A NMR investigation of adsorption/desorption hysteresis in porous silica gels

The purpose of this study is to investigate possible differences in geometry and connectivity of the liquid phase in a partially water-satured porous medium between the adsorption and the desorption branches, using a series of silica gels. This was done by comparing the T1 and T2 relaxation times (in 1H and 2H nuclear magnetic resonance (NMR) relaxation) as well as the water self-diffusion coefficient D (in 1H) along the two branches of the adsorption/desorption isotherms. The results show that the two relaxation times and the diffusion coefficient are strongly dependent on the water content (saturation level). When plotted in normalized coordinates, the T1 and T2 vs. P/P0 curves fit closely the adsorption/desorption isotherms, which validates the two-population, fast-exchange model. Furthermore, because at equivalent saturation levels, there is no difference between the relaxation times and diffusion coefficients obtained along the adsorption branch and those obtained along the desorption branch, one is led to the conclusion that despite different equilibrium conditions, the geometry and connectivity of the liquid phase are statistically the same along the two branches.