Effect of the nature of the hydrophobic group on the mode of association of amphiphilic molecules in aqueous solution

Abstract

The importance of the flexibility of the hydrophobic group in determining the association pattern of an amphiphilic molecule has been examined by comparing the solution properties of a flexible diphenylmethane derivative, adiphenine hydrochloride, with those of a similar molecule, pavatrine hydrochloride, in which the two phenyl rings of the hydrophobic group are linked in the form of a rigid fluorene group. The concentration dependence of the light scattering from aqueous solutions of pavatrine hydrochloride showed no significant discontinuity attributable to a critical micelle concentration and could not be simulated using the mass-action model of micellisation, indicating that the association pattern does not conform to that of a monodisperse micellar system. In contrast, adiphenine hydrochloride exhibited a typical micellar association pattern.

The association of pavatrine hydrochloride in water may be represented by a two-parameter stepwise association model in which the dimerization constant is treated as an independent variable and higher association constants, K_N, decrease with increasing aggregation number, N, according to the relationship K_N=K(N– 1)/(N– 2). Addition of sodium chloride causes not only an increase in the magnitude of the association constants but also a change in the pattern of association. In the presence of 0.1 mol dm^–3 NaCl the association is best represented by a two-parameter model in which higher stepwise association constants are identical. At higher electrolyte concentrations (0.2 and 0.5 mol dm^–3) the association follows a one-parameter scheme in which association constants increase with N according to K_N=K(N– 1)/N.