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

The hygroscopicity of nine protic ionic liquids (PILs) was first measured in air for 24 h at ambient temperature (ave. 28.9 degrees 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-infinity), sorption rate (R, kW(infinity), 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(infinity). 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 lambda(max) and was found to be positively correlated with their hygroscopicity capacity (W-24h, W-infinity) and initial rate (R-30min), while being negatively correlated with the average equilibrium (1/k), average rate (kW(infinity), 1/t(0.01)) and OH asymmetric stretching modes of water (nu(OH)) in PILs. The results show that the correlation between the hygroscopicity and polarity of PILs is complicated.