Water sorption in protic ionic liquids: correlation between hygroscopicity and polarity

Abstract

The hygroscopicity of nine protic ionic liquids (PILs) was first measured in air for 24 h at ambient temperature (ave. 28.9 °C) and relative humidity (ave. 56.6%). The hygroscopic process was qualitatively described by a sorption triangle, which connects well the three types of water sorption parameters, i.e., sorption capacity (C, W_24h, W_∞), sorption rate (R, kW_∞, 1/t_0.01, R_30min), and sorption equilibrium (E, 1/k). Then, the hydrophilicity of the PILs was derived by the steady-state sorption capacity 100W_∞. The results show that PILs are highly hygroscopic and have greater hydrophilicity compared to aprotic ionic liquids. Finally, the polarity of the PILs was indicated by Nile Red with λ_max and was found to be positively correlated with their hygroscopicity capacity (W_24h, W_∞) and initial rate (R_30min), while being negatively correlated with the average equilibrium (1/k), average rate (kW_∞, 1/t_0.01) and OH asymmetric stretching modes of water (ν_OH) in PILs. The results show that the correlation between the hygroscopicity and polarity of PILs is complicated.