H2O self-diffusion restricted by clay platelets with immobilized bound H2O layers: PGSE NMR study of water-rich saponite gels

The role of bound or less-mobile H2O layers near negatively charged clay platelets in water-rich saponite gels as obstacles to the diffusion of unbound H2O molecules in the pore-space was examined. Self-diffusion coefficients of H2O molecules, D, in gels of synthetic Na-rich saponite were measured by pulsed-gradient spin-echo proton nuclear magnetic resonance (PGSE proton NMR) to evaluate these obstruction effects. The clay weight fraction, w, of the gel samples ranged from 0.0 to 27.9 wt.% and the sample temperature was 20.0 to 60.6 °C. The NMR results showed that D normalized by D0 was independent of sample temperature and decreased with increasing w: ln(D/D0)=1.33[exp(−0.0290w)−1] where D0 was D in bulk water. This diffusion behavior was interpreted through random-walk computer simulations of a gel-structure model originally developed for montmorillonite, hectorite and stevensite. In the model, unbound H2O diffuses in the porous gel structure by avoiding randomly distributed obstacles representing clay platelets sandwiched between completely immobilized bound H2O layers. A quantitative dependence of the platelet volume fraction and the bound H2O layers thickness on D/D0 was obtained by the simulations. The ratio, χ=(volume of clay platelets and immobilized H2O layers)/(volume of clay platelets), was introduced as a measure of the thickness of the immobilized H2O layers. χ was estimated to be 5.7 for water-rich saponite gels (w≤7.99 wt.%) by fitting the results of the random-walk simulation to the PGSE NMR diffusion data. This value corresponds to an immobilized bound H2O layer thickness of 2.4 nm assuming that each clay particle in the gels consists of a single 1-nm-thick platelet. The thickness of immobilized H2O layers in water-rich gels of six smectite species was identified on this basis to decrease in the following order: montmorillonite (χ=18, 13)>saponite (χ=5.7)>hectorite (χ=4.2, 4.0)>stevensite (χ=3.6). This order correlates well with the order of the cation exchange capacity (CEC), suggesting that the thickness of the bound H2O layers increases with increasing layer charge. The present study confirmed that the gel-structure model developed previously for montmorillonite, hectorite and stevensite was also applicable to saponite.