Thermodynamic and Kinetic Anisotropies in Octane Thin Films

TL;DR

This study investigates how substrate-liquid interactions influence thermodynamic and kinetic anisotropies in octane nanofilms, revealing distinct freezing behaviors, dynamic regimes, and correlations near interfaces, with implications for confined molecular systems.

Contribution

It provides a detailed analysis of anisotropic thermodynamic and kinetic behaviors in octane thin films under various substrate interactions, highlighting novel interface effects and dynamic regimes.

Findings

Freezing occurs at low temperatures in octane nanofilms.

Frozen monolayers form at interfaces at intermediate temperatures.

Loose substrates accelerate, sticky substrates decelerate dynamics significantly.

Abstract

Confinement breaks the translational symmetry of materials. Such symmetry breaking can be used to obtain configurations that are not otherwise accessible in the bulk. Here, we explore the effect of substrate-liquid interactions on the induced thermodynamic and kinetic anisotropies. We consider n-octane nanofilms that are in contact with substrates with varying degrees of attraction. Complete freezing of octane nanofilms is observed at low temperatures, while at intermediate temperatures, a frozen monolayer emerges at both interfaces. By carefully inspecting the profiles of translational and orientational relaxation times, we confirm that the translational and orientational degrees of freedom are decoupled at these frozen monolayers. At sufficiently high temperatures, however, free interfaces and solid-liquid interfaces close to loose substrates undergo pre-freezing, characterized by mild peaks in several thermodynamic quantities. Two distinct dynamic regimes are observed. The dynamics is accelerated in the vicinity of loose substrates, while sticky substrates decelerate dynamics, sometimes by as much as two orders of magnitude. These two distinct dynamical regimes have been previously by us [JCP 141: 024506, 2014] for a model atomic glass-forming liquid. We also confirm the existence of two correlations proposed in the above-mentioned work in solid-liquid subsurface regions of octane films, i.e., a correlation between density and normal stress, and between atomic translational relaxation time and lateral stress. Finally, we inspect the ability of different regions of a film to explore the potential energy landscape, and observe no noticeable difference between the free surface and the bulk. This is unlike the films of model atomic glass formers that tend to sample their respective landscape more efficiently at free surfaces.