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Ionic liquids (ILs) based on 1-butyl-3-methylimidazolium (bmim) cation have proved to be promising solvents for the fractionation of plant biomass with the production of cellulose and lignin. This study deals with the characterization of lignins isolated from coniferous (spruce) wood using [bmim]OAc and [bmim]MeSO4 ionic liquids and their binary mixtures with DMSO (80:20). Molecular weight distributions, functional composition, and structural features of IL lignins were studied by size-exclusion chromatography, NMR spectroscopy (1D and 2D) and atmospheric pressure photoionization high-resolution mass spectrometry. It was shown that the interaction of ILs with lignin leads to significant chemical changes in the biopolymer; a decrease in the degree of polymerization and in the content of free phenolic hydroxyl groups due to alkylation, the disappearance (in the case of [bmim]OAc) of carbonyl groups and a significant destruction of β-O-4 bonds. The chemical reactions between lignin and 1-butyl-3-methylidazolium cation with covalent binding of ionic liquids or products of their decomposition is evidenced by the presence of a large number of nitrogen-containing oligomers in IL lignins.

Solvatochromic polarity parameters ( , α, β, and π*) for binary mixtures of 1-butyl-3methylimidazolium acetate-ionic liquid, which is a promising solvent of lignocellulosic materials with water, methanol, and dimethylsulfoxide, are determined via UV spectroscopy. A Redlich-Kister (CNIBS/R-K) equation modified for a quasi-ideal binary solvent is used to describe algebraically the dependences between the Dimroth-Reichardt energies, the Kamlet-Taft parameters, and the composition of binary mixtures. The addition of molecular solvents to the ionic liquid in significant amounts was shown to produce no radical changes in its polarity or donor and acceptor characteristics, due to the incorporation of a cosolvent into supramolecular clusters formed by alkylimidazolium cations.

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