Equilibrium properties and surface transfer coefficients from molecular dynamics simulations of two-component fluids

We report equilibrium and non-equilibrium molecular dynamics studies of an ideal and a non-ideal mixture of Lennard–Jones spline particles. Soave–Redlich–Kwong type equations of state for the vapor phases, and Clausius–Clapeyron type fits for the phase line were determined for the systems. The ideal mixture, that was made up by isotopes, had the same equilibrium properties as the pure component fluid. The isotope mixture was chosen for method development purposes, since its one-component system was known. The equilibrium properties were used to determine the position of the surface under non-equilibrium conditions, and thereafter the thermodynamic driving forces for transport of mass and heat. The temperatures and pressures were such that the systems on the average were closer to the critical line than to the triple line. It was shown that irreversible thermodynamic theory can be used to describe the transport phenomena. Non-zero coupling coefficients are essential in the description, since in the cases we studied, one component was transported against its main driving force. The surface transfer coefficients were next determined, using the composition and temperature variation in kinetic theory. The main coefficients of transfer that were derived, differed from the coefficients that were calculated from kinetic theory by a factor between 1.5 and 5, both systems considered, while the coupling coefficients differed much more. It was also found, in contrast to the results of kinetic theory, that the coupling coefficient(s) for heat and mass transfer were most of the time negative. This is unexpected, at least for the isotope mixture that behaves thermodynamically as a one-component system. On the average for the cases studied, the thermal resistivity of the non-ideal mixture was smaller, while the resistivities to mass transport were larger than those of the ideal mixture.