Unexpected cosolvency of water on poly(propylene glycol) in hydrophobic ionic liquid

By addition of small amount of water, lower critical solution temperature (LCST)–type cloud point (CP) of poly(propylene glycol) (PPG) in hydrophobic ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, was found to largely increase, while upper critical solution temperature–type CP appeared and increased at lower temperatures. Spectral measurements revealed that the LCST-type CP at higher temperature was caused by rupturing of hydrogen bonding between [C₂mim]⁺ cation and oxygen atom of PPG. Solvation of [C₂mim]⁺ cation would be enhanced after release from ion pair with [TFSI]⁻ anion by complex formation of [TFSI]⁻ anion with H₂O, which was supported by increase in endothermic enthalpy for the desolvation estimated by DSC measurements. This specific solvation would cause the energetic stabilization of PPG, which increased the LCST-type CP.

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