Optimization of micellar catalysis of nucleophilic substitution reactions in buffered solutions of cetyltrimethylammonium halide surfactants, part 2:buffers in the pH range 7-8

Binding of the phosphate, tris-(hydroxymethyl)-methylamine, aminomethylprop anediol, and glycinemethylester buffers by cetyltrimethylammonium chloride (CTAC1) in aqueous solutions has been probed by investigating: (1) the depe ndence of the buffer pH (starting pH 7.9) on [CTAC1], and (2) the micellar effect on the kinetics of dephosphorylation of p-nitrophenyldiphenylphospha te (PNDPP) by the anion of isonitrosoacetylacetone (INAA) in CTAC1 solution s in the presence of the same buffers. The pH-[CTAC1] profiles showed a mar ked dependence on the buffer employed and the coion, Y-, of its acidic comp onent, RNH3+Y- The sizeable pH decrease observed with phosphate buffer (0.4 3 pH units for [Buffer] = 10-2 M at [CTACI] = 2 X 10(-2) M) indicates that both buffer components, namely H2PO4- and HPO42-, exchange with the surfact ant counterion, Cl-. This ion exchange occurs at the expense of the nucleop hile (anion of INAA)-Cl- counterpart. Indeed, the micellar acceleration of the phosphate-buffered reaction is the smallest, k(max)/k(w) = 410 (k(max) and k(w) are the maximum pseudo first-order rate constants in buffered mice llar solutions and bulk water, respectively). Although CTAC1 micelles do no t seem to incorporate the neutral component of amino buffers, the pH-[CTAC1 ] profiles were found to depend on the nature in Y- (F-, Cl- or AcO-). The micellar accelerations (k(max)/k(w) approximate to 600), however, were not strongly altered by a change in the buffer coion, except where Y- = F-. In the interfacial region, the partially desolvated fluoride ion behaves as a nucleophile, competing with INAA anion for the dephosphorylation of PNDPP. The rate-[surfactant] profiles were interpreted in terms of the pseudophase ion-exchange model, as applied to a reaction scheme involving competitive exchanges of the oximate and Y- for the surfactant counterion. The second-o rder rate constant of the micelle-mediated reaction, smaller (ca one-third) than that in bulk aqueous solution, is discussed in terms of the propertie s (ionic strength and microscopic polarity) of interfacial water. Copyright (C) 2001 John Wiley & Sons, Ltd.