Fick Diffusion Coefficients
in Ternary Liquid Systems from Equilibrium Molecular Dynamics Simulations
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Abstract
An approach for computing Fick diffusivities directly
from equilibrium molecular dynamics (MD) simulations is presented
and demonstrated for a ternary chloroform–acetone–methanol
liquid mixture. In our approach, Fick diffusivities are calculated
from the Maxwell–Stefan (MS) diffusivities and the so-called
matrix of thermodynamic factors. MS diffusivities describe the friction
between different molecular species and can be directly computed from
MD simulations. The thermodynamic factor describes the deviation from
ideal mixing behavior and is difficult to extract from both experiments
and simulations. Here, we show that the thermodynamic factor in ternary
systems can be obtained from density fluctuations in small subsystems
embedded in a larger simulation box. Since the computation uses the
Kirkwood–Buff coefficients, the present approach provides a
general route toward the thermodynamics of the mixture. In experiments,
Fick diffusion coefficients are measured, while previously equilibrium
molecular dynamics simulation only provided MS transport diffusivities.
Our approach provides an efficient and accurate route to predict multicomponent
diffusion coefficients in liquids based on a consistent molecular
picture and therefore bridges the gap between theory and experiment