Structure in confined room-temperature ionic liquids

Solvation force profiles for three room-temperature ionic liquids (ILs), ethylammonium nitrate (BAN), propylammonium nitrate (PAN), and l-ethyl-3-methylimidazolium acetate (C₂mimAc) confined between Si₃N₄ tips and mica, silica, and graphite have been measured using an atomic force microscope. The measurements reveal oscillatory behavior in all cases, with the size of the oscillations corresponding to the physical dimension of the ion pair. The surface charge and roughness and the orientation of cations at the interface are critical determinants of solvation layer formation in ILs. The greatest number of solvation layers is observed for BAN on highly charged, atomically smooth mica. Fewer and more compressible layers are observed for PAN due to its increased molecular flexibility. The lower surface charge and increased roughness of silica produces fewer solvation layers for both BAN and PAN compared to mica. For the BAN- and PAN-graphite systems, any attractive interaction with the substrate is due to the alkyl groups of the amine. This attraction is greater for PAN due to the increased size of the alkyl moiety, leading to stronger solvation forces. Six or seven solvation layers are present in the CamimAc-graphite system. The C ₂mim⁺ ion adopts a flat orientation relative to this the substrate, which is more favorable for solvent layer formation for this IL, due to favorable interactions between the alkyl backbone of the cation and the substrate. Fewer layers are detected on mica and silica because cations in the interfacial layer are orientated with the ethyl group facing the bulk due to electrostatic interactions with these substrates.