Cellulose is the most abundant organic compound on earth, with an enormous potential to be used as a feedstock for sustainable fuels and chemicals. Its usage, however, is limited and one major reason for this is its resistance to dissolution in most common organic solvents. Many different ionic liquids (ILs) have been found to be efficient in dissolving cellulose, however these too suffer from a number of drawbacks such as cost, stability, high viscosity and ineffectiveness in the presence of water. One solvent that seems to circumvent many of these issues are aqueous systems of quaternary ammonium and phosphonium hydroxide, which dissolves cellulose despite being composed of roughly 50% water, a known anti-solvent. Moreover, it's been shown that the addition of urea to these systems has an enhancing effect on the overall solubility. Despite the ubiquity of cellulose, the underlying mechanisms of this processes are not well understood, which could lead us to further breakthroughs in effective pretreatments of biomass.
In this work, we use molecular dynamics simulations to study the effect of urea and water concentrations on cellulose dissolution in quaternary onium salts. Varying proportions of salt, water and urea were studied over the entire composition range to first elucidate the role that these had on the bulk properties of the solvent including solvent structure, transport properties and hydrogen bonding ability. Additionally, selected compositions of the solvent mixtures spanning both inside and outside of the dissolution range were simulated and the thermodynamics, solution state and properties of the interface analyzed. Through this we provide a deeper understanding of the molecular driving forces of dissolution and the enhancing role of urea in this process.