Degree of Counterion Binding of Alkylpyridinium Halide Micelles: Its Relation to Micelle Shape and Size

Abstract

From electromotive force measurements with surfactant and halide ion-selective electrodes, we obtained the degree of counterion binding (β) of dodecylpyridinium bromide (DPB) and iodide (DPI) and tetradecylpyridinium bromide (TPB) micelles in aqueous sodium halide solutions with concentrations of 0 - 20 mM. Under a geometrical consideration the micelle shapes and the surface densities of head group (s-1) of these micelles were deduced from aggregation numbers evaluated from their relationships to the added salt concentration previously reported. The spherical micelles of TPB having the shape of an oblate ellipsoid had a constant β, 0.76, independently of s-1. In the case of the spherical micelles of DPB, β was found to be 0.79 for sphere-shaped micelles and 0.82 for ellipsoidal ones. For DPI we obtained β values of 0.84 and 0.99 for the spherical micelles with the shape of an oblate ellipsoid and the rodlike micelles, respectively. From these dependences of β on micelle shape and s-1, we conclude that β of micelles investigated increases in the order of sphere-shaped < oblately ellipsoidal < rodlike micelles but is independent of the micelle size unless the micelle shape changes.