Synergistic Sorption of Mixed Solvents in Wood Cell Walls: Experimental and Theoretical Approach

Abstract

The behavior of poplar wood and its components in water‐ethanol mixtures is investigated through a coupled experimental and theoretical approach including physico‐chemical and dynamic mechanical analyses and molecular dynamics simulations. Affinity for water‐ethanol vapors, measured by isothermal gravimetric sorption experiments, features a maximum for mixed vapors. The longitudinal viscoelastic behavior of the same wood upon immersion in ethanol‐water solutions, measured by dynamic mechanical analysis, features a minimum in storage modulus and a maximum damping upon immersion in solutions of intermediate composition. Optical microscopy observation of solvent‐saturated samples evidences inter‐ and intra‐cellular disbonding in pure ethanol and ethanol aqueous solutions. Molecular dynamics simulations provide information on interactions of water‐ethanol solutions with models of cellulose microfibers and lignin. The relative solvation of cellulose microfibers by water and ethanol shows a nearly linear variation with the composition of the solution. In contrast, the accessibility of lignin dimers to the solvent presents a maximum at intermediate ethanol concentrations, in correspondence with a conformational transition of the dimer towards an extended conformation. The modelization of the interactions of cellulose and lignin in water‐ethanol solutions indicates a minimum of adhesion of the two components of wood in the presence of solutions with intermediate concentrations.

Molecular Insight into the Cosolvent Effect on Lignin–Cellulose Adhesion

“…Very recent experimental and atomistic simulations support the observation that lignin dissociates from cellulose in the presence of ethanol 64 and THF solvents. 64,[78][79][80][81] Our MD simulations 64 provided the first shreds of evidence that the lignin adsorption on cellulose in pure water solvent is disrupted upon the addition of 50 and 75 wt% ethanol co-solvent in line with the experimentally observed high swelling of wood after immersion in 44 wt% aqueous ethanol solution 64 .…”

“…In water, a bad solvent for lignin, the lignin dimer has the smallest average SASA, which is, however, broader than the lignin SASA of a single lignin dimer in water. 64 This reveals that cellulose stabilizes lignin in the adsorbed state by reducing its hydrophobic interactions with the water solvent. In comparison with the mixed and pure organic surfaces, the lignin interactions with water are the least favorable.…”

“…This indicates an improved intramolecular H-bonding in cellulose (vide supra), as the number of intramolecular cellulose H-bonds is better preserved in the pure organic solvent than in water. 64 Albeit this effect of organic co-solvents modifies the accessibility of different surface oxygens, it scantily affects the geometrical SASA of cellulose, as shown in Figure 9C.…”

“…45 Whereas extensive work on lignocellulosic components (mostly cellulose) in various monocomponent solvents has been carried out, only a few computational studies approach the molecular action of co-solvents in multicomponent liquids on the structural properties of lignocellulosic biomass and the interactions between the lignocellulosic polymers. 64,[72][73][74][75][76][77][78][79][80][81] Quantum chemical density functional theory-based method augmented with an implicit solvent (COSMO-RF) approach for binary solvent mixtures 82 evidenced the role of anions in the ionic liquids to dissolve lignin and cellulose from the computed excess enthalpies. The explicit treatment of solvent molecules, studied with all-atom MD simulations 72,73 showed that tetrahydrofuran (THF) -a polar aprotic ether -preferentially solvates lignin, which shifts the equilibrium conformational distribution from a crumpled globule to a coil lignin conformer.…”

Characterization of Organosolv Birch Lignins: Toward Application-Specific Lignin Production

“…The current work thus sets out to investigate the application of a promising mode of extraction, namely, the novel organosolv/steam-explosion method, 23 which combines the benefits of a catalytically active solvent allowing the dissolution of hydrolyzed lignin with the shear force of depressurization made more effective by solvent-softening of cell–cell adhesion. 24 While the characterization of birch lignin extracted under a certain organosolv condition has been reported, 25 there are no reports on how changing important process parameters employed during birch processing through a combined, novel organosolv/steam-explosion mode affect the chemistry of the extracted lignins. This chemistry of the extracted lignins will be discussed with respect to the suitability of the various lignins for specific, high-value, down-stream applications.…”