2D time-domain nuclear magnetic resonance (2D TD-NMR) characterization of cell wall water of Fagus sylvatica and Pinus taeda L.

Water dominates the wood cell wall in the hygroscopic range. The state and amount of cell wall water influence the physical and mechanical properties of wood. 2D time-domain nuclear magnetic resonance (2D TD-NMR) is capable of resolving cell wall water. To identify the relaxation property of cell wall water, 2D TD-NMR with saturation recovery Carr-Purcell-Meiboom-Gill pulse sequence was applied on Fagus sylvatica and Pinus taeda L. at equilibrium state under 12, 34, 47, 65, 84, and 97% relative humidity (RH), as well as during adsorption process from dry to 84% RH. The result showed that two pools of cell wall water were identified, the water molecules in disordered surface of cellulose microfibrils and their surrounding hemicellulose (Peak B), and the water molecules in the domain of matrix including lignin and surrounding hemicellulose (Peak C). B-water and C-water exhibited different relaxation properties with different moisture contents. With increasing RH, B-water gained more mobility than C-water. Through the calculation based on cluster theory, it was hypothesized that the formation of water clusters in high RH region may exist in the reservoir of B-water. During the sorption process from a dry state to equilibrium at 84% RH, the amount of B- and C-water gradually increased, whereas B-water appeared earlier than C-water but took a longer time to equilibrate. When the sub-equilibrium state was attained, the T1/T2 ratio stayed stable.