Luís A. Nunes Amaral

Monte Carlo simulations of the liquid-vapour interface for a site-site intermolecular potential of methylchloride are reported. The introduction of non-uniform samplings for computer modelling of non-uniform systems has been considered and it is suggested that they may be of interest when the transition region is an important portion of the whole system. The role played by dipolar coulombic interactions in the interfacial properties has been discussed and the results of the simulations for a system without charges (LJ system) are reported. The interfacial transition region has been studied from both the structural and the energetical point of view. The orientational order in the transition region reflects the competition between anisotropy of the site-site potential and the orientational ordering induced by dipolar interactions. The surface tensions (gammas) are not significantly dependent on the size of the system but are very sensitive to the long-range corrections. Our best estimate for gamma at 200 K is 36.6 +/- 2.4 erg cm-2 which is not in very good agreement with the experimental value (26.5 erg cm-2). The coulombic contribution to the surface tension accounts for 25% of the total value (7.7 erg cm-2). It is suggested that the differences between the results from simulations and experiments for gamma may be related to the inadequacy of effective pairwise additive intermolecular potentials parametrized to reproduce bulk properties to model the behaviour of non-uniform systems.