Monte Carlo study of the adsorption and aggregation of alkyltrimethylammonium chloride on the montmorillonite-water interface.

Organically modified clays exhibit adsorption capacities for cations, anions, and nonpolar organic compounds, which make them valuable for various environmental technical applications. To improve the understanding of the adsorption processes, the molecular-scale characterization of the structures of organic aggregates assembled on the external basal surfaces of clay particles is essential. The focus of this Monte Carlo simulation study was on the effects of the surface coverage and the alkyl chain length n on the structures of alkyltrimethylammonium chloride ((C(n)TMA)Cl) aggregates assembled on the montmorillonite-water interface.

Aggregation of alkyltrimethylammonium ions at the cleaved muscovite mica-water interface: a Monte Carlo study.

The precise molecular structure of organically modified mineral surfaces is still not well understood. To establish a relation between experimental observations and underlying molecular structure, we performed Monte Carlo simulations of the aggregation behavior of alkyltrimethylammonium surfactants (C(n)TMA(+)) at the interface between C(n)TMACl solution and cleaved K(+)-muscovite. The structures were examined with regard to the influence of varying alkyl chain length n (n = 8, 12, 16) and surface coverage of C(n)TMA(+) ions.

Potential of mean force for K+ in thin water films on cleaved mica.

The atomically smooth surface of cleaved K(+)-mica is a common substrate for investigations of various phenomena at the mineral-water interface. In this study, K(+) inner-sphere adsorption above tetrahedral substitutions in the mica structure is found to be more favorable than that in mica's ditrigonal cavities for both dehydrated and hydrated cleaved mica. The activation energy necessary to transfer K(+) from the inner-sphere adsorption state to the outer-sphere adsorption state, in which K(+) can presumably migrate parallel to the surface, is calculated to be 45-48 kJ/mol on hydrated cleaved mica.

Microhydration of the selenite dianion: a theoretical study of structures, hydration energies, and electronic stabilities of SeO(3)(2-)(H(2)O)(n) (n = 0-6, 9) clusters.

In extension of the ongoing investigations of oxyanion-water clusters, we studied energetically low-lying configurations of hydrated selenite dianion (and in select cases, SeO(3)(-)) clusters using density functional theory (B3LYP, M05-2X, PBE0) and second-order Møller-Plesset perturbation theory (MP2). Water molecules doubly hydrogen bond to the selenite oxygens for n View Article and Find Full Text PDF

Cetylpyridinium aggregates at the montmorillonite- and muscovite-water interfaces: a Monte Carlo study of surface charge effect.

Monte Carlo simulations of the interface between the external surface of montmorillonite and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported and compared with the preceding simulation study of muscovite. Simulation results reveal that a segregation of inorganic ions into the regions near the montmorillonite-water and bilayer-water interfaces leads to a nearly complete displacement of water molecules from the intermediate region of the bilayer aggregate. Such segregation does not occur for muscovite because of a considerably higher surface concentration of inorganic cations compensating its mineral charge.

Cetylpyridinium chloride at the mica-water interface: incomplete monolayer and bilayer structures.

Monte Carlo simulations of the interface between the cleaved surface of muscovite mica and aqueous cetylpyridinium chloride (CPCl) solution at ambient conditions are reported. Simulation results reveal that monolayer or bilayer aggregates of CP(+) ions at the muscovite-water interface remain incomplete up to a CP(+) coverage compensating the negative charge of muscovite. It is predicted that at this CP(+) coverage only a partial desorption of K(+) ions occurs and the two aggregates can be distinguished with help of the X-ray reflectivity technique.

Interlayer expansion and mechanisms of anion sorption of Na-montmorillonite modified by cetylpyridinium chloride: a Monte Carlo study.

To study the change of interlayer structure of a Wyoming-type Na-montmorillonite as a result of the replacement of interlayer Na+ ions by cetylpyridinium (CP+) ions, a series of NPT Monte Carlo simulations of the clay mineral with different contents of CP+, Na+, Cl- ions and water in its interlayer space is carried out. In agreement with conclusions from experimental studies, the simulations show that the CP+ ions form monomolecular, bimolecular, and pseudotrimolecular layers with increasing interlayer contents. Calculated potential energies reveal that clay-organic interactions are stronger than organic-organic interactions in CP+-modified montmorillonite, which is in conformity with observations of earlier thermogravimetric experiments.

Swelling of NaMg-montmorillonites and hydration of interlayer cations: a Monte Carlo study.

While the swelling behavior of laboratory-prepared homoionic montmorillonites has been studied extensively in numerous experimental and simulation works, far less attention has been given to much more abundant natural montmorillonites, containing a mix of monovalent and/or bivalent cations in interlayer spaces. We carried out a series of Monte Carlo simulations in order to investigate the reasons for the remarkable difference of experimental swelling patterns of a natural Na-rich/Mg-poor montmorillonite and a homoionic Na-montmorillonite. The simulations reproduced the swelling pattern of a natural montmorillonite, suggesting a mechanism of its hydration different from that of the homoionic montmorillonite.

The gap between crystalline and osmotic swelling of Na-montmorillonite: a Monte Carlo study.

Although the swelling of clay during moistening is a broadly experienced occurrence, the mechanisms driving it and especially the reason for the existence of a peculiar gap between crystalline and osmotic swelling of Na-montmorillonite are not yet fully understood. We obtained a deeper insight by means of Monte Carlo simulations of Na-montmorillonite swelling, which yield the swelling curve, interaction energies between and characteristic positions of structural atoms and water molecules. We find that a chainlike structure consisting of Na cations, water molecules, and oxygens of substituted tetrahedrons of neighboring mineral layers is formed in the interlayer space of Na-montmorillonite at a layer spacing of approximately 19 A, where experimental investigations show termination of crystalline swelling.