Alkali Metal and H₂O Dynamics at the Smectite/Water Interface

Molecular-scale dynamic processes involving ions and water at smectite–water interfaces play crucial roles in issues such as contaminant transport, reactivity of geochemical systems, and carbon sequestration, yet little is known about the specific manner in which interfacial ion and water dynamics influence one another, particularly at temperatures below 0 °C. In this work, we discuss the effects of the charge-balancing cation on the structure and dynamics of alkali metals and H 2 O at the mineral–water interfaces of alkali metal–smectite pastes over a broad range of temperatures. New variable-temperature 2 H and 23 Na NMR spectroscopic data for a Na–hectorite paste presented here in combination with the results from our previous studies of Cs– and K–hectorite pastes reveal a common anisotropic mechanism of 2 H motion for 2 H 2 O restricted by proximity to a surface or cation between −50 and −20 °C. This motion is well modeled by combined fast-limit C 2 librations about the 2 H 2 O molecular dipole moment and fast-limit octahedral-type jumps of 2 H 2 O molecules about the C 3 symmetry axis of a slightly compressed metal–H 2 O complex. At higher temperatures, 2 H 2 O dynamics are dominated by diffusion and/or chemical exchange of deuterons and differ for Na- and K-exchanged samples. Comparing our collective 39 K, 133 Cs, and 23 Na VT NMR results shows that Na + has less affinity for the smectite surface in pastes than K + or Cs +, that the influence of 2 H 2 O and 2 H 2 O content on alkali metal motion decreases from Na + to K + to Cs +, and that slow-to-intermediate (rate < 10 4 –10 5 Hz) two-site exchange is a significant dynamic process above −80 °C only for Cs + .