An approach for computing Fick diffusivities directly from equilibrium molecular dynamics simulations was presented and demonstrated for a ternary chloroform-acetone-methanol liquid mixture. In this approach, Fick diffusivities were calculated from the Maxwell-Stefan diffusivities and the so-called matrix of thermodynamic factors. MS diffusivities described the friction between different molecular species and could be directly computed from molecular dynamics simulations. The thermodynamic factor described the deviation from ideal mixing behavior and was difficult to extract from both experiments and simulations. Here, it was shown that the thermodynamic factor in ternary systems could be obtained from density fluctuations in small subsystems embedded in a larger simulation box. Since the computation used the Kirkwood-Buff coefficients, the present approach provided a general route toward the thermodynamics of the mixture. In experiments, Fick diffusion coefficients were measured, while previously equilibrium molecular dynamics simulation only provided MS transport diffusivities.

Fick Diffusion Coefficients in Ternary Liquid Systems from Equilibrium Molecular Dynamics Simulations. Liu, X., Martín-Calvo, A., McGarrity, E., Schnell, S.K., Calero, S., Simon, J.M., Bedeaux, D., Kjelstrup, S., Bardow, A., Vlugt, T.J.H.: Industrial and Engineering Chemistry Research, 2012, 51[30], 10247-58