Transport phenomena and microscopic structure in partially miscible binary fluids: A simulation study of the symmetrical Lennard-Jones mixture

TL;DR

This simulation study investigates the microscopic structure and transport properties of a symmetrical Lennard-Jones binary fluid with a miscibility gap, analyzing how these properties vary with temperature and concentration along the coexistence curve.

Contribution

The paper introduces a combined Monte Carlo and Molecular Dynamics approach to study microscopic structure and transport coefficients in a binary Lennard-Jones mixture with a miscibility gap, providing detailed insights.

Findings

Minority species diffuse faster than majority species

Stokes-Einstein relation remains a reasonable approximation despite concentration fluctuations

Transport coefficients and dynamic structure factors are characterized along the coexistence curve

Abstract

Static and dynamic structure factors and various transport coefficients are computed for a Lennard-Jones model of a binary fluid (A,B) with a symmetrical miscibility gap, varying both temperature and relative concentration of the mixture. The model is first equilibrated by a semi-grandcanonical Monte Carlo method, choosing the temperature and chemical potential difference $\Delta \mu$ between the two species as the given independent variables. Varying for $\Delta \mu=0$ the temperature and particle number $N$ over a wide range, the location of the coexistence curve in the thermodynamic limit is estimated. Well-equilibrated configurations from these Monte Carlo runs are used as initial states for microcanonical Molecular Dynamics runs, in order to study the microscopic structure and the behavior of transport coefficients as well as dynamic correlation functions along the coexistence curve. Dynamic structure factors $S_{\alpha \beta} (q,t)$ (and the corresponding static functions $S_{\alpha \beta} (q)$) are recorded ($\alpha, \beta, \in$ A,B), $q$ being the wavenumber and $t$ the time, as well as the mean square displacements of the particles (to obtain the self-diffusion constants $D_{\rm A}$, $D_{\rm B}$) and transport coefficients describing collective transport, such as the interdiffusion constant and the shear viscosity. The minority species is found to diffuse a bit faster than the majority species. Despite the presence of strong concentration fluctuations in the system the Stokes-Einstein relation is a reasonable approximation.