Slow diffusional dynamics at the mineral-water interface: Nanoscale view from the computational molecular modeling perspective
Abstract
Molecular-scale knowledge of the thermodynamic, structural, and transport properties of water and ions at mineral interfaces is crucial for quantitative understanding and prediction of many geochemical and environmental processes. At mineral interfaces, individual water molecules and hydrated ions simultaneously participate in several dynamic processes characterized by different, but equally important time- and length- scales. Most of these processes are very difficult to investigate experimentally and require a broad range of sophisticated analytical techniques, but many of them can be effectively studied by molecular dynamics computer simulations in a single MD run. On a relatively short time scale (~1-100 ps), the interfacial dynamics is dominated by the molecular librational and re-orientational motions. The librations (hindered rotations) of surface hydroxyls also occur on this time scale. These motions are responsible for the reformation and breaking of individual H-bonds, and the strength of these bonds can be directly correlated with the frequencies of intra-molecular O-H vibrations at an even shorter sub-ps time scale. However, the diffusional processes related to reformation of the entire interfacial hydrogen bonding network, surface adsorption of H2O molecules and ions are characterized by a much longer time scale (~0.1-10 ns). We quantify the slow diffusional motion of H2O molecules on the surfaces of quartz and tobermorite using van Hove self-correlation functions, GS(r,t). The results clearly show a distinct characteristic time, τ ~0.8 ns for the hopping diffusion of water molecules on the surface of tobermorite, in excellent qualitative and quantitative agreement with 1H NMR field cycling relaxometry results for surface-associated water in tobermorite (Korb et al., 2007a,b; Kalinichev et al., 2007). Similar hopping diffusional dynamics is several times faster on the ideal (001) surface of quartz, but in reality can be significantly slowed down by the atomic- scale roughness of the surface. These results are also consistent with recent neutron spectroscopic data for water dynamics on rutile and cassiterite surfaces (Mamontov et al., 2007, 2008).
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V23F2191K
- Keywords:
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- 3610 Geochemical modeling (1009;
- 8410);
- 3620 Mineral and crystal chemistry (1042);
- 3947 Surfaces and interfaces