Crystalline Structuring of Confined Ionic Liquids at Room Temperature

Ionic liquids (ILs) are widely used as electrolytes and lubricants in many energy storage devices and microsystems because of their low vapor pressure, thermal stability, and also their advantageous mechanical and electrochemical properties. Despite this, they often display substantially different behaviors in confined systems where they are employed, compared to their bulk counterparts. Much details of glassy or solid-like behavior, concerning peculiar structural and dynamic properties of the ILs confined in a constrained geometry, are provided in the scientific literature. Here, we report the presence of correlations of the liquid phase of [Bmim][Tf2N] revealed by X-ray scattering experiments and supported by molecular dynamics (MD) simulations, which stress the leading role of hydrogen-bonding interactions between the anion and cation already formed in the bulk IL. The organization of the IL at the nanoscale characterizing the liquid phase provides the architecture for the formation of solid-like IL terraces formed at the interface with oxidized substrates in a confined geometry, whose crystalline structuring at room temperature has been observed for the first time. Furthermore, the employment of transmission electron microscopy (TEM) to study the crystal nature of such local surface-induced IL solid-like structures is an innovative strategy and powerful benchmark for the investigation of such systems confined at the nanoscale.