Phase Transitions in Hexane Monolayers Physisorbed onto Graphite

TL;DR

This study uses molecular dynamics simulations to explore phase transitions in hexane monolayers on graphite, identifying solid, nematic, and isotropic phases and analyzing the molecular mechanisms behind these transitions.

Contribution

It provides detailed simulation-based insights into the phase behavior and molecular mechanisms of hexane monolayers on graphite, including the effects of molecular tilting and defects.

Findings

Identified two phase transitions at 138K and 176K.

Transitions involve molecular tilting and gauche defect formation.

Results align with experimental observations.

Abstract

We report the results of molecular dynamics (MD) 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 $T_1 = 138 \pm 2$K followed by another transition at $T_2 = 176 \pm 3$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 through. 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.