Adsorption Isotherms of Water on Mica: Redistribution and Film Growth

Abstract

Adsorption isotherms of water on muscovite mica are obtained using grand canonical Monte Carlo simulations over a wide range of relative vapor pressures, p/p(0) at 298 K. Three distinct stages are observed in the adsorption isotherm. A sharp rise in the water coverage occurs for 0 < p/p(0) < 0.1. This is followed by a relatively slow increase in the coverage for 0.1 < or = p/p(0) < or = 0.7. Above p/p(0) = 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 < or = p/p(0) < or = 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 liquidlike 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/p(0) > 0.7.

On the Hydrogen Bonding Structure at the Aqueous Interface of Ammonium-Substituted Mica: A Molecular Dynamics Simulation

“…The congruence with the experimental distribution has been attributed to the predominant role played by the basal surface bridging oxygen atoms on the mica surface [19]. Our atomic density profiles are also in generally good agreement with the results of earlier simulations [20,23,24,26], and any discrepancies could be attributed to the random distribution of the surface cations and isomorphic aluminium substitution at the tetrahedral layer, compared to the more ordered distributions assumed in the earlier studies.…”

“…Concurrently, molecular computer simulations have become one of the most important tools in the study of such interfacial systems and phenomena by providing the atomistic information in many ways complementary to the experimental data. For instance, the results of Monte Carlo (MC) simulations by Park and Sposito [23] and Malani and Ayappa [20] on the adsorption of water adjacent to muscovite mica are consistent with the X-ray reflectivity measurements [31]. On the other hand, molecular dynamics (MD) simulations by Wang et al [12,26] on the hydrated muscovite (001) surface suggest the probable orientational ordering and the diffusive restriction of water molecules normal to the surface.…”

X-ray reflectivity reveals equilibrium density profile of molecular liquid under nanometre confinement

“…Since both mica membranes have the same thickness, a symmetric arrangement is obtained. Permanently adsorbed water is found on the inner and outer mica surfaces as is expected from their hydrophilic character [15][16][17]. Even in a dry environment, an adsorbed water layer is present at the surface planes of potassium ions which we assume to be immobile in our study.…”

The nature of the air-cleaved mica surface

“…A difficulty when comparing the results of the different studies is that some of these, experimental as well as simulations, strictly speaking deal with the interface between mica and bulk water [167; 168] whereas others consider a system of mica with a thin film of water in vapour [169][170][171][172][173][174]. These two systems may be quite different for films that are only a few water layers thick, when the mica-adsorbed film and the adsorbed film-vapour interface are not independent.…”