Molecular-dynamics simulation of mutual diffusion in nonideal liquid mixtures

Abstract

The mutual-diffusion coefficients, D ₁₂, of n-hexane, n-heptane, and n-octane in chloroform were modeled using equilibrium molecular-dynamics (MD) simulations of simple Lennard-Jones (LJ) fluids. Pure-component LJ parameters were obtained by comparison of simulations to experimental self-diffusion coefficients. While values of “effective” LJ parameters are not expected to simulate accurately diverse thermophysical properties over a wide range of conditions, it was recently shown that effective parameters obtained from pure self-diffusion coefficients can accurately model mutual diffusion in ideal, liquid mixtures. In this work, similar simulations are used to model diffusion in nonideal mixtures. The same combining rules used in the previous study for the cross-interaction parameters were found to be adequate to represent the composition dependence of D ₁₂. The effect of alkane chain length on D ₁₂ is also correctly predicted by the simulations. A commonly used assumption in empirical correlations of D ₁₂, that its kinetic portion is a simple, compositional average of the intradiffusion coefficients, is inconsistent with the simulation results. In fact, the value of the kinetic portion of D ₁₂ was often outside the range of values bracketed by the two intradiffusion coefficients for the nonideal system modeled here.