Structural and dynamical relationships of Ca2+ and H2O in smectite/2H2O systems

We present an X-ray diffraction and multi-nuclear (H-2 and Ca-43) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature Ca-43 MAS NMR. and controlled humidity XRD indicate that Ca2+ occurs as proximity-restricted outer-sphere hydration complexes between -120 and +25 degrees C in a two-layer hydrate and at T <= -50 degrees C in a 2:1 water/solid paste. Changes in the Ca-43 NMR peak width and position with temperature are more consistent with diffusion-related processes than with dynamics involving metal-surface interactions such as site exchange. The H-2 NMR signal between -50 and +25 degrees C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents (H2O)-H-2 molecules experiencing anisotropic motion describable using the H-2 C-2/C-3 jump model we proposed previously. H-2 T-1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for (H2O)-H-2 dynamics, yielding GHz-scale rotational reorientation rates compatible with the C-3 component of the C-2/C-3 hopping model. The apparent activation energy for (H2O)-H-2 rotational diffusion in the two-layer hydrate is greater for Ca-hectorite than Na-hectorite (25.1 vs. 21.1 kJ/mol), consistent with the greater affinity of Ca2+ for H2O. The results support the general principle that the dynamic mechanisms of proximity-restricted H2O are only weakly influenced by the cation in alkali metal and alkaline earth metal smectites and provide critical evidence that the NMR resonances of charge-balancing cations in smectites become increasingly influenced by diffusion-like dynamic processes at low temperatures as the charge density of the unhydrated cation increases.