Interfacial compositions of cationic and mixed non-ionic micelles by chemical trapping: a new method for characterizing the properties of amphiphilicaggregates

A specially synthesized arenediazonium ion bound to amphiphilic aggregates decomposes spontaneously via rate determining loss of N-2 to give a highly reactive, unselective, aryl cation intermediate. This intermediate is trapp ed competitively by weakly basic nucleophiles in the interfacial region of aggregates such as micelles and other association colloids. Product yields, analyzed by HPLC with UV detection, are used to estimate, simultaneously, the interfacial concentrations of a number of different nucleophiles, inclu ding water, that are commonly found at the surfaces of biomembranes and in many commercial products. Two applications of the method are discussed. Fir st, we show that the interfacial concentrations of X- (X = Br, Cl) increase steadily with increasing cetyltrimethylammonium halide (CTAX) and tetramet hylamnlonium halide (TMAX) concentrations and that the interfacial concentr ations of these counterions increase continuously with their aqueous phase concentrations at a constant degree of micelle ionization. Interfacial Br- and Cl- concentrations also show marked increases at their respective spher e-to-rod transitions. This steady increase in interfacial counterion concen tration with increasing aqueous counterion concentration contradicts a basi c assumption of the pseudophase ion exchange (PIE) model of chemical reacti vity in aggregates, i.e. that the total concentrations of ions at aggregate interfaces is constant and independent of the amphiphile and salt concentr ations. The consequences for the PIE model are discussed. Second, the chemi cal trapping reaction is used to estimate: (a) distributions of terminal OH groups of non-ionic amphiphiles in mixed non-ionic micelles composed of am phiphiles with different lengths of oligoethylene oxide chains and (b) hydr ation numbers of the inner layers of interfacial region next to the hydroca rbon core in these mixed micelles. Terminal OH groups distributions are wel l fitted by a radial one-dimensional random walk model. The average hydrati on number for the inner layers at 40 degreesC is about 3, in agreement with estimates from NMR water (D2O) self-diffusion measurements and with the hy dration number of 3 for aqueous solutions of polyethylene oxide. The result s suggest that the hydration states of the ethylene Oxide (EO) units near t he micellar core are near their minimum value. Recent and potential applica tions of the chemical trapping method are briefly discussed. (C) 2001 Elsev ier Science B.V. All rights reserved.