Effect of micellization on acid dissociation and headgroup conformation of hexadecyl(2-hydroxyethyl)dimethylammonium bromide

The acid dissociation constant of the hydroxy group of micellized hexadecyl(2-hydroxyethyl)dimethylammonium bromide, 1, was estimated from variations of the 1H and 13C NMR chemical shifts of the hydroxyethyl group with pH, and by monitoring the pH increase on addition of OH−. These methods give pKa in the range 12.4-12.6, which is similar to the value of 12.4 estimated kinetically from rate constants of dephosphorylation and aromatic nucleophilic substitution with functional micelles of 1 over a range of [OH−]. These values are similar to the literature value of pKa = 12.8 for dilute choline measured conductimetrically and micellization does not have a major effect on the deprotonation equilibrium of 1. The hydroxyethyl group of 1 is largely in the gauche conformation in both the micelle and the monomer due to favorable O⋯N+ interactions, as shown earlier for choline. The headgroups in micelles of 1 behave largely independently as regards their conformation and deprotonation of the hydroxy group.