Alkali Metal and H₂O Dynamics at the Smectite/Water Interface

Abstract

ARTICLEsurface normal, most likely compression in the case of our paste samples due to electrostatic interactions between the alkali metal ions and the smectite surfaces. On the basis of these results and the higher hydration energies of the alkaline earth metals, we expect a stronger influence of alkaline earth metals on high-temperature 2 H 2 O dynamics and increased importance of cation diffusion as a dynamic mechanism at lower temperatures in alkaline earth metalÀ smectite pastes.' ASSOCIATED CONTENT b S Supporting Information. Details of the NMR experiments, TGA results, and supplementary figures referenced in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

NMR and computational molecular modeling studies of mineral surfaces and interlayer galleries: A review

“…The results confirm that cation diffusion and site hopping do not occur in fully dehydrated interlayers, because of coordination to the basal O atoms. This conclusion is in agreement with the 133 Cs NMR results of Weiss et al (1990) and with more recent 23 Na, 39 K, and 43 Ca NMR results (Bowers et al 2008a(Bowers et al , 2011(Bowers et al , 2014b. In interlayers hydrated to one water layer or more, however, the MD simulations show that both the cations and water diffuse at observable rates (e.g., Marry et al 2011;Morrow et al 2013;Ngouana and Kalinichev 2014).…”

“…Characterization of the interlayer and surface regions of clay minerals, LDHs and NOM and the fluid near NOM molecules requires understanding of the distribution of local [nearest neighbor (NN) and next nearest neighbor (NNN)] atomic arrangements; the average structure (e.g., average coordination and bond distances and angles); the diffusional, site exchange, vibrational, rotational, and translational dynamics; and the energetics of the interactions. The important molecular processes that control these interactions have characteristic frequencies that vary over many orders of magnitude at ambient temperatures, from at least as small as 10 2 -10 4 Hz to >10 12 Hz (e.g., Cygan 2001;Wang et al 2005Wang et al , 2006Bowers et al 2011) The importance of dynamical behavior in these systems is the result of the dominance of electrostatic [Coulombic and hydrogen-bond (H-bond)] and dispersive (van der Waals) interactions, rather than the stronger covalent bonding that dominates the aluminosilicate structure of many minerals (Cygan and Kubicki 2001;Cygan 2001;Gale 2001;Kalinichev 2001;Wang et al 2001;Cygan et al 2004Cygan et al , 2009. H-bonding is very significant for hydrous systems, and H-bond strengths typically range from 10 to 25 kJ/mol.…”

“…The combination of nuclear magnetic resonance spectroscopy (NMR) and computational, molecular-scale modeling has been particularly effective in assessing the structures and dynamics of disordered geological materials across this range of temporal and spatial scales (Kalinichev et al 2000;Kirkpatrick et al 2005c;Bowers et al 2011;Morrow et al 2013). This paper reviews results that illustrate how the combination of these two methods provides otherwise unobtainable structural and dynamical information about interlayer galleries and near-surface fluid regions.…”

“…T 2 (transverse) relaxation and line shape analysis are sensitive to lower frequencies, typically in the 1-100 kHz range. Both of these frequency ranges are much lower than those probed by vibrational spectroscopy but are often relevant to processes in interlayers and at fluid-mineral interfaces (e.g., Wang et al 2005Wang et al , 2006Bowers et al 2008aBowers et al , 2011Bowers et al , 2014b. Unlike quasieleastic neutron scattering that can simultaneously probe the structure and dynamics in these frequency ranges for hydrous systems through observation of hydrogen isotopes (e.g., Michot et al 2012), NMR can observe many, although not all, elements, including 1 H, 2 H, many alkali and alkaline earth elements, many group III-VIII elements, and some transition metals.…”

“…In many cases interpretation is based on data for well-understood reference compounds and on the variation of the observed parameter with composition, temperature, and water content (relative humidity). Because the interaction among the (alumino)silicate or hydroxide layers and the interlayer/ interfacial ions, H 2 O, and organic molecules is dominated by electrostatic and dispersive forces, it is often necessary to investigate 2 H 2 O, cation, and anion line shapes over temperatures from -80 or -100 °C to near or above room temperature to obtain a clear picture of the dynamical behavior (Weiss et al 1990;Kim et al 1996;Bowers et al 2008aBowers et al , 2011Bowers et al , 2014b.…”