Molecular Dynamics Simulations of Halomethane Adsorption on Two Dissimilar Surfaces

In this study, atomic-scale MD simulations were used to study multilayer adsorption of three different halomethane compounds (CF$_4$, CF$_3$Cl and CF$_3$Br) onto two different surfaces (graphite and hydroxylated $\alpha$-quartz) to show how orientation, layer structure, packing and dynamics vary as a function of temperature and substrate surface characteristics. In this way, non-polar CF$_4$ on hydroxylated $\alpha$-quartz is shown to exhibit a highly ordered packing arrangement only weakly dependent on temperature variation. In contrast, interactions between polar adsorbate groups and hydroxyl groups on the surface give CF$_3$Cl and CF$_3$Br a very different layer structure at the surface. Analysis of the orientations of polar molecules in the first adsorbed layer shows a lightly ordered arrangement of dipoles, and residence time and dipole correlation calculations suggest that this ordering is subject to frequent shifts in orientation and position within the film.