How to dissolve cellulose in water: Alkaline conditions

For centuries there has been great interest in the dissolution of cellulose. Regarding industrial uses, water-based solvents remain the primary focus. It has long been known that concentrated solutions of sodium hydroxide can dissolve significant amounts of cellulose, but only at temperatures well below ambient conditions. This review discusses a general thermodynamic framework for cellulose dissolution in aqueous systems, emphasizing the balance between electrostatic and hydrophobic contributions. In alkaline aqueous systems, cellulose dissolution is governed by hydroxide-induced ionization via deprotonation of glucopyranose hydroxyl groups and the resulting balance between electrostatic repulsion and hydrophobic interactions, with ionization providing the primary driving force. Whereas the electrostatic driving force is now better established, the factors opposing dissolution have remained controversial, although recent studies increasingly support an important role of hydrophobic effects. The strong temperature dependence of cellulose solubility, favored at low temperatures, has long lacked a satisfactory explanation. A recent thermodynamic analysis could demonstrate that temperature-dependent electrostatic and hydrophobic interactions can quantitatively reproduce experimental observations, including a well-known phase map. This treatise discusses thermodynamics of cellulose dissolution and phase separation in aqueous media, including differences between counterions, effects of additives, cellulose crystallization, and solution instability.