Nonequilibrium thermodynamic material transport equations for thermodiffusion of isotopes and isomers in multicomponent systems

Abstract

We extend our nonequilibrium thermodynamic model of thermodiffusion in binary systems to multi-component mixtures. The fundamental parameter is the difference in molecular entropy of the components, which can be obtained in one of three ways; (i) derived as temperature derivatives of the respective equilibrium chemical potentials at constant pressure using equilibrium statistical mechanics; (ii) obtained in the literature from computer simulations; or (iii) obtained as empirical values in the literature. The model is used to relate thermodiffusion in multicomponent mixtures of related isomers or isotopes to isomer/isotope effects in binary mixtures that are commonly enumerated in one of two ways: (i) as a difference in the Soret coefficients measured on two binary mixtures, each containing one of two related isomers/isotope in a common solvent; or (ii) as this difference from two binary mixtures, each consisting of a common solute dissolved in one of the two related isomers/isotopes as the solvent. The model is used to estimate the concentration profiles established in neat multicomponent mixtures of hexane isomers, and for the prediction and optimization of separating isotopes of cyclohexane in various organic solvents by thermodifusion.