Structural and phase properties of tetracosane (C24H50) monolayers adsorbed on graphite. Explicit Hydrogen Molecular Dynamics study

TL;DR

This study uses explicit hydrogen molecular dynamics simulations to investigate the phase behavior of tetracosane monolayers on graphite, revealing the importance of electrostatic interactions in accurately modeling melting temperatures and phase transitions.

Contribution

It introduces fully atomistic MD simulations with explicit hydrogens to accurately reproduce experimental phase behavior of tetracosane monolayers on graphite, highlighting the role of electrostatics.

Findings

The low-temperature phase is a rectangular incommensurate solid.

A smectic mesophase appears above 200 K due to gauche defects.

Melting temperature is highly sensitive to electrostatic interaction modeling.

Abstract

We discuss Molecular Dynamics (MD) computer simulations of tetracosane (C24H50) monolayer physisorbed onto a graphite surface. The alkane molecules are simulated with explicit hydrogens, and the graphite substrate is represented as an all-atom struc-ture having six graphene layers. The tetracosane dynamics modeled in the fully ato-mistic manner agree well with experiment. The low-temperature ordered solid orga-nizes in rectangular centered structure, incommensurate with underlying graphite. Above T = 200 K, as the molecules start to lose their translational and orientational order via gauche defects formation, a weak smectic mesophase (observed experimentally but never reproduced in United Atom (UA) simulations) appears. The phase behavior of the adsorbed layer is critically sensitive to the way the electrostatic interactions is included in the model. If the electrostatic charges are set to zero (as it is in UA force field), the melting temperature increases by ~70 K with respect to the experimental value. When the non-bonded 1-4 interaction is not scaled, the melting temperature decreases by ~90 K. If the scaling factor is set to 0.5, the melting occurs at T = 350 K, in very good agreement with experimental data. PACS number: 82.45.Mp, 64.70.dj