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

Effect of drying on the hydroxyl accessibility and sorption properties of pressurized hot water extracted wood

“…Similar findings were reported by Salmén and Stevanic (2018), who showed that a reduction in water retention and an increase in cellulose microfibril size due to drying were not accompanied by changes in relative OH accessibility in never-dried pulp fibres. Overall, the current observations were well in line with a number of recent studies, which showed that the accessible OH groups as sorption sites are not the main factor that determines the amount of absorbed water in native wood (Altgen and Rautkari 2021), thermally treated wood (Willems et al 2020;Lillqvist et al 2019) or chemically modified wood (Thybring et al 2020).…”

“…In line with previous studies on hydrothermal treatments of wood (Altgen et al 2016(Altgen et al , 2018bKymäläinen et al 2018;Altgen and Rautkari 2021), the sample MC decreased with increasing treatment intensity, i.e. the HWE treatment time (Fig.…”

“…The nearly complete re-opening of the dried cell wall may be achieved by vacuum-impregnation or prolonged soaking in liquid water (Hoffmeyer et al 2011;Fredriksson and Thybring 2019). However, a small, yet measurable difference in MC of dried and re-soaked samples compared to never-dried samples has recently been reported by Penttilä et al (2020) and Altgen and Rautkari (2021). In line with these previous studies, a slightly lower MC at 95% RH during desorption was observed for DR compared to ND reference samples (ΔMC Fig.…”

“…The relative hydroxyl accessibility of deuterated pre-test samples was only 30.3 ± 1.8%, which is about 5-10%-points lower than the value observed in other studies (Fackler and Schwanninger 2011;). Altgen and Rautkari (2021) obtained similar results and assigned the low relative hydroxyl accessibility to re-protonation of easily accessible OD groups, which occurred shortly after sample removal from the automated sorption balance. Since such easily accessible OD groups would already be re-protonated during the H 2 O vapour exposure in the sorption balance, this effect was less critical during the actual test series.…”

“…The relative hydroxyl accessibility, which was calculated from the OH and OD stretching bands, ranged between 0 and 2.7% (see Supplementary Table S.4). Small amounts of OD groups that persisted even after long H 2 O exposure periods have been reported recently in native wood by Altgen and Rautkari (2021). After re-protonation of deuterated wood in H 2 O vapour, they measured relative OH accessibilities of less than 3%, while gravimetrically determined amount of residual OD groups was ca.…”