Vapour-liquid phase equilibria and interfacial properties of fatty acids methyl esters from molecular dynamics simulations
We have determined the phase equilibria and interfacial properties of methyl esters homologous series (from methyl acetate to methyl heptanoate) from direct simulation of the vapour-liquid interface. Methyl esters are modelled using the united atom approach in combination with the TraPPE force fields for alkanes, alkenes, carbon dioxide, ethers, and carboxylic acids in a transferable way. This allows to take into account explicitly both dispersive and coulombic interactions. Simulations are performed in the NVT or canonical ensemble using molecular dynamics. Vapour-liquid surface tension is determined using the virial route, i.e., evaluating the normal and tangential components of the pressure tensor along the simulation box. We have also calculated density profiles, coexistence densities, vapour pressures, surface entropies and enthalpies, interfacial thickness, and critical temperatures, densities, and pressures as functions of temperature. Special attention is paid to the comparison between experimental data taken from the literature and our results obtained from molecular dynamic simulations. We also analyze the effect of increasing the molecular weight of the methyl esters (at fixed temperature) on all the properties considered, with special emphasis on phase equilibria envelopes and surface tension. The TraPPE force fields transferred from other molecules and chemical families are able to predict very accurately the experimental vapour-liquid phase envelopes of methyl esters. We also compare the results obtained from simulations for the surface tension with experimental data taken from the literature. To our knowledge, this is the first time that vapour-liquid phase equilibria and interfacial properties, and particularly surface tension, of the methyl esters homologous series are obtained from computer simulation.