Nano-scale porosity of water-swollen wood cell walls: analysis of solute exclusion data from a new perspective

The solute exclusion technique (SET) is often used to characterise the nano-porous structure of water-swollen cell walls. SET is based on the immersion of water-saturated wood samples in solutions of probe molecules of known size. Based on determined concentration differences in the solution before and after immersion, the accessible water within the wood is determined for each probe. However, this assumes that the concentration of probe molecules is the same in the pores of the material as in the surrounding bulk solution, but the concentration in narrow pores is actually lower than in the bulk solution. This study investigated the nano-porous structure of water-swollen wood cell walls by incorporating these known effects of concentration differences in narrow pores into the analysis. Based on solute exclusion measurements on both untreated and hydrothermally treated Norway spruce wood, the study explored the effect of modification on the nano-porous cell wall structure as well as potential sources of uncertainties such as soaking time, osmotic effects and probe molecule adsorption. The results suggested that the water-swollen, nano-porous structure of untreated and hydrothermally treated Norway spruce was dominated by one characteristic pore size which increased by hydrothermal treatment. The exact size depended on the assumed geometry of the pores.