In situ monitoring the moisture absorption of three ionic liquids with different halogen anions by ATR-FTIR spectroscopy

Ionic liquid is a promising cellulose solvent in the future, while moisture absorption hinders the cellulose dissolution. In the present work, we in situ monitored the procedure of moisture absorption of three 1-butyl-3-methylimidazolium cation based ionic liquids with variation of halogen anions. According to the peak position shifts across the moisture absorption process, we analyzed the structure variation of ionic liquids during the process. Besides, we analyzed the hydrogen bond interaction between halogen anion and imidazolium cation CHs, including ring CHs and side alkyl chain CHs, based on infrared absorption bands position shifts of the three ionic liquids. In addition, two dimensional correlation spectroscopy was applied to enhance the spectra resolution, unveiling the transformation of ionic liquids after absorbing moisture for the split of peak attributed to ring C2H stretching. In summary, we provide reliable evidence to analyze the structure variation of ionic liquid during moisture absorption process and hydrogen bond interaction of halogen anion with imidazolium cation ring/side alkyl CHs from the perspective of infrared spectroscopy.