t, 6H, 2CH3), 1.05 (t, 6H, 2CH3), 1.10-1.38 (m, 36H, 18CH2), 1.53-1.80 (m, 8H, 4CH2), 3.25-3.50 (m, 8H, 4CH2). DDBAX: 1H NMR δ 0.88 (t, 6H, 2CH3), 1.04 (t, 6H, 2CH3), 1.12-1.41 (m, 40H, 20CH2), 1.51-1.85 (m, 8H, 4CH2), 3.28-3.51 (m, 8H, 4CH2). Kinetics. Reactions were followed spectrophotometrically in 0.01 M NaOH at 25.0 °C as described.10 In some conditions, concentrations were solubility limited, but solutions, which were made up without sonication, were stable over extended periods with no precipitation in the course of reaction. We avoided use of even low power sonication because it might induce formation of metastable vesicles, which is, however, unlikely in our dilute solutions.25,26 Integrated first-order rate plots were linear for up to 3-4 half-lives. In nearly all our reactions, the existence of rate maxima was demonstrated, although because of the sharp increase of kobs with increasing surfactant, the position was sometimes imprecise. The value of kplat corresponding to the reaction of fully micellar-bound substrate could not be determined in all conditions due to limited solubility.
Acknowledgment. Support of the Ministero dell’Istruzione, Universita` e Ricerca (MIUR), Rome, is gratefully acknowledged. LA061807T
