NMR Evidence for Grotthuss-like Proton Diffusion on the Surface of N-Alkyl-ammonium Micelles in Acidic Aqueous Solution

Abstract

Dramatic enhancements of proton exchange rates, by a factor of up to ca. 10 × 104, are observed in acidic aqueous solution on formation of micelles of N-alkyl-ammonium chlorides C n H2n+1NH x Me3− x , with n = 8–12 and x = 1 or 2. Exchange rate/concentration plots at constant pH yield the cmc value as the break point abscissa and the micellar NMR rate constant k mic as the asymptotic limit. The linearity of k mic as a function of 1/[H+] and the saturation factor of ammonium hydrogens have put in evidence a pH increase in the immediate surrounding of the micelle surface by ca. two units, and a Grotthuss-like pH-independent proton transfer from one polar head to the next. The number of diffusion steps is shown to increase with the length of the carbon chain and the nature of the polar head, due to a higher compactness of micelles and a greater counter-ionic screening effect up to 0.82. The cmc data obtained in the present work have the peculiarity of being obtained in highly acidic solutions, allowing for an in-depth discussion of pH-cmc relationships, and are in fair agreement with those obtained by chemical shift, fluorescence and conductivity measurements, with the advantage of an improved sensitivity.