Alkali Metal and H2O Dynamics at the Smectite/Water Interface

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• 作者：Geoffrey M. Bowers ; Jared W. Singer ; David L. Bish ; R. James Kirkpatrick
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：December 1, 2011
• 年：2011
• 卷：115
• 期：47
• 页码：23395-23407
• 全文大小：1132K
• 年卷期：v.115,no.47(December 1, 2011)
• ISSN：1932-7455

文摘

Molecular-scale dynamic processes involving ions and water at smectite鈥搘ater 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 掳C. In this work, we discuss the effects of the charge-balancing cation on the structure and dynamics of alkali metals and H₂O at the mineral鈥搘ater interfaces of alkali metal鈥搒mectite pastes over a broad range of temperatures. New variable-temperature ²H and ²³Na NMR spectroscopic data for a Na鈥揾ectorite paste presented here in combination with the results from our previous studies of Cs鈥?and K鈥揾ectorite pastes reveal a common anisotropic mechanism of ²H motion for ²H₂O restricted by proximity to a surface or cation between 鈭?0 and 鈭?0 掳C. This motion is well modeled by combined fast-limit C₂ librations about the ²H₂O molecular dipole moment and fast-limit octahedral-type jumps of ²H₂O molecules about the C₃ symmetry axis of a slightly compressed metal鈥揌₂O complex. At higher temperatures, ²H₂O dynamics are dominated by diffusion and/or chemical exchange of deuterons and differ for Na- and K-exchanged samples. Comparing our collective ³⁹K, ¹³³Cs, and ²³Na VT NMR results shows that Na⁺ has less affinity for the smectite surface in pastes than K⁺ or Cs⁺, that the influence of ²H₂O and ²H₂O content on alkali metal motion decreases from Na⁺ to K⁺ to Cs⁺, and that slow-to-intermediate (rate < 10⁴鈥?0⁵ Hz) two-site exchange is a significant dynamic process above 鈭?0 掳C only for Cs⁺.