| Abstract
| - Solvation force profiles for three room-temperature ionic liquids (ILs), ethylammonium nitrate (EAN),propylammonium nitrate (PAN), and 1-ethyl-3-methylimidazolium acetate (C2mimAc) confined between Si3N4tips and mica, silica, and graphite have been measured using an atomic force microscope. The measurementsreveal oscillatory behavior in all cases, with the size of the oscillations corresponding to the physical dimensionof the ion pair. The surface charge and roughness and the orientation of cations at the interface are criticaldeterminants of solvation layer formation in ILs. The greatest number of solvation layers is observed forEAN on highly charged, atomically smooth mica. Fewer and more compressible layers are observed for PANdue to its increased molecular flexibility. The lower surface charge and increased roughness of silica producesfewer solvation layers for both EAN and PAN compared to mica. For the EAN− and PAN−graphite systems,any attractive interaction with the substrate is due to the alkyl groups of the amine. This attraction is greaterfor PAN due to the increased size of the alkyl moiety, leading to stronger solvation forces. Six or sevensolvation layers are present in the C2mimAc−graphite system. The C2mim+ ion adopts a flat orientationrelative to this the substrate, which is more favorable for solvent layer formation for this IL, due to favorableinteractions between the alkyl backbone of the cation and the substrate. Fewer layers are detected on micaand silica because cations in the interfacial layer are orientated with the ethyl group facing the bulk due toelectrostatic interactions with these substrates.
|
