Solvent behavior of an ionic liquid set around a cellulose Iβ crystallite model through molecular dynamics simulations

A set of imidazolium-based ionic liquids: [C(4)mim][PF6], [C(4)mim][BF4], [C(4)mim][Cl], [C(4)mim][CF3COO], [C(4)mim][NTf2], [C(4)mim][OMs], [C(4)mim][Br], and [C(4)mim][OAc], was studied by molecular dynamics simulations to elucidate their solvent behavior around a crystallite model of cellulose I beta, through atomistic interactions and the degree of departure of its thermodynamic properties from their solvent pure phase. These departure changes were correlated with experimental values of the Kamlet-Taft solvent basicity parameter, and it was found that, even at room temperature, density changes, and vaporization enthalpy changes can be correlated with the capacity of ionic liquids for the preconditioning of the cellulose crystallite. Hydrogen bond occupancies indicate that ionic liquids can disrupt external chains of the cellulose crystallite by replacing and reducing the strong O6-H center dot center dot center dot O2/O3 hydrogen bonds by weak hydrogen bonds such as O6-H center dot center dot center dot O4 along the interchain etwork. Also, radial distribution functions indicated that structural changes in the cellulose-ionic liquid mixtures did not depart significantly with respect to the pure IL structure. The results of the free energy of solvation calculations for a cellulose chain, presented the following trend: [C(4)mim][Cl] > [C(4)mim][OAc] > [C(4)mim][CF3COO] > [C(4)mim][Br] > [C(4)mim][OMs] > [C(4)mim][BF4] > [C(4)mim][PF6] > water > [C(4)mim][NTf2]. It is important to emphasize, that the focus of this work was not the cellulose dissolution, but instead, the solvent behavior and cellulose preconditioning within each IL at room temperature. Our results can provide insights about the preconditioning stage of cellulose at low temperature, useful in the development of lignocellulosic materials and valuable cellulose derivatives by means of low energy requirements.

Automated summary

This study investigated the solvent behavior of a set of imidazolium-based ionic liquids around a crystallite model of cellulose I beta through molecular dynamics simulations. Results showed that the ionic liquids can disrupt the external chains of the cellulose crystallite and affect its properties such as density and vaporization enthalpy, contributing to cellulose preconditioning. Solvation free energy calculations for the cellulose chain revealed a specific trend among the different ionic liquids in their capacity for cellulose preconditioning.