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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.

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The ionic liquid choline arginate [Ch][Arg] is more effective for biomass pretreatment than other choline based amino acid ILs, but the underlying mechanism has been unclear. In the present work we use the highlevel CCSD(T)/CBS(MP2) and G4(MP2) thermochemical protocols to probe the H-bonding interactions of [Ch][Arg] with water, and organic functional groups commonly found in biomass. We show that the [Ch][Arg] IL forms unusually strong H-bonding interactions with prototypical H-bond donors. For example, we obtain H-bonding interactions of 76.6, 80.0, and 103.6 kJ mol −1 with water, methanol, and phenol, respectively. Our theoretical results shed light on the capacity of [Ch][Arg] to dissolve biomass, and they demonstrate the importance of ion conformation, in addition to speciation, for IL performance more generally. As a point of reference, we compare the H-bonding interactions of [Ch][Arg] with those of a related IL, choline glycinate ([Ch][Gly]), which does not dissolve biomass as effectively as [Ch][Arg].