Criticality in an imidazolium ionic liquid fully wetting a sapphire support

TL;DR

This study uses molecular dynamics simulations to investigate how an imidazolium ionic liquid monolayer on sapphire undergoes a temperature-dependent phase transition around 300K, revealing critical behavior in its structural properties.

Contribution

The paper introduces a detailed simulation approach to analyze the critical behavior of ionic liquid monolayers on sapphire, highlighting the impact of temperature and confinement on phase transitions.

Findings

Identified a phase transition at ~300K from solid-like to liquid-like behavior.

Observed crystalline order below critical temperature and isotropic pattern above.

Detected dynamic defects and structural changes related to temperature.

Abstract

Hypothesis: Ionic liquids have various applications in catalytic reaction environments. In those systems, their interaction with interfaces is key to their performance as a liquid phase. We hypothesize that the way a monolayer ionic liquid phase interacts with interfaces like a sapphire substrate is significantly dependent on temperature and that critical behavior can be observed in the structural properties of the liquid film. Methods and simulations: We perform molecular dynamics simulations of imidazolium-based ionic liquid monolayers deposited on a sapphire substrate at temperatures from 200K to 400K. We develop computational tools to analyze structural properties of molecular arrangement in the monolayer, the structure of the film and the defects spontaneously forming and healing. Findings: We observe a clear structural phase transition at around 300K from a solid-like to a liquid-like behavior of a film. Below the critical point an alternating crystalline structure of cations and anions with alignment of periodic vectors with the underlying substrate grid is observed, with frozen defects. Above the critical temperature, the pattern becomes isotropic within the contact layer that displays dynamic defects of a characteristic size. Our results highlight the importance of confinement to the phase behavior of the system.