Humidity-dependence of the hydroxyl accessibility in Norway spruce wood
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ORIGINAL RESEARCH
Humidity-dependence of the hydroxyl accessibility in Norway spruce wood Michael Altgen
. Lauri Rautkari
Received: 12 August 2020 / Accepted: 14 October 2020 Ó The Author(s) 2020
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 lm) 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.
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-020-03535-6) contains supplementary material, which is available to authorized users. M. Altgen L. Rautkari Department of Bioproducts and Biosystems, Aalto University, P.O. Box 16300, 00076 Aalto, Finland M. Altgen (&) Department of Biology, Institute of Wood Science, Wood Physics, Universita¨t Hamburg, Leuschnerstraße 91 c, 21031 Hamburg, Germany e-mail: [email protected]
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Cellulose
Graphic abstract
Keywords Deuterium exchange DVS FT-IR spectroscopy Sorption Wood
Introduction Lignocelluloses, such as wood, interact with water vapor from the surroundings because they contain a large number of polar groups that attract water molecules, in particular hydroxyl (OH) groups (Berthold et al. 1996). Water accessible OH groups in lignocelluloses can be studied using hydrogen–deuterium exchange that occurs when deuterium oxide (D2O) forms hydrogen bonds with OH groups in wood or other cellulosic materials. The exchanged hydrogen can be quantified either gravimetrically by a dry mass increase of 1 g per mol exchanged hydrogen (Po¨nni et al. 2014; Uimonen et al. 2020), or spectroscopically by the shift of the OH stretching band to lower wavenumbers (Mann and Marrinan 1956; Hofstetter et al. 2006; Salme´n and Stevanic 2018). This provides an estimate for the number of accessible OH groups in
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