Molecular Simulation Algorithms for Predicting Solid-Liquid Phase Equilibria
Advancing the chemical engineering fundamentals
Thermodyanmics: Molecular Simulation & Related Approaches (T2-1d)
Keywords: molecular dynamics simulation, solid-liquid equilibria, phase behaviour, intermolecular potentials
The phase behaviour of both pure systems and binary mixtures has been widely studied by molecular simulation, using techniques such as the Gibbs ensemble , Gibbs-Duhem or histogram reweighing algorithms. The common aim of many of these investigations is to accurately predict the phase diagram using effective intermolecular potentials, most notable the Lennard-Jones potential. In contrast, other studies have used molecular simulation techniques, in conjunction with genuine two- and three-body intermolecular potentials, to determine the influence of various intermolecular interactions on phase behaviour. These studies have concluded that three-body interactions have a significant influence on phase behavior. Three-body interactions decrease the density of the liquid phase of pure fluids and they contribute significantly to the vapor-liquid critical point. In binary mixtures, three-body interactions are required to obtain good agreement between theory and experiment for the pressure-composition behaviour. There is also some evidence that three-body interactions have a pivotal role in the transition between the different global phase behavior types of binary mixtures.
Previous investigations of three-body interactions on phase equilibria have been confined largely to pure fluids. Theoretical studies have been reported which indicate three-body interactions are important in solid phases. However, the direct molecular simulation of solid-liquid equilibria for both pure fluids and mixtures has mainly focused on predicting phase coexistence using an effective intermolecular potential.
The solid-liquid phase transition is difficult to determine accurately using traditional molecular simulation techniques. The high densities mean that it is not practical to use the Gibbs ensemble because of the difficulty of exchanging particles between the phases. Although this limitation is avoided by the Gibbs-Duhem technique, it is not self-starting which means it requires prior knowledge of one pair of coexistence data. Therefore, its ability to predict the phase boundary largely depends on the accuracy of the starting point data. In this work, we employed a novel approach for locating the solid-liquid phase boundary which combines elements of both equilibrium and non-equilibrium molecular dynamics techniques. The approach yields reliable calculations and it avoids the problems encountered in both Gibbs ensemble and Gibbs-Duhem methods.
Presented Tuesday 18, 11:00 to 11:20, in session Thermodyanmics: Molecular Simulation & Related Approaches (T2-1d).
