Water sorption hysteresis in wood: III physical modeling by molecular simulation

Abstract

Molecular simulation has been successfully applied to sorption and hysteresis studies of various nanoporous materials, revealing underlying mechanisms that neither theoretical nor experimental approaches can achieve. In this work, the grand canonical Monte Carlo approach is used in a simplified wood-water system to simulate sorption isotherms and hysteresis at 25°C and 40°C. Wood is represented by a cell wall model composed of a solid substance and evenly distributed independent cylindrical nanopores with diameters in the range of 0.6–2.2 nm. Polysaccharides and lignin pore-wall compositions are considered. Hydroxyl groups are modeled as negative energy pits attached to walls and water is represented by the extended simple point charge model. Capillary condensation in the wide hygroscopic range and metastable states are well demonstrated in the simulations, thus supporting the independent domain model discussed in the first paper of this series. The size of simulated hysteresis loops increases with pore size, less hydrophilic lignin composition and reduced temperature. The trends shown by the model are consistent with experimental findings. The larger hysteresis can be explained by more metastable states due to weaker wall-water interaction or smaller thermal fluctuation.

Role of hydrogen bonding in hysteresis observed in sorption-induced swelling of soft nanoporous polymers

“…This swelling is accompanied by changes in the internal structure of the cellulosic system which modify the experience of the water molecules and feedback on the internal structure. This coupling between sorption and swelling in nanoporous materials has been investigated experimentally and with atomistic simulations 10 – 14 but also with phenomenological approaches such as the domain theory 15 , 16 (see ref 17 . for a recent review on water adsorption in wood).…”

Nanopore-Level Wood-Water Interactions—A Molecular Simulation Study

“…The simulation model was taken from our previous publication [25]. The wood cell wall-water system was modeled as a collection of infinitely long cylindrical nanopores of different sizes d j with water confined in them (Figure 1a).…”

“…The magnitude of simulated sorption hysteresis increased with the cell wall pore size, the lignin content of cell walls, and the reduced temperature, which is consistent with experimental observations. Details can be found in [25]. Furthermore, the model had been quantitatively examined by comparing the predicted cell wall pore size distributions at fully saturated states with the experimental ones derived from the solute exclusion method.…”

“…The studies on the origin of sorption hysteresis in wood (more specifically, reproducible hysteresis in [17]) are quite diverse. These include, but not limited to the roles of accessible sorption sites during sorption and their possible coupling with sorption-induced swelling [18][19][20][21], the rigidity or softening of wood cell walls [22][23][24], and the metastable states of adsorbed and desorbed water associated with the cell wall nanopore filling [25]. Experimentally, the effects of the wood-water interactions, accessible sorption sites and cell wall pore size on sorption and hysteresis are often investigated by modifying wood using various methods, e.g., thermal or chemical, and then relating the hygroscopic property change to changes of the number of accessible hydroxyl groups, wood-water interaction strength (WWIS) and cell wall pore size distributions [26][27][28][29][30][31][32][33].…”

“…However, it is difficult to quantitatively evaluate the aforementioned factors and possible couplings among them. In [25], the effect of cell wall pore size on sorption and hysteresis was explored using molecular simulation methods based on a nanoscale cell wall pore model (details can be found in the following Simulation Model and Method section), and promising results were reported. Comparing to the purely theoretical and experimental approach, the molecular simulation approach has the advantage of conducting systematic and quantitative investigations at the nanoscale using a model that is close to the real system.…”