Equilibrium properties and surface transfer coefficients from molecular dynamics simulations of two-component fluids Marie-Laure Olivier a , Jean-Damien Rollier b , Signe Kjelstrup b, * a Department of Refrigeration and Air Conditioning, Norwegian University of Science and Technology, N-7491 Trondheim, Norway b Department of Chemistry, Norwegian University of Science and Technology, N-7491 Trondheim, Norway Abstract 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. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Equilibrium properties; Surface transfer coefficients; Two-component fluids; Molecular dynamics 1. Introduction Coefficients for transfer of heat and mass from the liquid state to the vapor state are of large technical importance. Non-equilibrium transitions occur when fluids are transported in pipe-lines, in distillation columns and in evaporators. It is known that the film between a liquid and a gas may have a significant resistance to transport of heat and mass, but it is less clear whether the resistance is due to the bulk parts of the film or to the surface itself. Transfer rates have been mod- * Corresponding author. Tel.:  /47-735-94178; fax:  /47- 735-91676 E-mail address: signekj@chembio.ntnu.no (S. Kjelstrup). Colloids and Surfaces A: Physicochem. Eng. Aspects 210 (2002) 199  /222 www.elsevier.com/locate/colsurfa 0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0927-7757(02)00378-3