Insights on the Cellulose Pretreatment at Room Temperature by Choline-Chloride-Based Deep Eutectic Solvents: An Atomistic Study

Abstract

The pretreatment or disruption of a cellulose Iβ crystallite, was described from the atomistic interactions evaluated at room temperature within four deep eutectic solvents: choline-chloride ethylene glycol, choline-chloride oxalic acid, choline-chloride urea, and choline-chloride levulinic acid. The analysis was based on the obtained correlations for the Kamlet-Taft α and β parameters with a series of thermodynamic, structural, and energetic properties. The Kamlet-Taft parameters correlated with the thermodynamic properties of the solvent and their changes upon addition of the cellulose crystallite. Structural analysis revealed that the weaker the interactions within the molecules of solvent, the stronger the interactions between the hydroxyl group from cellulose with the chloride anion and with the hydrogen bond donor. Further analysis indicated that the R-CO-R moieties in the hydrogen bond donor within the solvent, interacted better with the cellulose. The hydrogen bond occupancies within the cellulose crystallite, displayed that the main O6-H6···O2/O3 and O2-H2···O6 interchain hydrogen bonds in the glucan located at the edge of the solute were replaced by weak hydrogen bonds in all solvents. This effect was related to the O···H-Cl , C-H···O , and O···H-O HBs between cellulose and DES molecules and confirmed by the non-covalent interactions obtained through DFT calculations. Finally, the energetic interactions and the atomistic degree of disruption of the cellulose crystallite were not completely described by the Kamlet-Taft β or α parameters separately. Surprisingly, by using the net basicity ( β - α) definition, such correlations were improved suggesting that both parameters must be considered in order to develop new, green, and sustainable solvents for cellulose pretreatment process.