Analyzing moisture self-diffusion coefficient and pore characteristics in Mongolian scots pine using single-sided NMR

In this study, single-sided nuclear magnetic resonance (NMR) spectroscopy was used to detect the changes of axial and radial tangential moisture self-diffusion coefficient with diffusion time of Mongolian Scots pine (Pinus sylvestris var. mongolica). The result shows that, the self-diffusion coefficient values ranked as axial > radial > tangential. The axial self-diffusion coefficient exhibited free diffusion, averaging 2.0 × 10^–9 m²/s, while radial and tangential directions showed restricted diffusion, decreasing with time. Based on the restricted diffusion theory, the results are as follows, radial and tangential tracheid surface-to-volume ratios (S/V) were approximately 203,000 ± 10,600/m and 265,000 ± 25,000/m, average size of lumen ends and pits 6.4 ± 0.33 μm and 6.2 ± 0.49 μm in radial and tangential direction respectively, tortuosity values τ_R = 3.96 ± 0.02 and τ_T = 6.59 ± 0.45. Combining S/V with the form factor (Fs) and the T₂ relaxation mechanism yields the following results, average pore sizes for radial and tangential tracheids were 19.7 ± 1.44 μm and 15.09 ± 1.3 μm, cell water transverse surface relaxation rates were ρ_2R = 0.103 ± 0.005 μm/ms and ρ_2T = 0.082 ± 0.007 μm/ms. The pore size obtained above is within an acceptable range with the results of SEM. This study provides a systematic method for wood moisture self-diffusion analysis.