Self-diffusion coefficient of bound water in longleaf pine wood investigated with pulsed-field-gradient 1H-NMR and molecular simulation

The self-diffusion coefficient of bound water in longleaf pine wood was studied using pulsed-field-gradient nuclear magnetic resonance (PFG-NMR) and molecular dynamic techniques. The PFG stimulated-echo pulse sequence with different parameter settings was used for the diffusion measurements of fully saturated solid wood samples and samples conditioned to the equilibrium states at a constant relative humidity of 90 and 100% at 45 °C. Additional 20–40 mesh wood powder samples were conditioned at a humidity of 90% at 45 °C. Three coexisting water phases were found for fully saturated samples, indicating self-diffusion from free water, cross-relaxation of bound and free water, and bound water, with estimated longitudinal self-diffusion coefficients in the order of 10^–9, 10^–9, and 10^–10 m²/s, respectively. The cross-relaxation of bound and free water remains inclusive and requires further investigation. Only one detectable homogeneous water phase was found for samples conditioned at 90 and 100% humidity at 45 °C, with an estimated longitudinal self-diffusion coefficient in the order of 10^–11 m²/s. The coefficient from the wood powder sample is one order smaller than the one from the solid wood sample at similar moisture contents. The coefficient was found to be independent of moisture content when close to or above the fiber saturation point, but decreases rapidly with a decrease in moisture content below saturation. The estimated coefficients for cell wall pore sizes of 1.2, 1.6, 2.0 and 2.4 nm from molecular dynamic simulations are generally larger than those determined experimentally, but with errors within an order of magnitude. The PFG-NMR technique and simulation method used here may be applied to reveal the intrinsic self-diffusion behavior of bound water in various wood species or more general cellulosic materials.