Fluid phase coexistence by molecular simulation

Potter, Simon Christopher (1997) Fluid phase coexistence by molecular simulation. University of Southampton, Doctoral Thesis.

Abstract

Molecular dynamics (MD) computer simulations have been performed on a variety of molecular systems at the liquid/vapour and liquid/liquid interfaces.

A potential model has been developed for the liquid/vapour equilibria of fluoromethanes using the direct MD method of simulating the two phases in contact. This potential has been shown to be transferable by giving a good fit to the experimental coexistence properties for the three fluids CH₂F₂, CHF₃ and CF₄. The models show good agreement with the coexisting densities, latent heats and vapour pressures. Additionally, a reasonable fit to the radial distribution function g(r), obtained from coherent neutron scattering, is observed for CH₂F₂ and CHF₃. The model fits the experimental data especially well for CHF₃.

The transferable model has been applied to the study of the liquid/vapour coexistence of the CHF₃/CF₄ mixture. It was found that the MD timescale is far too slow for the determination of phase equilibria using the direct method. While the total bulk densities of the liquid and vapour phases are quickly established there are fluctuations in the local concentrations which do not average out quickly. Unfortunately the model does not appear to 'transfer' to the case of the mixture: the mole fractions of the two coexisting densities poorly match those of experiment at the same temperature and pressure. The simulations have been used to determine the surface tension and orientational ordering at the surface and to predict the experimental neutron reflectivity profile.

A simulation has been performed of the liquid/liquid interface of water/1,2-dichloroethane. This work has revealed some insights into the ordering of molecules at the interface. Comparisons have been made in particular with the results of MD simulations at the liquid/vapour interface of pure water. The distribution of the water dipoles is very similar to that at the free liquid/vapour interface and shows too different preferred orientations either side of the surface. We have also calculated the surface tension which is found to be significantly larger than the experimental value and must be due to a deficiency in the model.

This record has no associated files available for download.

Contributors

Download statistics