Chapter 10 - Isothermal Diffusion and Intradiffusion in Surfactant Solutions

Abstract

This chapter discusses isothermal diffusion and intradiffusion in surfactant solutions. Diffusion is the effect of the average displacement of molecules in a fluid system. Displacement is a simple consequence of Brownian motion in a system at thermodynamic equilibrium. If displacement happens in a solution in which concentration gradients are present, it has mutual diffusion, which is a classical linear irreversible process. As diffusion is closely related to molecular mobilities, it should be treated according to statistical mechanics of timedependent processes in fluid systems. The most general function that contains complete information regarding correlation of particle motion in liquid phases is the van Hove density-density correlation function, which describes correlation in space and time among the motions of different as well as of identical particles. This chapter states that the diffusion coefficient value depends on the extent of the thermodynamic driving force. If this force approaches zero, the diffusion coefficient also does. The gradients of chemical potentials approach zero around a critical mixing point in systems where a phase separation is present. The same condition is verified along the spinodal curve although this condition is difficult to achieve, as it is an unstable condition for solutions. In multicomponent systems, thermodynamics requires that the determinant of diffusion coefficients becomes zero at a critical mixing point. This chapter concludes that intradiffusion is a phenomenon that describes the diffusive motions of particles at equilibrium caused by their thermal excitation.