Unraveling the proton transfer of cellulose dissolution in aqueous NaOH/Urea solution: insights from thermodynamics by molecular dynamics simulation

Abstract

Sodium hydroxide (NaOH) plays a crucial role in the coordination structure and enthalpy contribution of cellulose dissolution. This paper utilized classical molecular dynamics (MD) simulation to reveal that NaOH mainly bound to the cellulose chain through electrostatic interaction, which reduced the electrostatic interaction between cellulose and water, but increased the London dispersion interactions between water and cellulose. In addition, the mixing of NaOH solution and cellulose solution was an exothermic process, with the largest contribution coming from cellulose and hydroxide ions (OH−). NaOH also affected the free energy change by reducing the enthalpy change of the system. The hydroxyl group at the C2 losed protons, "temporarily" transforming the molecular chain into a polyelectrolyte. Subsequently, protons were recaptured from the surrounding water molecules to achieve the diffusion of OH− around cellulose. This "activation" phenomenon involving deprotonation followed by reduction can provide additional "reaction enthalpy" for the dissolution of cellulose and increase the free energy of dissolution. With regard to Na+, it complexed with three oxygen atoms (O3, O5 and O6) near the glycosidic bond of cellulose simultaneously, and additionally complexes with around 3 water molecules. The total oxygen coordination number of Na+ on the surface of the cellulose chain was 0.2 more than that of Na+ in the bulk phase, leading to a certain enthalpy reduction effect. This study highlights that the enthalpy effect of NaOH may be a key step in the low-temperature dissolution of cellulose using NaOH solvent, and it may also be a necessary step in the cellulose derivatization reaction process.