Simulations of the Chain Length Dependence of the Melting Mechanism in Short-Chained n-alkane Monolayers on Graphite

TL;DR

This study uses molecular dynamics simulations to explore how chain length affects the melting mechanism in short-chained n-alkane monolayers on graphite, revealing the role of gauche defects in phase transitions.

Contribution

It provides new insights into the chain length dependence of melting behavior and the role of gauche defects in short-chained n-alkane monolayers, extending previous experimental and simulation studies.

Findings

Gauche defect formation increases with chain length and influences melting.

Octane and dodecane monolayers depend similarly on gauche defects for melting.

Differences in phase transition behavior compared to hexane monolayers suggest greater gauche defect influence.

Abstract

The melting transition in solid monolayers of a series of short-chained n-alkanes, n-octane (n-C8H18), n-decane (n-C10H22), and n-dodecane (n-C12H26) physisorbed onto the graphite basal plane are studied through use of molecular dynamics simulations. Utilizing previous experimental observations of the solid phase behavior of these monolayers, this study investigates the temperature dependence of the phases and phase transitions in these three monolayers during the solid-fluid phase transition, and compares the observed melting behavior to previous studies of hexane and butane monolayers. In particular, this study seems to indicate a greater dependence of the melting transition on the formation of gauche defects in the alkyl chains as the chain length is increased. In light of the previously proposed footprint reduction mechanism and variations where the formation of gauche defects are energetically negated, simulations seem to suggest that decane and dodecane monolayers are generally equally as dependent upon the formation of gauche defects for the melting transition to take place, whereas octane monolayers seem to have less dependence, but follow a trend that is established in previous studies of melting in butane and hexane monolayers. Also, the phase transition from a solid herringbone phase into an orientationally ordered intermediate phase is found to exhibit some differences as compared to a recent study of hexane monolayers, which may be interpreted as originating from the greater influence of gauche defects. Comparison to experimental melting temperatures is provided where possible, and applications involving thin film manipulation and lubrication is discussed.