Mapping the Three-Dimensional Nanostructure of the Ionic Liquid-Solid Interface Using Atomic Force Microscopy and Molecular Dynamics Simulations

Ionic liquids (ILs) are a widely investigated class of solvents for scientific and industrial applications due to their desirable and "tunable" properties. The IL-solid interface is a complex entity, and despite intensive investigation, its true nature remains elusive. The understanding of the IL-solid interface has evolved over the last decade from a simple 1D double layer, to a 2D ordered interface, and finally a liquid-specific, complex 3D ordered liquid interface. However, most studies depend solely on one technique, which often only examine one aspect of the interfacial nanostructure. Here, a holistic study of the protic IL-solid interface is presented, which provides a more detailed picture of IL interfacial solvation. The 3D nanostructure of the ethylammonium nitrate (EAN)-mica interface is investigated using a combination of 1D, 2D, and 3D amplitude modulated-atomic force microscopy and molecular dynamics simulations. Importantly, it is found that the EAN-mica interface is more complex than previously reported, possessing surface-adsorbed, near-surface, surface-normal, and lateral heterogeneity, which propagates at relatively large distances from the solid substrate. The work presented in this study meaningfully enhances the understanding of the IL-solid interface.