Distribution and Existing States of Water in Wood:A Review

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Wood-water relationship has been studied since the beginning of wood research. With the development of new technology, vast modern analytical method are emerging, and the study of wood-water relationship is changing from macro to micro scale and from cell to molecular level. The main objective of this paper was to review the modern analytical techniques which have been applied to study the wood-water relationship. This paper was divided into four parts. Firstly, the extremely heterogeneity of the wood cell wall structure and chemical composition and the highly complex states of water were discussed, and then the distribution and existing states of water in wood was summarized as an active field of the research about wood-water relationship. Secondly, the application of four kinds of analytical techniques, such as magnetic resonance imaging (MRI), computed tomography (CT), neutron imaging (NI) and vibrational spectroscopic imaging, in the study of water distribution were summarized. Meanwhile, the advantages and limitations of these four techniques were provided. Some of the latest research progress was:Vibrational spectroscopic imaging techniques such as micro-FTIR and confocal Raman could offer visual examination and spectral information of chemical functional groups in situ, and owned high spatial resolution on the micrometer length scale. The spatial resolution of micro-FTIR imaging technique was 6.25 μm by the instrumental parameters, and the spectral changes indicated that the adsorbed water concentration was nonuniform at the cell structure level. Meanwhile, the spatial resolution of confocal Raman imaging technique was higher than 1 μm, and the spectral changes indicated that the amount of water in the cell corner (CC) was less than that in the middle layer of secondary wall (S2) throughout the entire range of relative humidity (RH) levels. Thirdly, the recent advances in the application of four kinds of analytical techniques, such as near infrared (NIR) spectroscopy, nuclear magnetic resonance (NMR), fourier transform infrared spectroscopy (FTIR) and Raman spectroscopy, were discussed in the study of existing states of water in wood. The advantages and limitations of these four techniques were also provided. Some of the latest research progress was:The micro-FTIR spectroscopy and a specially designed sample cell were used to examine the molecular association of adsorbed water with wood during adsorption process. It was confirmed that carboxyl C=O, C-O groups as well as OH groups were active sites for water adsorption. Meanwhile, strongly, moderately and weakly hydrogen-bonded water molecules were identified and assigned. What's more, according to the variation trend of these hydrogen-bonded water molecules, three sections were divided for adsorption process. Furthermore, the existing states of water in each section was demonstrated as C=O…(HOH)…OH or OH…(OH2)…OH、WATER…HOH…WATER, and tetrahedral structure. Finally, some future research subjects in the research about wood-water relationship were proposed, such as applying new approaches with greater accuracy and higher resolution, developing the convincing component band analysis.