Interface-Induced Ordering of Gas Molecules Confined in a Small Space

TL;DR

This study reveals how gas molecules confined at interfaces between water and hydrophobic solids form ordered structures, influenced by interfacial interactions, challenging classical thermodynamics of bulk gases.

Contribution

It demonstrates that interfacial structures and ordering of gases are primarily governed by surface interactions, providing new insights into confined gas thermodynamics.

Findings

Ordered epitaxial layers of gases observed

Gas molecules form nanostructures under confinement

Interfacial water stabilizes gas structures

Abstract

The thermodynamic properties of gases have been understood primarily through phase diagrams of bulk gases. However, observations of gases confined in a nanometer space have posed a challenge to the principles of classical thermodynamics. Here, we investigated interfacial structures comprising either O2 or N2 between water and a hydrophobic solid surface by using advanced atomic force microscopy techniques. Ordered epitaxial layers and cap-shaped nanostructures were observed. In addition, pancake-shaped disordered layers that had grown on top of the epitaxial base layers were observed in oxygen-supersaturated water. We propose that hydrophobic solid surfaces provide low-chemical-potential sites at which gas molecules dissolved in water can be adsorbed. The structures are further stabilized by interfacial water. Gas molecules can agglomerate into a condensed form when confined in a sufficiently small space under ambient conditions. The ordering and thermodynamic properties of the confined gases are determined primarily according to interfacial interactions. The crystalline solid surface may even induce a solid-gas state.