9762
J. Phys. Chem. B 2007, 111, 9762-9769
Specific Anion Binding to Sulfobetaine Micelles and Kinetics of Nucleophilic Reactions Luisa Marte,† Rosane C. Beber,‡ M. Akhyar Farrukh,‡ Gustavo A. Micke,‡ Ana C. O. Costa,‡ Nicholas D. Gillitt,§ Clifford A. Bunton,§ Pietro Di Profio,† Gianfranco Savelli,† and Faruk Nome*,‡ CEMIN, Dipartimento di Chimica, UniVersita` di Perugia, 06100 Perugia, Italy, Departamento de Quı´mica, UniVersidade Federal de Santa Catarina, Floriano´ polis-SC 88040-900, Brazil, and Department of Chemistry and Biochemistry, UniVersity of California, Santa Barbara, California 93106-9510 ReceiVed: April 10, 2007; In Final Form: June 14, 2007
With fully micellar bound substrates reactions of OH- with benzoic anhydride, Bz2O, and of Br- with methyl naphthalene-2-sulfonate, MeONs, in micellized sulfobetaines are strongly inhibited by NaClO4 which displaces the nucleophilic anions from the micellar pseudophases. Micellar incorporations of ClO4- and Br- are estimated with an ion-selective electrode and by electrophoresis, and partitioning of Br- between water and micelles is related to changes in NMR spectral 79Br- line widths. Extents of inhibition by ClO4- of these nucleophilic reactions in the micellar pseudophase are related to quantitative displacement of the reactive anions from the micelles by ClO4-. The kinetic data are correlated with physical evidence on the strong interactions between sulfobetaines and ClO4-, which turn sulfobetaine micelles anionic and effectively provoke displacement of OH- and Br-.
1. Introduction CnHn+1N+R2-
Micelles of aqueous sulfobetaine surfactants, (CH2)mSO3-, have been studied extensively. The alkyl chain length, n, and that of the methylene tether group, m, can be varied, and the alkyl ammonium substituents, R, are usually methyl or short chains. We designate the surfactants as SBm-n for methyl groups and SBRm-n for larger alkyl groups, e.g., Et, Pr, and Bu for R ) ethyl, propyl, and butyl, respectively.1-4 Sulfobetaine micelles are formally neutral, but they bind anions, although much less strongly than cationic micelles. There is a charge gradient at the surface, and its contribution to anion binding has been treated theoretically.5 However, ion binding follows the Hofmeister series; for example, OH- is bound weakly, and Br- and ClO4- are more strongly bound, and this anion specificity has also been treated theoretically.5b,c,6 Interionic competition in ionic micelles has been examined quantitatively, in detail, but there is less evidence on competition in sulfobetaine than in cationic micelles, although anion orders are similar.5-7 Added NaClO4 effectively displaces reactive anions, e.g., OH-, Cl-, and Br- from sulfobetaine micelles, and the effect of added ClO4- in manipulating separation selectivity for polarizable anions was demonstrated by electrochromatography.5e,5f Physical evidence for anion binding to ionic micelles, and other association colloids, comes from independent methods such as conductance, use of ion-selective electrodes (ISEs), electrophoresis, and NMR spectroscopy. The results are consistent with kinetic data on counterion reactions, e.g., of nucleophilic anions in cationic micelles and reactive cations in anionic micelles.4,6-8 Dediazonation trapping has been used to estimate local concentrations of anions or molecules in colloidal * To whom correspondence should be addressed. E-mail: faruk@ qmc.ufsc.br. † Universita ` di Perugia. ‡ Universidade Federal de Santa Catarina. § University of California.
regions, e.g., micellar interfaces, but this method is limited to nucleophilic, weakly basic solutes.8 The competitive binding of counterions to ionic association colloids has been treated quantitatively by the pseudophase ion exchange, PIE, model which allows kinetic evidence to be related to physical evidence on ion binding4 in terms of exchange constants which reflect relative ion affinities to the micelle. This simple model, or modifications of it, assumes extensive or complete charge neutralization and is therefore not applicable to betaine micelles, although binding to them is anionspecific.5-7 The aim of the present work is development of a treatment allowing results on kinetic salt effects to be related quantitatively to physical evidence on anion transfer between water and sulfobetaine micelles. Anion distributions between water and sulfobetaine micelles, monitored with ISEs is highly specific, fits Langmuir isotherms, and is discussed elsewhere.7,9 Signal NMR line widths of quadrupolar nuclei increase markedly on association of an anion with cationic species, e.g., a hydrophobic ammonium ion or a cationic surfactant, and signals of Cl-, Br-, and ClO4- show this behavior on binding to cationic and betaine micelles, where solubility permits.6b,10 Chemical shifts also change, but less so than line widths. Changes in NMR line width in sulfobetaine micelles correlate qualitatively with ionic transfer between water and micelles and with rate constants for SN2 reactions of hydrophobic substrates, e.g., of Br- with methyl naphthalene2-sulfonate (MeONs), or reaction of benzoic anhydride (Bz2O) with OH-, which take place in the micellar pseudophase (Scheme 1).6,10 There is physical evidence that ClO4- binds much more strongly to sulfobetaine micelles than anions such as Br- or OH-, and our hypothesis is that it should quantitatively displace these anions from micellar pseudophases, although solubility precludes this experiment in cationic micelles. Interionic competition in ionic micelles is often treated by a variety of ion-exchange equations, similar to those applied to ion-exchange resins.4,5 The formalisms are applicable to ionic
10.1021/jp0727897 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/28/2007
Sulfobetaine Micelle Anion Binding and Reaction
J. Phys. Chem. B, Vol. 111, No. 33, 2007 9763
SCHEME 1
micelles whose surface headgroup charge is largely neutralized by counterions,4b but not, as noted, to zwitterionic micelles, which are formally nonionic, although there is a charge gradient from the hydrocarbon-like core to the surface.1,2,5 Kinetic salt effects upon reactions of nucleophilic anions with nonionic substrates in cationic micelles are due largely to interionic competition, and not to changes in micellar structure, but this generalization does not apply to some micellar-mediated reactions.4 In the present work we examined transfer of Br- between water and sulfobetaine micelles quantitatively by ISEs, capillary electrophoresis, or 79Br- NMR spectroscopy, and for the latter, we examined the relationship between the increasing line width of the 79Br- signal and rate constants of an SN2 reaction as a function of [SBPr3-14] (Scheme 1) and the effects of NaClO4. The reaction of OH- with Bz2O, in solutions of sulfobetaine micelles (Scheme 1), is strongly inhibited by ClO4-. 2. Experimental Section 2.1. Materials. We used the sulfobetaines N-decyl-, Ndodecyl-, and N-tetradecyl-N,N-dimethylammonio-1-propanesulfonate (SB3-10, SB3-12, and SB3-14), from Sigma, and N-tetradecyl-N,N-di-n-propylammonio-1-propanesulfonate (SBPr314) was prepared as described.6 Preparation and purification of MeONs have been described.6 NMR measurements were made in 99.9 at. % D2O, and H2O was purified by deionization (Milli-Q system, Millipore) and was used in all experiments, except for NMR spectroscopy. Sodium tetraborate (STB) was from Merck (Darmstadt, Germany). The pH of solutions for reactions of Bz2O was maintained with borate buffer, pH 9.0 (0.01 M). Solutions were prepared immediately before use. All measurements were at 25 °C. Other reagents and solvents were of analytical grade and were used without further purification. 2.2. Potentiometric Measurements. Potentiometric and pH measurements were made with a Metrohm model 713, pH meter or a Hanna model 200, digital pH meter calibrated with standard buffers, pH 7.00 and 9.00 (Carlo Erba). In general, potentiometric measurements with ISEs were made with a standard calomel electrode as reference and a membrane type ISE, prepared as described.7 Perchlorate ion standards were prepared in borate buffer (0.01 M), and aqueous solutions of the different sulfobetaines (generally 0.05 or 0.1 M) containing NaClO4 were monitored in the concentration range of the standards, in order to quantify the free and bound anionic, e.g., ClO4-, concentrations. 2.3. Capillary Electrophoresis. Experiments were carried out on an Agilent capillary electrophoresis system (Agilent CE3D), with on-column diode-array detection and temperature control at 25 °C. Samples were introduced into the capillary by hydrodynamic injection at 50 mbar/(5 s). Fused-silica capillaries (Polymicro Technologies) total length of 60.0 cm, effective length of 51.5 cm, and 50 µm i.d. were used. The electrophoresis system was operated under normal polarity and constant voltage of 30 kV. The capillary was
Figure 1. Langmuir plots showing concentrations of bound ClO4- as a function of total NaClO4 (b) in 0.05 M SB3-14, pH 9.0 and 25 °C.
initially conditioned by flushes of 1 M NaOH (5 min), deionized water (5 min), and electrolyte solution (10 min). Between experiments, the capillary was reconditioned by a pressure flush with the electrolyte containing 3 mM sodium tetraborate (2 min). The mobility of the zwitterionic micelles was monitored by following the migration of micellar bound pyrene (1 µM), and acetone (0.1%) was used as an electroosmotic flow marker. 2.4. Kinetics and NMR Spectroscopy. Kinetics was followed spectrophotometrically in aqueous sulfobetaines, SB310, SB3-12, SB3-14, and SBP3-14, as described earlier under conditions such that the organic substrate is almost wholly micellar bound and [surfactant] is much higher than the critical micellar concentration, cmc6,11,12 The 79Br line widths (B, Hz) were measured on a Varian Inova instrument (500 MHz for 1H) with Varian line broadening software. All NMR measurements were at 25 °C in D2O. Kinetic solutions were made up from bulk stock solutions, but, in order to reduce waste of D2O, solutions for the NMR work were made up on small scales from D2O, with or without NaClO4, which involved weighing small amounts of solids. The disadvantage of this procedure is that some concentrations in the kinetic and NMR experiments differed slightly and then we applied minor ( Br- > Cl- > OH- and specific anion binding saturates far from a 1:1 anion to surfactant headgroup ratio. We note that although, in the derivation of eq 5, nonelectrostatic interactions are not included, the results indicate ion specificity because experimental ζ potentials implicitly include ion specificity in the ψo term which determines the binding order. In fact the extent of incorporation of anions to the zwitterionic micelles is controlled by the balance of two opposing forces: (i) the increase in ζ potential which effectively decreases the rate of entry of anions to the anionoid micelle and (ii) the ability of the anions to decrease the rate of exit from the micelle, most probably via ion pair formation with the cationic moiety of the headgroup. Clearly, the less hydrated anions are those that have advantage in ion pair formation and, therefore, show greater micellar incorporation. To demonstrate the importance of these effects, we examined rate constants for reactions of different nucleophilic anions, (OH- and Br-) with hydrophobic substrates in sulfobetaine micelles and 79Br- NMR line widths in the presence of ClO4which is known to inhibit reactions of anionic nucleophiles and bases in sulfobetaine micelles by displacing the reactive anions.4a 3.3. Kinetics of the Reaction of Hydroxide Ion with Bz2O. The spontaneous hydrolysis of Bz2O in water is only slightly inhibited (
