Investigation of capillary waves and thermal diffusion at the air–liquid interface of imidazolium-cation-based ionic liquids using transient grating spectroscopy

Abstract

The capillary wave and thermal diffusion at the gas–liquid interface of 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [Cnmim][NTf2] (n = 4, 8, 12; where n indicates the number of carbons in the alkyl chain) were investigated using interface-selective transient grating (TG) spectroscopy. At the interface, the TG signal exhibited capillary wave damping (with no capillary wave oscillation). Moreover, the capillary wave damping constant was greater than that predicted by the hydrodynamic theory; the deviation increased with the cation alkyl-chain length. The thermal diffusivities at the gas–liquid interface of [C8mim][NTf2] and [C12mim][NTf2] were lower than those in the bulk, whereas both the values at the interface and in the bulk were similar for [C4mim][NTf2]. The difference is possibly owing to the formation of a layered structure at the interface.