(5bh) Structure and Dynamics of Water near Mica Surface

Mica is representative of a naturally occurring hydrophilic solid substrate. Due to its atomically smooth structure mica has been widely used in a variety of experiments which seek to probe the structure and dynamics of water near mica surface. Understanding the structure and dynamics of molecularly thin films is an active field of research and has wide applications in areas ranging from biology to geology.

Adsorption isotherms of water on muscovite mica are obtained using grand canonical Monte Carlo simulations over a wide range of relative vapor pressures, p/p0 at 298 K. Three distinct stages are observed in the adsorption isotherm. A sharp rise in the water coverage occurs for 0 0 < 0.1, followed by a relatively slow increase in the coverage for 0.1 0 < 0.7. Above p/p₀ = 0.7, a second increase in the coverage occurs due to the adsorption of water with bulklike features. The derived film thickness and isotherm shape for the simple point charge (SPC) water model is in excellent agreement with recent experiments of Balmer et al. [Langmuir 2008, 24, 1566]. A novel observation is the significant redistribution of water between adsorbed layers as the water film develops. This redistribution is most pronounced for 0.1 0 < 0.7, where water is depleted from the inner layers and film growth is initiated on the outer layer. Isosteric heats of adsorption were found to be in good agreement with the differential microcalorimetric data of Rakhmatkariev (Clays Clay Miner. 2006, 54, 402), over the entire range of pressures investigated.

The dynamics of interfacial water near mica surface is evaluated using molecular dynamics simulations. The analysis shows that water in the contact layer has a high residence time near the surface. The potassium ions have a strong influence on the structure and dynamics of interfacial water. The presence of potassium ions, gives rise to a non-monotonic decoupling behaviour in the translational and orientational motions toward the mica surface. It was observed that orientational motions are much more suppressed when compared with translational motions. Despite slowing down of water relaxation near the surface, dynamics of water in the hydration shell of potassium ion is comparable to that observed in bulk water. The analysis also shows the presence of orientational jumps in the contact layer of mica surface. Recently these molecular jumps have been observed in molecular dynamics simulations of bulk water.¹

References

1. Laage, D. and Hynes, J. T., A molecular jump mechanism of water reorientation. Science 311, 832?835, 2006.