Abstract:

In the ionic liquid （IL）-1-butyl-3-methimidazole methylsulfonate （［C₄C₁im］CH₃SO₃） aqueous solutions， the solubility of the model lignin phenolic monomer 2，6-dimethoxyphenol （2，6-DMP） is the highest at IL molar fraction（x_IL） of 0.2. This phenomenon is similar to dissolution of lignin. Thus， dissolution of 2，6-DMP in ［C₄C₁im］CH₃SO₃ aqueous solution is studied using two-dimensional correlated infrared spectroscopy （2D-IR） in order to help us deeply understand the dissolution mechanisms of lignin in IL aqueous solutions. In the case of x_IL =0.02 ~ 0.2， adding ILs could destroy the weak hydrogen bonding structure of water， and the anion and cation ions of ILs are in the form of hydrated ionic pairs； relative hydrophobic 2，6-DMP dissolution is greatly enhanced by the interactions between 2，6-DMP and ［C₄C₁im］⁺， CH₃SO₃^-. In the case of x_IL=0.2 ~ 1.0， ILs form dense ionic clusters structure which is unfavourable for the interactions between 2，6-DMP and the anion， cation ions of ILs. The 2D-IR analyses with 2，6-DMP molar fraction （x_DMP） as disturbance show that adding 2，6-DMP slightly influences the ILs structure， but greatly affects the microstructure of water. For the IL aqueous solutions with ionic pairs structure （x_IL=0.1）， water molecules preferentially interactes with CH₃SO₃^-； for the IL solutions with ionic clusters （x_IL=0.6）， water forms the self-associated small clusters. The microstructure of IL aqueous solutions affects the interactions of IL-water-2，6-DMP and solubility， and at the same time 2，6-DMP dissolution influences the variation tendency of the solution microstructure.

Key words: Ionic liquid, Water, 2, 6-Dimethoxyphenol, Solubility, Two-dimensional correlation of infrared spectroscopy