Structural change of ionic association in ionic liquid/water mixtures:A high-pressure infrared spectroscopic study

摘要

High-pressure infrared measurements were carried out to observe the microscopicstructures of two imidazolium-based ionic liquids, i.e., 1-ethyl-3-methylimidazoliumbis(trifluoromethylsulfonypamide EMI~+(CF_3SO_2)_2N~ˉ,EMI~+TFSA-and 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide EMI~+(FSO_2)_2N~ˉ, EMI~+FSAˉ. The results obtained atambient pressure indicate that the imidazolium C—H may exist in two different forms, i.e., isolatedand network structures. As the sample of pure EMI~+FSA- was compressed, the networkconfiguration is favored with increasing pressure by debiting the isolated form. ForEMI+TFSA-/ H_2Omixtures, the imidazolium C—H peaks split into four bands at high pressures. Thenew spectral features at approximately 3117 and 3190 cm~(-1),being concentration sensitive, can beattributed to the interactions between the imidazolium C—H and water molecules. The alkyl C—Habsorption exhibits a new band at approximately 3025 cm~(-1)under high pressures. This observationsuggests the formation of a certain water structure around the alkyl C—H groups. The 0—H stretchingabsorption reveals two types of O—H species, i.e., free O—H and bonded 0—H. ForEMI~+TFSAˉ / H_2Omixtures, the compression leads to a loss of free O—H band intensities, andpressure somehow stabilizes the bonded O—H configurations. The results also suggest thenon-negligible roles of weak hydrogen bonds in the structure of ionic liquids.