Lignocellulosic biomass is an abundant and underutilized resource which could be used to create fuels, plastics, composites, and commodity chemicals. Cellulose, the main structural component, has an extensive hydrogen bonding network and strong intersheet van der Waals interactions, requiring amphiphilic solvents for solubility. Aqueous quaternary ammonium hydroxides (QAH) show high solubilities of cellulose at room temperature, high water concentrations and even enhanced solubility in the presence of the cosolvent urea. This study investigates cellulose solubility in aqueous QAH/urea solutions using two-state molecular dynamics simulations. A fixed concentration of cellulose was added to predetermined concentrations of QAH, water and urea in both crystalline and dissolved states. Points were purposefully chosen both inside and outside of the experimentally determined solubility envelope, with the aim of understanding the molecular-level driving forces of solubility. Through these simulations, both the enthalpy and the entropy differences between the two states were calculated, allowing us to estimate the free energy of dissolution. We observe an excellent agreement between the calculated free energy of dissolution and the experimentally determined maximal solubility of cellulose at these specified concentrations lending confidence in our explanation of the molecular-level driving forces. This talk explores how the structure of QAH, the concentration of water and the presence of urea all contribute the favorability of dissolution in these systems at the molecular-level. We show that cosolvents such as urea can enhance the effects that drive dissolution, and explore how this effect can be leveraged to design cheaper, more effective solvents, allowing for the full potential of cellulose to be realized.

Cellulose solubility in aqueous quaternary ammonium hydroxide and urea using molecular dynamics simulations

2019 Spring National Meeting

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