Role of interlayer porosity and particle organization in the diffusion of water in swelling clays

This study focuses on the role played by the interparticle and interlayer porosities, and the preferred orientation of the particles on water diffusion in dual-porosity clayey media (i.e., swelling clay minerals). For this purpose, we use Na-vermiculite, a swelling clay that does not exhibit osmotic swelling and therefore allows a clear discrimination between the interparticle and interlayer porosities. Two samples were prepared with an equal proportion of interparticle and interlayer porosities (i.e., 0.25 each) but different degree of preferred particle orientation (i.e., isotropic vs anisotropic). Through-diffusion and pulsed gradient spin echo attenuation measurements by nuclear magnetic resonance of protons techniques were used to probe water mobility, while the orientation of the particles was quantified by X-ray scattering analysis. Experimental water diffusion results obtained with these two Na-vermiculite samples were compared to those with samples made of Na-kaolinite particles (non-swelling clay mineral) having only interparticle porosities equal to 0.25 and 0.5, corresponding respectively to the interparticle and the total porosity of the Na-vermiculite samples. In addition, these experimental results were compared to simulated data using Brownian dynamics with virtual porous media representative of the real samples. For the range of porosities investigated, a good agreement was observed between measured and simulated water mobilities. The obtained results confirmed the important role played by the preferential orientation of the particles on water dynamics in clayey media, through an important reduction of overall water mobility between the isotropic and anisotropic Na-vermiculite samples. These results also showed that for the same total porosity, the presence of interlayer porosity and associated nano-confinement led to a logical reduction in the pore diffusion coefficient of water in Na-vermiculite in comparison to Na-kaolinite. Moreover, in comparison with Na-kaolinite having the same interparticle porosity, results showed that the contribution of the interlayer volume on the traversing flux was small compared to the interparticle volume. Finally, the computed results revealed that the anisotropy in water diffusion can be directly predicted based on the degree of particles preferred orientation, irrespective of the total or the distribution of the different porosity types.