Phase transitions in hexane monolayers physisorbed onto graphite

Abstract

We report the results of molecular dynamics simulations of a complete monolayer of hexane physisorbed onto the basal plane of graphite. At low temperatures the system forms a herringbone solid. With increasing temperature, a solid-to-nematic liquid-crystal transition takes place at T1=138±2 K followed by another transition at T2=176±3 K into an isotropic fluid. We characterize the different phases by calculating various order parameters, coordinate distributions, energetics, spreading pressure, and correlation functions, most of which are in reasonable agreement with available experimental evidence. In addition, we perform simulations where the Lennard-Jones interaction strength, corrugation potential strength, and dihedral rigidity are varied in order to better characterize the nature of the two transitions. We find that both phase transitions are facilitated by a “footprint reduction” of the molecules via tilting and to a lesser degree via creation of gauche defects in the molecules.