Abstract

We present an X-ray diffraction and multi-nuclear (2H and 43Ca) NMR study of Ca-exchanged hectorite (a smectite clay) that provides important new insight into molecular behavior at the smectite-H2O interface. Variable-temperature 43Ca MAS NMR and controlled humidity XRD indicate that Ca2+ occurs as proximity-restricted outer-sphere hydration complexes between −120 and +25 °C in a two-layer hydrate and at T ≤ −50 °C in a 2:1 water/solid paste. Changes in the 43Ca 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 2H NMR signal between −50 and +25 °C for a two-layer hydrate of Ca-hectorite is similar to that of Na- and other alkali metal hectorites and represents 2H2O molecules experiencing anisotropic motion describable using the 2H C2/C3 jump model we proposed previously. 2H T1 relaxation results for Ca- and Na-hectorite are well fit with a fast-exchange limit, rotational diffusion model for 2H2O dynamics, yielding GHz-scale rotational reorientation rates compatible with the C3 component of the C2/C3 hopping model. The apparent activation energy for 2H2O 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.

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