Liquid Nanostructure of Cholinium Argininate Biomass Solvents

Abstract

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.

Design and applications of biocompatible choline amino acid ionic liquids

“…Structural and computational studies suggested they can be further polarised by nearby choline groups, to become even stronger H-bond donors. 28 This may be important to intercept the existing lignin–cellulose H-bonds in biomass through a competitive process, making strong H-bond donors a desirable feature to have on the anion sidechain for this application.…”

“…Between the two comprehensive studies, 23,27 values reported by De Santis et al are significantly higher than those of Liu et al The former is more widely supported by other studies such as ChAsp, Ch 2 Asp and ChArg being room temperature solids. 28,29 Despite the differences in reported values, it is generally agreed that most ChAAILs are highly viscous, making them difficult to handle under room conditions. The presence of additional hydrogen bonding sites on the amino acid side chains, such as Asp − , Arg − , Glu − , His − , etc ., seems to further increase viscosity.…”

“…Table 1 Published physical properties of ChAAILs at 25 °C. 23,27,28,35 Including viscosity (η), density (ρ), molar volume calculated from ChAAIL density (V m ), molar volume estimated by the sum of molar volume of respective ion pair (V sum m ), [36][37][38] While all species are charged in ILs, Table 1 shows that measured conductivities of ChAAILs vary widely, and their prediction from ionic strength is by no means trivial. Oppositely charged ions exhibit concentration-dependent ion-pair formation even in aqueous solutions in which the resultant ion pair behaves as one unit.…”

“…Unlike many conventional ILs, numerous studies have shown that ChAAILs can maintain their high specificity and efficiency as extraction solvents upon water dilution. 28,34,82,86,90 Their high effectiveness at dissolving coal and biomass is attributed to their ability to form strong and extensive H-bonds by the amino acid anion and the solute. 28,90 This is only slightly affected by the addition of water up to 40 wt%.…”

Evolution of microstructures and hydrogen bond interactions within choline amino acid ionic liquid and water mixtures

“…17 However, to date, compared with tremendous experimental and theoretical investigations of imidazolium based IL–water mixtures, 18 only a few reports have focused on the interactions and microstructures of [Cho][AA]–water mixtures. And these studies only involve the single AA − anion (such as glycine, 16 lysine 19 and arginine 17 ), the effect of the AA structure on dynamic interactions between [Cho][AA] and water molecules, and on the resulting microstructures are still blank. Based on the fact that the anion structure of the IL plays a predominant role in its interaction ability and application efficiency, 20 the influences of the side chain length and residue functional group of the AA − anion on interactions and microstructures of [Cho][AA]–water mixtures need to be extensively explored to achieve various applications.…”