Insights on the cellulose pretreatment at room temperature by choline-chloride-based deep eutectic solvents: an atomistic study

The pretreatment or disruption of a cellulose I beta crystallite by four deep eutectic solvents (DES): choline-chloride ethylene glycol, choline-chloride oxalic acid, choline-chloride urea, and choline-chloride levulinic acid, was described from the atomistic interactions observed in molecular dynamics and ab-initio (NCI-AIM) studies. The cellulose I beta disruption was studied considering plausible correlations between the Kamlet-Taft (alpha and beta) solvent parameters, and a series of thermodynamic, structural, and energetic properties. It was found that the Kamlet-Taft parameters correlated with the thermodynamic properties of the DES, as well as their variations after the addition of the cellulose crystallite. Structural analysis revealed that the weaker the interactions within the molecules of the 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, displayed best interplaying with the cellulose. The hydrogen bond occupancies within the cellulose crystallite, evidenced that the main O6 - H6 center dot center dot center dot O2/O3 and O2 - H2 center dot center dot center dot O6 interchain hydrogen bonds in the glucan located at the edge of the solute, were replaced by weak O6 - H6 center dot center dot center dot O4 hydrogen bonds in all solvents. This effect was related to the O/Cl-DES center dot center dot center dot H - donor and O-DES - H-DES center dot center dot center dot acceptor HBs between cellulose and DES molecules, and it was confirmed by the noncovalent interactions obtained through DFT calculations. The energetic interactions and the atomistic degree of disruption of the cellulose crystallite, were not completely described by the Kamlet-Taft beta or alpha parameters when considered separately. Surprisingly, by using the net basicity (beta - alpha) definition such correlations were improved, suggesting that both parameters must be considered together to develop new, green, and sustainable solvents for cellulose pretreatment process. [GRAPHICS] .

Automated summary

This study investigated the pretreatment and disruption of cellulose crystallite using molecular dynamics and ab-initio simulations. It was found that the thermodynamic properties of the solvents and their interactions with cellulose are correlated with the solvent parameters. The disruption of cellulose crystallite depends on the strength of interactions within the solvent and the hydrogen bonding with cellulose.