Towards Better Understanding of Molecular Solvent Behavior in Ionic Liquid-Biopolymer Mixtures

Abstract

Some ionic liquids (ILs) are notable for their ability to solubilize traditionally difficult to dissolve natural materials such as cellulose, silk, and wool keratin. NMR studies have shown that this ability is due predominantly to the interaction of chaotropic anions present in the IL, such as Cl- or acetate, with the hydroxyl groups present on polymeric materials. When molecular solvents are added to such mixtures, differing behaviors occur depending on the nature of the added solvent. Polar protic solvents generally serve as antisolvents, inducing the precipitation of dissolved materials. In contrast, polar aprotic solvents behave as cosolvents and are able to be added in significant fractions without causing the reconstitution of dissolved materials. It is generally understood that this differing behavior is the result of whether or not the molecular solvent preferentially interacts with the IL anion, thus preventing the anion from interacting with the polymer hydroxyl groups. However, little experimental work has been published to verify this hypothesis. In this study, the physical and transport properties (density, viscosity, ionic conductivity, and ion diffusion coefficients) are reported for 1-ethyl-3-methylimidazolium acetate (EMIAc) containing dissolved cellulose. The properties of these EMIAc-cellulose solutions were studied with no added molecular solvent, with added acetonitrile, or with added water. The fraction of ions present in the IL that are “free” (the Ionicity values) were calculated for each solution. The addition of molecular solvents to an EMIAc-cellulose solution results in significant changes in the mobility of all species present, and offers a means to better understand the molecular interactions that govern biopolymer solvation in ILs.