The surface structure of dicationic ionic liquids (DILs) of (1, n- (bis(3-methyl imidazolium-1-yl) alkane bis{(trifluoromethyl) sulfonyl}amide ([Cn(mim)2][TFSA]2, (n=4~8)) has been investigated using in-situ IR-visible sum-frequency generation vibrational spectroscopy (IV-SFVS). Polar orientations of dications and anions have been clearly observed: imidazolium rings of dications are lying flat on the surface while TFSA anions of C1 conformer point SO2 groups to the bulk side, which are all common to mono-cationic ionic liquids. A new phenomenon found in DIL is that the SF signal of the CH2,as mode of dications increases very slowly and saturates after some hours. This slow relaxation of DIL configurations also exhibits a strong dependence of the length of the alkyl chain connecting two imidazolium rings. A kinetic study of the SF signal reveals that the slow relaxation is well described by a self-catalytic behavior, indicating the presence of the surface specific aggregation of DIL. Activation energy is obtained as 13 kJ/mol, which is about the same as a gauche-trans conformational barrier height. Alkyl chain length dependence allows us to deduce the model configuration of DILs at surfaces in relation to the bulk crystal structures. Ultra slow and self-catalytic aggregation process of DIL surface is driven by the linker-chain induced strong hindrance to the bulk aggregated structure.

Ultra slow relaxation process at surfaces of dicationic ionic liquids

248th National Meeting (2014)

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