Assessing Cellulose Dissolution Using Kamlet-Taft Parameters of [Emim][DEP]-Molecular Solvent Mixtures

Ionic liquids (ILs), defined as salts with melting points below 100°C, are being investigated for their ability to dissolve cellulose, a renewable polymer which is soluble in few conventional solvents because of its tendency to preferentially hydrogen bond with itself. Cellulose can be converted to a variety of valuable chemical products, motivating interest in effective, green solvents for the chemical processing of cellulose. The IL of interest in this study, 1-ethyl-3-methylimidazolium diethyl phosphate ([EMIm][DEP]), possesses one of the highest cellulose solubilities known for ILs (~20% by mass at 100°C). However, two primary disadvantages of [EMIm][DEP] is its high viscosity—negatively impacting mass transfer and momentum transfer properties—and high cost. A potential solution to these problems is to use mixed IL-organic solvent systems instead of pure ILs for cellulose dissolution. It has been shown that aprotic organic solvents are suitable for this purpose, in that they do not appreciably affect cellulose dissolution. Alternatively, protic organic solvents decrease—sometimes drastically—the cellulose dissolution capabilities of the mixture, relative to the pure [EMIm][DEP]. Organic solvents which only marginally affect cellulose solvation are designated “cosolvents”, while those that decrease cellulose solubility are designated “antisolvents”

Motivated by the desire to understand the specific and non-specific interactions occurring in [EMIm][DEP]-cosolvent systems versus those occurring in [EMIm][DEP]-antisolvent systems, the solvatochromic probes 4-nitroaniline (4-Na), N,N-diethyl-4-nitroaniline (NNDE4Na) and Reichardt’s dye (Re. D) were utilized in conjunction with UV/Vis spectroscopy to calculate the empirical α, β, and π* solvatochromic solvent parameters of Kamlet and Taft (KT parameters) for a variety of systems. Systems studied include binary mixtures of [EMIm][DEP] and the aprotic organic solvents 3-dimethyl-2-imidazolidinone (DMI), dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), acetone, acetonitrile (ACN) and two protic solvent, ethanol (EtOH) and water (H2O). For each binary system, the parameters were obtained across the range of compositions. The KT parameters α and β measure the hydrogen bond donating and accepting ability of a mixture. Similarly, π* corresponds to a systems polarity and polarizability. Plots of the three KT parameters as a function of composition will be presented to demonstrate preferential solvation and solvation mechanisms occurring in these systems; these plots correlate with analogous cellulose dissolution plots.