IWNET12
IWNET12
Fick Diusion Coe cients in Ternary Liquids from Equilibrium Molec-
ular Dynamics
Thijs J.H. Vlugt1
1 Department of Process and Energy, Delft University of Technology, Leeghwaterstraat 44, 2628 CA Delft, The
Netherlands
Abstract
An approach for computing ternary Fick diusivities directly from equilibrium Molecular Dynamics (MD)
simulations is presented and demonstrated for a liquid mixture of chloroform-acetone-methanol. In our approach,
Fick diusivities are obtained from Maxwell-Stefan (MS) diusivities and the so-called matrix of thermodynamic
factors. MS diusivities describe the friction between dierent molecular species and can be directly computed
from equilibrium MD simulations. The thermodynamic factor is the concentration derivative of the activity
coe cient describing the deviation from ideal mixing behaviour. It is important to note that all mutual diusion
experiments measure Fick diusion coe cients, while previous equilibrium molecular dynamics simulation did
only provide MS diusivities. The required thermodynamic factor to convert MS into Fick diusivities and
vice versa, however, has been di cult to extract from both simulations and experiments leaving a gap between
theory and applications. Here, we extend an e cient computing scheme for the thermodynamic factor recently
proposed by the us for ternary complex mixtures [1]. The novel computing scheme extracts the thermodynamic
factor from density uctuations in small subsystems embedded in a larger simulation box. The computed
thermodynamic factor is in excellent agreement with experimental VLE data as well as calculations using the
COSMO-SAC method. Our approach provides an e cient route to obtain multicomponent diusion coe cients
in liquids based on a consistent molecular picture and therefore bridges the gap between theory and experiment.
References
[1] X. Liu, S.K. Schnell, J-M Simon, D. Bedeaux, S. Kjelstrup, A. Bardow, T.J.H. Vlugt J. Phys. Chem. B,
2011, 115, 12921-12929.
E-mail: T.J.H.Vlugt@tudelft.nl