Probing Molecular Interactions in 1-Butyl-3-methylimidazolium Chloride-Water and 2,6-Dimethoxyphenol Mixtures Using Attenuated Total Reflection Infrared Spectroscopy

Molecular interactions of the ternary mixtures of 1-butyl-3-methylimidazolium chloride (C₄C₁imCl)-water-2,6-dimethoxyphenol (2,6-DMP, a phenolic monomer lignin model compound) were investigated in comparison with the C₄C₁imCl-water binary systems through attenuated total reflection infrared spectroscopy. Results indicated that the microstructures of water and C₄C₁imCl changed with varying mole fraction of C₄C₁imCl (^xIL) from 0.01 to 1.0. This change was mainly attributed to the interactions of C₄C₁imCl-water and the self-aggregation of C₄C₁imCl through hydrogen bonding. The band shifts of C-H on imidazolium ring and the functional groups in 2,6-DMP indicated that the occurrence of intermolecular interactions by different mechanisms (i.e., hydrogen bonding or π-π stacking) resulted in 2,6-DMP dissolution. In the case of ^xIL=0.12, the slightly hydrogen-bonded water was fully destroyed and C₄C₁imCl existed in the form of hydrated ion pairs. Interestingly, the maximum 2,6-DMP solubility (238.5 g/100 g) was achieved in this case. The interactions and microstructures of C₄C₁imCl-water mixtures influenced the dissolution behavior of 2,6-DMP.