Different melting behavior in pentane and heptane monolayers on graphite: Molecular dynamics simulations

Abstract

Molecular dynamics simulations are utilized to study the melting transition in pentane (C 5 H 12 ) and heptane (C 7 H 16 ), physisorbed onto the basal plane of graphite at near-monolayer coverages. Through use of the newest, optimized version of the anisotropic united-atom model (AUA4) to simulate both systems at two separate coverages, this study provides evidence that the melting transition for pentane and heptane monolayers are significantly different. Specifically, this study proposes a very rapid transition from the solid crystalline rectangular-centered (RC) phase to a fluid phase in pentane monolayers, whereas heptane monolayers exhibit a transition that involves a more gradual loss of RC order in the solid-fluid phase transition, accompanied by uniaxial motion of sublattices and molecules within sublattices. Through a study of the melting behavior, encompassing variations where the formation of gauche defects in the alkyl chains are eliminated, this study proposes that this melting behavior for heptane monolayers is a result of less orientational mobility of the heptane molecules in the solid RC phase, as compared to the pentane molecules. This idea is supported through a study of a nonane monolayer, which gives the gradual melting signature that heptane monolayers also seem to indicate. The results of this work are compared to previous experiment over pentane and heptane monolayers, and are found to be in good agreement.

Molecular Thin Films on Solid Surfaces: Mechanisms of Melting

“…Thus, we characterized melting in MD studies of two model systems representing pentane and hexane ML adsorbed on the basal plane of graphite. The trends that we observe here are consistent with experimental studies 19−24 as well as with the most recent MD studies for pentane 13 and hexane. 11 However, the underlying mechanisms of melting appear to be unique to our study.…”

“…11 However, the underlying mechanisms of melting appear to be unique to our study. We observe melting trends similar to those observed by Pint, 13 who adopted the AUA4 model for pentane chains.…”

“…Another point that arises in this study is that the melting in these systems arises from an intricate combination of interactions, and by comparing our study with those of Pint 13 and Wexler and colleagues, 11 we see that this combination is not unique. This indicates that to achieve a firm understanding of the structure and function of organic thin films, a holistic view is needed, with a focus that is broader than a single experimental measure.…”

“…From a theoretical perspective, difficulties associated with handling large molecules and the significant role played by van der Waals interactions make first-principles studies challenging for many organic thin-film systems. Although some progress has been made in understanding organic thin-film structures using molecular dynamics (MD) simulations based on force fields, − achieving sufficiently large system sizes and adequate simulation times can be challenging. Additionally, the reliability of the force fields for these simulations, especially those for the interaction of the molecules with the solid surface, can limit their capabilities.…”

The Mixing Behavior of Alkanes Adsorbed on Hexagonal Boron Nitride

“…To fully understand this behavior will require further experiments to understand how the coverage modifies the phase behavior and computational studies to help explain the qualitative description outlined above. There have been a number of simulation studies of alkane interactions with graphite in recent years, ,− but relatively few of these have considered such subtle substrate effects as those observed experimentally in this work. We hope that the results presented herein will stimulate work in this area and allow for a more complete understanding of the behavior observed.…”