Molecular-Resolution Imaging of Ionic Liquid/Alkali Halide Interfaces with Varied Surface Charge Densities via Atomic Force Microscopy

Ionic liquids (ILs) in contact with a charged solid surface are known to form layered solvation structures consisting of alternating cation and anion layers. Understanding the response of these layers to surface charge density is vital for IL-based energy storage systems, but it remains inadequately understood. This is partly due to inconsistent experimental conclusions regarding low surface charge density and scarce experimental results for high surface charge density. Here, we probe the solvation structure of ILs on alkali halide surfaces with varied surface orientations: slightly charged RbI(100) and highly charged RbI(111) surfaces to shed light on the above issues, by employing frequency modulation atomic force microscopy (FM-AFM) with a high spatial resolution. Two commonly used ILs, 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C3mpyr][NTf2]) and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) are selected for this study. On the slightly charged RbI(100) surface, we observe alternating cation and anion layers, diverging from the previously proposed monolayer model for IL/alkali halide(100) interfaces. On the highly charged RbI(111) surface, we find crowded layers, which is hardly observed in commonly used ILs due to typically unreachable surface charge densities in electrochemical IL/electrode systems. Our data experimentally elucidates the response of ILs to surface charge density, being expected to deepen our understanding of IL solvation structures on charged solid surfaces. Specifically, observing multilayers on alkali halide(100) surfaces could reconcile some controversial results regarding low surface charge density, whereas detecting crowded layers on alkali halide(111) surfaces offers a new platform for exploring the crowding phenomenon.