The influence of relative humidity on the physicochemical environment of moisture in wood cell wall

The intrinsic hygroscopic nature of lignocellulosic materials allows water molecules to interact with cell wall polymers, and thus affects their performances. The presences of nano-porous structure and hydroxy groups create physiochemical environment facilitating water sorption in cell wall, which is highly humidity level dependent. This study examined the changes of available hydroxy groups and cell wall pore architectures of Fagus sylvatica (beech) and Pinus taeda L. (pine) under different relative humidity (RH) conditioned by saturated salt solutions. The physical environment, including pore volume and pore size distribution, was determined using differential scanning calorimetry thermoporosimetry (DSCT) and time–domain nuclear magnetic resonance (TD-NMR). The application of attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) allowed for the examination of the influence of RH on the sorption sites. Additionally, heavy water was utilized to isotopically label the accessible hydroxy groups in the wood cell wall, enabling the assessment of the chemical environment using differential spectrum analysis. The results indicated that the lower RH levels had a greater effect on pore diameter but affected pore volume less. When the RH exceeded 60%, the impact of RH on pore volume became more pronounced, while pore diameter remained relatively constant. On the other hand, the number of available hydroxy groups gradually increased as RH rose till 60%. Therefore, a turning point of 60% RH was proposed for physicochemical environment of cell wall moisture in wood, below and above this point the instantaneous pores and hydroxy groups were primarily responded to the RH variations, respectively. Moreover, moisture accumulated in high RH is more likely to be constrained by the physical surroundings and has less strong interactions with the cell wall. Elucidating the coupling effect of water induced physiochemical environment changes enable the further understanding of the interplay of wood and water, and give insights in the design of advanced materials.