Liquid Nanostructure of Cholinium Argininate Biomass Solvents

Cholinium argininate ([Ch][Arg]) and water mixtures are biocompatible solvents which are efficient for the dissolution of various kinds of biomass, but the molecular origins of their efficacy are unresolved. Here, we use neutron diffraction experiments and empirical potential structure refinement fits to reveal the liquid nanostructure of 1:3 [Ch][Arg]/water, 1:10 [Ch][Arg]/water, and 1:10:0.5 [Ch][Arg]/water/guaiacol. Guaiacol addition is studied to probe solvation of a model biomass residue found in lignin. In all three systems, [Ch][Arg] and water form separate domains. Radial distribution functions reveal that cation-anion electrostatic interactions are complemented by a multitude of hydrogen bond interactions, dominated by the interactions between the argininate carboxylate group and the cholinium hydroxyl group. In 1:3 and 1:10 [Ch][Arg]/water without guaiacol, the cation charge group tends to occupy regions of space around the anion that will polarize any hydrogen bonds with the guanidine group as previously predicted in a computational study of [Ch][Arg]. This could explain the outstanding performance of [Ch][Arg] aqueous solutions for biomass breakdown. However, the 1:10:0.5 [Ch][Arg]/water/guaiacol system shows that guaiacol is solubilized primarily by the argininate carboxylate and water - so strong polarized hydrogen bonds may be responsible for biomass breakdown but are not key for guaiacol dissolution.