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Water sorption hysteresis and wood cell wall nanopore structure Shi, Jingbo


The origin of sorption hysteresis in the wood-water system is still under debate. In this study, cell walls are considered as micro-mesoporous materials and capillary condensation in the entire hygroscopic region is proposed as an alternative sorption mechanism. Initially, the pore connectivity was investigated by observing five experimentally generated hysteresis patterns at 25 and 40oC. Consistent patterns were found for the species-temperature combinations. Further, the satisfactory congruency and wiping-out properties indicate the dominance of independent cell wall pores. After this experimental phase, the geometric interpretations derived from the Preisach model, the mathematical form of the independent domain model, was used tο explain the observed hysteresis patterns. Additionally, a modification to the aforementioned model was suggested that involves a numerical implementation, which avoids the use of unknown parameters. The low prediction errors and well-maintained wiping-out property support the suitability of our approach. In the next phase, grand canonical Monte Carlo (GCMC) technique was applied in a simplified wood-water system to simulate sorption isotherms and hysteresis at 25 and 40°C. In the simulation system, wood is represented by a cell wall model that is composed of solid substances and evenly distributed independent cylindrical nanopores with sizes in the range of 0.6 – 2.2nm. Two types of pore-wall compositions regarding polysaccharides and lignin have been considered. The hydroxyl groups are modeled as negative energy pits attached to walls whereas water is represented by the SPC/E model. Results demonstrated that hysteresis can be well explained by the existence of metastable states associated with capillary condensation and evaporation of water in cell wall pores. The alternative sorption mechanism driven by capillary condensation is also strongly supported by the simulation. In the last phase, the cell wall pore size distributions in the hygroscopic range were explored for the three species from a “trial and error” calculation approach. This approach was indirectly examined by comparing derived volumetric strain of cell walls and the density of adsorbed water in the hygroscopic range with literature data. The qualitative agreement indicates the soundness of assumptions made on the cell wall swelling process and proposed calculation procedures.

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