(700c) Water Adsorption On Mica: Redistribution and Film Growth
AIChE Annual Meeting
2009
2009 Annual Meeting
Separations Division
Molecular Simulation of Adsorption II:
Friday, November 13, 2009 - 1:00pm to 1:15pm
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 of water near mica surface. 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. This is followed by a relatively slow increase in the coverage for 0.1 ≤ p/p0 ≤ 0.7. Above p/p0 = 0.7, a second increase in the coverage occurs due to the adsorption of water with bulk-like 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 ≤ p/p0 ≤ 0.7, where water is depleted from the inner layers and film growth is initiated on the outer layer. During this stage, potassium hydration is found to play a dominant role in the rearrangement of water near the mica surface. The analysis of structural features reveals a strongly bound first layer of water molecules occupying the ditrigonal cavities between the potassium ions. In-plane structure of oxygen in the second layer, which forms part of the first hydration shell of potassium, reveals a liquid-like structure with the oxygen-oxygen pair correlation function displaying features similar to bulk water. 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. Both SPC and extended simple point charge (SPC/E) water models were found to yield qualitatively similar adsorption and structural characteristics, with the SPC/E model predicting lower coverages than the SPC model for p/p0 > 0.7.