On the Behavior of Hexane on Graphite at Near-Monolayer Densities

TL;DR

This study uses molecular dynamics simulations to explore how hexane molecules behave on graphite surfaces at various coverages, revealing phase transitions, molecular orientations, and the effects of density on melting and solid phases.

Contribution

It provides new insights into the molecular orientations, phase behavior, and transition temperatures of hexane on graphite at near-monolayer densities, emphasizing the role of molecular rolling and tilting.

Findings

Formation of uniaxially incommensurate herringbone solid at low temperatures.

Presence of a nematic mesophase at certain coverages during heating.

Melting temperature remains largely insensitive to coverage variations.

Abstract

We present the results of molecular dynamics (MD) studies of hexane physisorbed onto graphite for eight coverages in the range $0.875 \le \rho \le 1.05$ (in units of monolayers). At low temperatures the adsorbate molecules form a uniaxially incommensurate herringbone (UI-HB) solid. At high coverages the solid consists of adsorbate molecules that are primarily rolled on their side perpen-dicular to the surface of the substrate. As the coverage is decreased, the amount of molecular rolling diminishes until $\rho$ = 0.933 where it disappears (molecules become primarily parallel to the surface). If the density is decreased enough, vacancies appear. As the temperature is increased we observe a three-phase regime for $\rho > 0.933$ (with an orientationally ordered nematic mesophase), for lower coverages the system melts directly to the disordered (and isotropic) liquid phase. The solid-nematic transition temperature is very sensitive to coverage whereas the melting temperature is quite insensitive to it, except for at low coverages where increased in-plane space and ultimately vacancies soften the solid phase and lower the melting temperature. Our results signal the importance of molecular rolling and tilting (which result from an the competition between molecule-molecule and molecule-substrate interactions) for the formation of the intermediate phase, while the insensitivity of the system's melting temperature to changing density is understood in terms of in-plane space occupation through rolling. Comparisons and contrasts with experimental results are discussed.