Coarse-grained models for fluids and their mixtures: Comparison of Monte Carlo studies of their phase behavior with perturbation theory and experiment

TL;DR

This paper compares Monte Carlo simulations, perturbation theory, and experimental data to model phase behavior of simple fluids and their mixtures using coarse-grained models, highlighting their effectiveness and limitations.

Contribution

It introduces simple coarse-grained models fitted with Lennard-Jones potentials that accurately reproduce phase behavior and critical points of various fluids and mixtures, including polar molecules.

Findings

Coarse-grained models successfully predict phase diagrams of pure fluids.

Lorentz-Berthelot rule works well for mixtures, with some exceptions.

Models are computationally efficient and useful for polymer solution simulations.

Abstract

The prediction of the equation of state and the phase behavior of simple fluids (noble gases, carbon dioxide, benzene, methane, short alkane chains) and their mixtures by Monte Carlo computer simulation and analytic approximations based on thermodynamic perturbation theory is discussed. Molecules are described by coarse grained (CG) models, where either the whole molecule (carbon dioxide, benzene, methane) or a group of a few successive CH_2 groups (in the case of alkanes) are lumped into an effective point particle. Interactions among these point particles are fitted by Lennard-Jones (LJ) potentials such that the vapor-liquid critical point of the fluid is reproduced in agreement with experiment; in the case of quadrupolar molecules a quadrupole-quadrupole interaction is included. These models are shown to provide a satisfactory description of the liquid-vapour phase diagram of these pure fluids. Investigations of mixtures, using the Lorentz-Berthelot (LB) combining rule, also produce satisfactory results if compared with experiment, while in some previous attempts (in which polar solvents were modelled without explicitly taking into account quadrupolar interaction), strong violations of the LB rules were required. For this reason, the present investigation is a step towards predictive modelling of polar mixtures at low computational cost. These very simple coarse-grained models of small molecules developed here should be useful e.g. for simulations of polymer solutions with such molecules as solvent.