Diffusion of Alkyltriphenylphosphonium Bromides in Aqueous Micellar Solutions

Dynamic light scattering, conductometry, and capillary viscometry have been used to study aqueous micellar solutions of dodecyl-, tetradecyl-, and hexadecyltriphenylphosphonium bromides in a wide range of concentrations covering the first and second critical micelle concentrations (CMC₁ and CMC₂). It has been shown that the concentration curves for the diffusion coefficients of the ionic surfactants increase above CMC₁ and, then, pass through a maximum. As the alkyl chain length increases, the slopes of the concentration curves within the range of the linear growth in the diffusion coefficient rise, the height of the maximum increases, and its position shifts toward lower concentrations. The obtained results have been explained in terms of a theory previously developed for ideal micellar systems. It has been shown that the mobility factor plays the predominant role in the range of the linear increase in the diffusion coefficient and the effect of the viscosity of a micellar surfactant solution is enhanced with a rise in its concentration.