RISM-assisted analysis of role of alkali metal hydroxides in the solvation of cellulose in alkali/urea aqueous solutions

The three-dimensional reference interaction site model theory with the Kovalenko–Hirata closure (3D-RISM–KH) combined with the Kirkwood–Buff integral (KBI) was used to clarify the role of alkali metal hydroxides (MOHs) in cellulose solvation in alkali/urea aqueous solutions. Pair distribution functions, KBI, and the excess number of MOHs showed that M⁺ hydrates were formed close to cellulose and that their distance was the same as the distance between M⁺ ions and water molecules in the hydrates. The most stable Li⁺ hydrate due to the highest Li⁺ charge density was the closest to the cellulose resulting in the most electrostatic interaction and possibly hydrogen bonding with the cellulose. However, K⁺ had the lowest charge density, formed the least stable hydrate, and had the least interaction with the cellulose. Hence, the direct solvation energy, which is part of the cellulose solvation energy and accounts for the solute–solvent interaction, was the most negative in the LiOH/urea solution. The solvent reorganization energy—which is another part of the cellulose solvation energy and arises from the clustering of urea, water, and MOH (i.e., ion hydrates) around cellulose—was the most negative in the LiOH/urea solution because of the highest probability and the closest positioning of the Li⁺ hydrate to the cellulose. Therefore, the calculation results obtained using 3D-RISM–KH and KBI explained the difference among the cellulose solubilities in the LiOH/urea, NaOH/urea, and KOH/urea aqueous solutions.