Temperature and composition dependence of the Soret coefficient in Lennard-Jones mixtures presenting consolute...

Nonequilibrium molecular dynamics calculations have been carried out o Lennard-Jones binary mixtures with the aim to investigate the dependence of the Soret coefficient on the temperature and on the composition for systems presenting phase transitions. By an appropriate choice of the cross interaction parameter, epsilon(12) (0 < epsilon(12) < min{epsilon(11),epsilon(22)}), these systems show a mixing/demixing (consolute) phase transition. The other parameters are those of a binary mixture of Argon and Krypton. This system has been considered over a wide range of temperatures (up to approximately or equal to 1000 K), of compositions (0.1 < or = x(1) < or = 0.9), and of cross interaction parameter (0 < epsilon(12) < min{epsilon(11),epsilon(22)}). The study allows the formulation of a very simple expression for the Soret coefficient, S(T), as a function of temperature and composition. Indeed the computed values of S(T) in the one phase region outside the critical region are closely fitted by the function [T-T(c)(x(1))](-1) where T(c)(x(1)) is the demixing temperature of the mixture under study. This result indicates for this type of systems a dependence of S(T), as a function of the temperature, on a unique characteristic property of the fluid mixture, the demixing temperature T(c), which, in turn, is a function of the binary mixture composition x(1).