20018
J. Phys. Chem. C 2008, 112, 20018–20026
Aqueous Solution Structure at the Cleaved Mica Surface: Influence of K+, H3O+, and Cs+ Adsorption Artur Meleshyn Center for Radiation Protection and Radioecology (ZSR), Leibniz UniVersita¨t HannoVer, Herrenha¨user Str. 2, 30419 HannoVer, Germany ReceiVed: July 23, 2008
Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous solution containing K+, Cs+, and/or H3O+ ions compensating negative charge of muscovite and Cl- ions at ambient conditions are reported. Simulation results reveal that, in dependence on surface concentration, K+ is preferentially adsorbed above ditrigonal cavity centers or substituted tetrahedrons ∼1.7 Å or ∼2.15 Å from the surface, respectively, whereas Cs+ is preferentially adsorbed above ditrigonal cavity centers ∼2.0-2.2 Å from the surface. Adsorption of K+ and Cs+ in excess of the surface concentration compensating the negative charge of muscovite leads to a coadsorption of Cl- as an outer sphere complex on the solution side of the Stern layer. This study also suggests that H3O+ is the only species adsorbing as close as ∼1.3 Å from the mica surface in contact with deionized water and contributing to the first adsorbed layer observed recently with X-ray reflectivity. On the basis of available experimental and the presented simulated electron density profiles, it is concluded that KCl (or CsCl) solution concentrations of 0.01 and 0.5 M in contact with muscovite result in an adsorption of 0-0.5 and 1-1.375 of K+ (or ∼0.5 and ∼1.25 of Cs+) ions per unit cell area, respectively. 1. Introduction Muscovite mica is a clay mineral with a basal surface structure very similar to that of illite and montmorillonite, which are two other clay minerals of high importance in various environmental and industrial applications. In contrast to the latter two minerals, however, muscovite has cleavage properties, which make it an excellent model system for studying phenomena occurring at solid-liquid interface in general and for investigating colloid properties of clays in particular. Muscovite can be cleaved along a basal plane to produce an atomically flat surface over a large area sufficient for use in X-ray reflectivity,1,2 electrokinetic,3 direct force,4 atomic force microscopy,5-8 and shear force microscopy9 measurements. These studies resulted in a considerable progress in understanding the solution structure and mechanisms of cation adsorption at the muscovite-water interface. It has been found that water hydrating muscovite surface has an ordered layered structure with layer periodicity of 2.5 ( 0.3 Å,1,4 which rationalized previous observations on clay-water systems. Recent experimental studies of water films adsorbed on the muscovite surface additionally revealed the formation of twodimensional water islands with polygonal edges in registry with the mica lattice directions at room temperature.6,7 These findings motivated molecular simulation studies in confined systems with one up to eight water layers sandwiched between two muscovite layers10-13 as well as at the cleaved muscovite surface,14-17 which provided more detailed information about water structure and water properties, as well as water, K+, and H3O+ adsorption sites at the muscovite-water interface. However, previous molecular simulation studies paid no attention to Cs+ adsorption at the cleaved mica surface and related changes in the structure of adsorbed water. A recent X-ray reflectivity study2 provided valuable data on the solution structure at the muscovite-water interface and, on the basis of
the structural models, derived the positions of the adsorbed K+ and Cs+ ions. These derived positions predominantly in the ditrigonal cavities are in agreement with the earlier assumption that K+ and Cs+ can fit ditrigonal cavities3 and suggest a high degree of stability of K+ and Cs+ adsorption in accordance with their observed specificity for the surface of muscovite and clay minerals with similar basal surfaces. Schlegel et al.2 have also investigated 0.5 M KCl and CsCl solutions in contact with the muscovite surface, so that the question arises about the positions of Cl- ions within a diffuse double layer for these relatively high solution concentrations. The present study aims at obtaining detailed molecular-scale data on water structure and positions of the adsorbed water molecules as well as K+, Cs+, H3O+, and Cl- ions at the muscovite-water interface using Monte Carlo (MC) simulations as described in the next section. 2. Simulation Details A layer of 2M1-muscovite mica with the formula unit KAl2(Si3Al)O10(OH)2 consists of two tetrahedral sheets with one out of four Si atoms substituted by Al, which sandwich an octahedral sheet with two out of three octahedrally coordinated positions occupied. The Al substitutions in the tetrahedral sheet are arranged in accordance with the Lo¨wenstein’s rule of avoidance of Al-O-Al linkages, so that hexagonal rings of Si4Al2 and Si5Al1 compositions are equally represented in the modeled muscovite layer. Six basal oxygen atoms bridging Si and Al atoms of the same hexagonal ring are in the vertices of the two equilateral triangles with side lengths of ∼4 Å and ∼5 Å featuring a ditrigonal cavity in the mica surface.5 Parameters of the muscovite unit cell were taken from the X-ray reflectivity study by Schlegel et al.2 Although the name muscovite refers to the natural K+ form of this clay mineral, K+-muscovite is used instead further in the text for clearness as opposed to Cs+exchanged muscovite referring to the cleaved surface of muscovite with K+ ions exchanged by Cs+ ions.
10.1021/jp806524d CCC: $40.75 2008 American Chemical Society Published on Web 11/19/2008
Influence of K+, H3O+, and Cs+ Adsorption The simulation cell consists of the two muscovite layers of a total thickness of 20.059 Å separated from the next two layers through a cleavage along the plane of the interlayer K+ ions and pulling apart the cleaved surfaces to 100 Å. Hence, whereas the interlayer space between the two muscovite layers within the simulation box is identical with the bulk muscovite and contains K+ ions at a coverage corresponding to 2 K+ ions per unit cell area (Auc; Auc) 46.72 Å2), their two external surfaces have a coverage of 1 K+/Auc as a result of cleavage. Lateral dimensions of the simulation cell of ∼20.75 Å by ∼18.01 Å enclose eight unit cells. To simulate the structure of water films adsorbed on muscovite surface, 16, 24, and 48 water molecules (or, respectively, 2, 3, and 6 H2O per unit cell area, Auc; Auc) 46.72 Å2) were randomly distributed in a slab of a thickness of 5 Å within the simulation cell near one of the cleaved muscovite surfaces in the first series of simulations. The K+ ions at this surface, remaining after cleavage and amounting to one monolayer coverage (1 K+/Auc), were moved to positions 3.5 Å away from the surface. To simulate Cs+-exchanged muscovite or cleaved muscovite surface rinsed free of K+ in contact with deionized water, K+ ions were replaced by Cs+ or H3O+ ions, respectively. In the second series of simulations, 278 water molecules corresponding to water coverage of ∼35 H2O/Auc were randomly distributed in a slab of a thickness of 30 Å within the simulation cell near a cleaved muscovite surface. After equilibration, the water film thickness was in the range of ∼24-26 Å for all studied systems, which is large enough to take into account possible effects of water wetting in addition to those of water adsorption.18 Four different K+ and Cs+ coverages of 0.25, 0.5, 1, and 2 K+/Auc or Cs+/Auc and H3O+ coverage of 1 H3O +/Auc with ions in the positions 3.5 Å above the surface were considered. After equilibration in the systems with coverages of 2 K+/Auc or Cs+/Auc, only 1.375 K+/Auc and 1.25 Cs+/Auc were adsorbed within 7 Å from the mica surface. Removal of 0.75 or 0.5 K+/Auc from the simulation cell at the two former K+ coverages was compensated by the addition of 0.75 or 0.5 H3O+/Auc (positioned 3.5 Å above the same cleaved surface), respectively, in accordance with the experimental observations for muscovite in contact with deionized water or water solution at lower KCl concentrations.1,2 Similarly, addition of 1 K+/Auc at the K+ coverage of 2 K+/Auc or 1 Cs+/Auc at the Cs+ coverage of 2 K+/Auc was compensated by the addition of 1 Cl-/Auc (positioned 7.65 Å above the same cleaved surface). The Monte Carlo simulations were carried out in canonical, constant-NVT ensemble with a temperature fixed at 298 K. Three-dimensional periodic boundary conditions were applied to the simulation cell to model the interface between a muscovite platelet and water. Mineral layers were considered as rigid bodies with atomic charges assigned according to Skipper et al.19 The imposed rigidity of the mica layers is a reasonable approximation considering that mineral atoms show relaxations of
