Viscosity and Rheology of Ionic Liquid Mixtures Containing Cellulose and Cosolvents for Advanced Processing

Select ionic liquids (ILs) are capable of dissolving significant quantities of biomass (cellulose, hemicellulose, and lignin) but are limited by prohibitively high viscosities even for relatively small biomass loadings. This may be rectified by using mixtures of ionic liquids with certain polar aprotic solvents, which can improve thermodynamic solubility, reduce transport properties, and decrease solvent cost compared to,using pure ionic liquids. Here, the viscosity and rheology of microcrystalline cellulose dissolved in IL/cosolvent mixtures were measured at 40 degrees C and 60 degrees C using 1-ethyl-3-methylimidazolium diethyl phosphate [EMIm][DEP] and dimethyl sulfoxide (DMSO) as a model ionic liquid (IL) and cosolvent. Increasing the DMSO cosolvent composition significantly reduced the apparent viscosity of IL/cellulose mixtures. The zero-shear viscosity and Power Law (Ostwald-de Waele) parameters are reported for the various mixtures. The cellulose solution exhibits increasingly Newtonian behavior with higher cosolvent loadings. The cost of a mixed solvent system for processing cellulose was analyzed. Optimal ratios of cosolvent to ionic liquid based on cost and solubility were found to be between 25-50 mass% of cosolvent, which corresponds to viscosities that were lower than the pure IL system by up to 80%.