The Impact of Carbohydrate Solutes on the Ionicity of 1-Ethyl-3-Methylimidazolium Acetate Ionic Liquid Solutions

Abstract

Selected ionic liquids have been shown to be effective solvents for biopolymers. This aspect of ILs has been leveraged in a process called Natural Fiber Welding (NFW) which enables the reconfiguration of biopolymer materials while retaining much of their native structure. Biopolymer dissolution in ionic liquids is largely driven by the interaction of a chaotropic anion and the hydrogen bonds present in biopolymers. This suggests that increasing the concentration of an IL should lead to a greater degree of biopolymer mobilization. However, previous work in our lab has shown that the addition of large mole fractions of polar, aprotic solvents, up to 75 mole-%, can actually increase the degree to which NFW modifies a biopolymer substrate. We have speculated this improvement in the NFW ability of an IL solvent could be the result of the impact of the solvent and/or biopolymer on the anion availability. Ions present in an ionic liquid usually have a high degree of ion aggregation between cations and anions, with 20 to 50% of ions being aggregated. Any anions that are present in such aggregates are less available to interact with biopolymer chains and thus will contribute less to biopolymer mobilization. Ion aggregation can be adjusted through a number of approaches, including changing temperature or introducing a co-solvent to solvate ions. Therefore, the addition of aprotic solvents may, in reality, result in an increase in the availability of anions to disrupt hydrogen bonding, even if the overall concentration of anions actually decreases. The ionic character of a solvent mixture can be characterized by determination of its ionicity. Ionicity is the ratio of diffusion-based estimated ion conductivity to measured ion conductivity, where ionicity values less than unity indicate the presence of ion-aggregation. For highly aggregated ionic liquids, ionicity values less than 0.5 are typical. By monitoring changes in ionicity, the impact of co-solvents and solution on the ionic character of an ionic liquid can be assessed. Changes in ionicity were evaluated for dissolved cellulose, cellobiose, and glucose in 1-ethyl-3-methylimidazolium acetate (EMIAc) and 1:1-mole-ratio mixtures of EMIAc with acetonitrile (1:1 EMIAc:AN) and water (1:1 EMIAc:H2O). Test solutions were made with these three solvent types by dissolving 0.1, 0.5, 1.0, and 3.0 wt.-% of either cotton, microcrystalline cellulose (MCC), cellobiose, or glucose. EMIAc’s ionicity decreased with the 1:1 addition of AN by 27 and 19 % at 20 and 60°C, respectively, and EMIAc’s ionicity increased with the 1:1 addition of H2O by 13 and 22 % at 20 and 60°C, respectively. No significant changes in ionicity were observed with the addition of the carbohydrate solutes. Solutes’ negligible changes to ionicity are consistent with how these solutes produce almost indistinguishable decreases in molar conductivity at given solute content for a given solvent type, suggesting the ion aggregation and conductivity behavior remain unchanged with solute up to 3.0 wt.-%.