A Comprehensive Theory of the Soret Effect in a Multicomponent Mixture

Abstract

A comprehensive theory for the Soret effect (also called: Ludwig-Soret effect and thermal diffusion) is presented which incorporates both the thermodynamic contribution and the kinetic contribution. The new theory is an extension of a theory presented in our previous paper (J. Chem. Phys. (1989) 90, 6541) in which the thermodynamic contribution only was modelled. The single assumption of the theory is that the Soret effect in the steady state is the macroscopic state accomplished by a maximum number of microstates with respect to the ideal gas state. The methodology of the new theory, which can be used to model other cross-effects in irreversible thermodynamics, is further validated by application to the textbook case of gravity equilibrium in an isothermal fluid column. As a result, the Soret effect in a multicomponent mixture can be calculated by using input from an equation-of-state of the mixture and kinetic gas theory.