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

Abstract The role of bound or less-mobile H 2 O layers near negatively charged clay platelets in water-rich saponite gels as obstacles to the diffusion of unbound H 2 O molecules in the pore-space was examined. Self-diffusion coefficients of H 2 O 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 D 0 was independent of sample temperature and decreased with increasing w : ln( D / D 0 )=1.33[exp(−0.0290 w )−1] where D 0 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 H 2 O diffuses in the porous gel structure by avoiding randomly distributed obstacles representing clay platelets sandwiched between completely immobilized bound H 2 O layers. A quantitative dependence of the platelet volume fraction and the bound H 2 O layers thickness on D / D 0 was obtained by the simulations. The ratio, χ =(volume of clay platelets and immobilized H 2 O layers)/(volume of clay platelets), was introduced as a measure of the thickness of the immobilized H 2 O 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 H 2 O 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 H 2 O 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 H 2 O 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.