Molecular dynamics simulation of argon and ammonia physisorbed on graphite.

摘要

Molecular dynamics simulations are reported for argon and ammonia adsorbed on graphite.;Potential models for the Ar-Ar and Ar-graphite interactions are taken to be sums of site-site Lennard-Jones(12-6) energies. Ammonia-ammonia and ammonia-graphite interactions are modeled by site-site interaction, with discrete fractional charges used to describe the electrostatic interactions and the Lennard-Jones(12-6) model used for the non-electrostatic part.;The simulations of tri-layer and monolayer film of Ar/Gr were performed for temperatures ranging from 50K to 120K. Layer energies, layer densities, pair distribution functions and orientational bond correlation functions are calculated to characterize the melting transitions and the structures. The results show that the tri-layer film melts layer by layer.;The chemical potential of the tri-layer film of argon is calculated at T = 103K by using the test particle, the real particle and the ratio methods. Chemical potentials are calculated for small bins in z direction perpendicular to the surface. The variation of Boltzmann factor with z provides a criterion of the accuracy of the chemical potential obtained. The ratio method provides more precise values than the test particle and real particle methods.;Simulations of the monolayer, partial bilayer and complete bilayers at T = 96K were carried out. The results show that the variation of the molecular orientations with coverage are dramatic. The centers of mass of ammonia form a triangular lattice with lattice spacing equal 3.55 A. The unit cell for a monolayer contains four molecules. Electrostatic interactions played an important part in determining the orientations of the molecules. These interactions include both dipole-dipole and dipole-quadrupole interactions, which were roughly equal in magnitude.;Melting transition for the submonolayer and monolayer NH