Moisture-dependent transition from strong to weak hydrogen bonding in wood polysaccharides

Near-infrared (NIR) spectroscopy combined with multivariate analysis was employed to investigate moisture-dependent hydrogen bonding of water in softwood cell walls. Spectral variations at equilibrium moisture contents were analyzed using principal component analysis and perturbation-correlation moving-window two-dimensional correlation spectroscopy to distinguish water populations with varying hydrogen-bond strengths. A distinct transition in hydrogen-bonding behavior was identified at approximately 10% wood moisture content. Below this threshold, water was found to predominantly engage in strong hydrogen bonds within wood cell walls. Above 10% moisture content, the relative contribution of strong hydrogen bonds to the overall wood-water hydrogen bond network decreased, whereas weakly hydrogen-bonded population became increasingly dominant. Experiments on modified wood and isolated polymers indicated that this transition originates mainly from wood polysaccharides, while lignin plays a minor role. These findings provide strong evidence for distinct water populations in wood cell walls, distinguished by their intermolecular hydrogen-bonding energies and predominant at specific wood moisture contents.