Thermophysical properties of simple molecular liquid mixtures: On the limitations of some force fields

Abstract

In this work, we explore the ability of two force fields, a united atom one (TraPPE-ua: Transferable Potential for Phase Equilibria united atom) and a coarse grained one (MCCG: Mie Chain Coarse Grained), to describe simultaneously equilibrium derivative properties, such as isothermal compressibility and speed of sound, excess property in mixtures and transport properties such as shear viscosity in binary liquid mixtures composed of n-hexane + n-dodecane over a wide range of thermodynamics conditions (from 293.15 to 353.15 K and pressure up to 100 MPa). To do so on a consistent and controlled set of experimental data, we have measured accurately density, speed of sound and shear viscosity of these mixtures. Numerically, we computed the aforementioned thermophysical properties at the same thermodynamic conditions using both classical Monte Carlo and Molecular Dynamics simulations. Comparisons between experimental data and molecular simulations of volumetric and acoustic properties indicate a fair agreement for both force fields, with an overall advantage to the MCCG force field. In addition, both approaches, combined with classical Lorentz-Berthelot combining rules, are able to capture reasonably well the small excess properties of the studied thermodynamic properties. However, non-negligible deviations, up to around 50%, are observed on viscosity for the densest systems. Such deviations confirm that, even on simple molecular systems, force fields may be limited to yield precise transport properties at high densities.