Temperature-dependent structural evolution from order to disorder in sapphire-supported imidazolium ionic liquid films

Hypothesis: Imidazolium-based ionic liquids supported on alumina are central to various technological processes. We hypothesize that molecular interactions—both ion-ion and ion-surface—govern film structure and stability as a function of temperature and concentration. Methods and simulations: We aim to optimize these systems through control of thermodynamic parameters in molecular dynamics simulations of 1,3-Dimethylimidazolium Bis-(trifluormethylsulfonyl)- imid ionic liquid monolayers spreading on hydroxylated alumina substrate at temperatures from 200 K to 400 K. We develop computational tools to analyze structural properties of molecular arrangement in the emergent monolayer, the structure of the film and the defects spontaneously forming and healing. Findings: We find that the structure of the film is sensitive to temperature, with the appearance of a crystalline-like phase within the expanding film while the bulk IL is still deep in the liquid phase. We furthermore show that surface coverage affects the level of order at low temperatures and the number of defects within the film at high temperatures. We relate these results to absolute and relative changes in cohesion energy between ions in the film and adhesion energy generated by hydrogen bonds with the surface. • When wetting a surface, liquids can form monolayers or bi-layers depending on surface-liquid and liquid-internal interaction forces. • The competition between adhesion, cohesion and thermal excitation energies causes the film to undergo a solid-like to liquid-like transition. • The transition impacts the frequency, size and shape of defects appearing in monolayers. • We provide a new analysis technique to detect shape parameters of periodic monolayers and use it to investigate these shape changes. • The surface saturation impacts the tendency to form monolayer or bi-layer structures and is a crucial factor during the initial wetting of a surface.