Abstract
Ionic liquids (ILs) with an upper critical solution temperature (UCST) are feasible candidates as draw solutions in the forward osmosis process. However, the factors controlling the phase separation of UCST-type ILs in aqueous solutions remain unclear. Here, we focused on the role of hydrogen bonding in the phase separation behavior of UCST-type ILs. The phase separation behavior of aqueous solutions of UCST-type ILs is affected by the hydrogen-bonding functional groups of the ILs. A thermodynamic analysis of the mixing of water and ILs indicated that the phase separation of the aqueous solution of UCST-type ILs is characterized by enthalpy changes of mixing. To investigate the state of hydrogen bond formation in aqueous solutions of ILs, 1H nuclear magnetic resonance spectra were acquired. The data suggest that the hydrophobic parts of the ILs broke the water–water hydrogen bonds, and the hydrogen-bonding functional groups of the IL formed hydrogen bonds in the aqueous IL solution. These hydrogen-bonding states strongly affect the enthalpy changes of mixing. Therefore, water–water and water–IL hydrogen bonds play an important role in the phase separation behavior of UCST-type ILs in aqueous solutions.
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