Effect of solid-like nitrogen contact layers on graphite: anisotropy of tangential pressure and orientational order

Abstract

Molecular simulation has provided a significant progress in understanding behavior of two-dimensional molecular layers of various gases deposited on a solid surface. Substantial efforts have been made to describe the state of noble gases, nitrogen, methane, carbon dioxide etc. on the graphite surface. As a result, mechanism of solidification and melting of molecular layers, formation of structures commensurate with substrate, orientational ordering of non-spherical molecules and other distinctive features of 2D layers are mainly clear for at least 2 decades. Some quantitative refinements in modeling of such systems and adjusting parameters basing on experimental data are still continuing, but the activity in fundamental study in this area has been weakened. In part, this is due to difficulties in reliable evaluating of thermodynamic functions of 2D systems, especially in the crystalline form, which hinders exact evaluation of the 2D liquid–solid transition parameters. Recently new possibilities were demonstrated in the framework of a methodology based on kinetic Monte Carlo and explicit accounting for the 2D gas–solid and gas–liquid interface with an adjustable external potential imposed on the gas phase (Ustinov and Do in J Colloid Interface Sci 366:216–223, 2012; Ustinov in J Chem Phys 140: 074706, 2014a, in J Chem Phys 141:134706, 2014b, in J Chem Phys 142: 074701, 2015). This study aims at an extension of the approach to adsorption of nitrogen on the graphite surface. A special attention is paid to effect of commensurate nitrogen monolayer on graphite, manifesting itself as the appearance of a significant tangential pressure directly in the graphite layer. This pressure has to be accounted for in all thermodynamic equations such as the Gibbs–Duhem equation and involved into simulation schemes realizing the NPT ensemble. In the case of nitrogen at low temperatures the herringbone structure of the crystalline layer has proved to result in a significant anisotropy of that pressure. The developed approach can be extended to 3D systems and various fluids confined in nanoporous materials.