Humidity-dependence of the hydroxyl accessibility in Norway spruce wood

Abstract: This study aimed at a better understanding of the wood-water interaction, in particular the role of the hydroxyl accessibility during the humidity-dependent change in moisture content. Thin sections (80 µm) of never-dried Norway spruce sapwood that contained early- and latewood were used for the experiments. Sorption isotherm measurements confirmed the humidity-dependent moisture content changes and the effect of the first drying of the wood sections. Changes in hydroxyl accessibility were then determined by deuteration of the sections using deuterium oxide, followed by their re-protonation in water (H₂O) vapor at different relative humidity: 15, 55 or 95%. The deuteration and re-protonation of the wood sections were quantified by dry mass changes as well as by changes in the OH and OD stretching vibrations in the Fourier transform infrared spectra. The results showed that the deuterated sections could be almost completely re-protonated in H₂O vapor, nearly irrespective of the applied relative humidity. Therefore, changes in hydroxyl accessibility were not the driving force for the humidity-dependent changes in moisture content. However, a slow re-protonation rate at low relative humidity had to be considered. Nonetheless, a small quantity of OD groups persisted the re-protonation in H₂O vapor and liquid H₂O, which was not related to the drying of the wood. Graphic abstract: [Figure not available: see fulltext.].