Molecular Modeling of the Nanoscale Complexity of Clay-Water Interfaces: Recent Results and Further Challenges
Molecular-level understanding of mineral-water interfaces is extremely important for many geochemical, environmental, and technological problems, such as geological carbon sequestration, nuclear waste disposal in geological formations, heterogeneous catalysis, etc. Experimental nanoscale studies of such systems are not always feasible, and their results often require considerable interpretation in the efforts to extract quantitative surface-specific and confinement-specific information from the measurements. Molecular computer simulations significantly complement such efforts by providing invaluable atomic scale background understanding of the specific effects of mineral substrate structure and composition on the structure, dynamics and reactivity of interfacial and nano-confined aqueous phase.
Based on the successful development and implementation of the CLAYFF force field (Cygan et al., 2004), we have performed a series of molecular dynamics simulations of aqueous interfaces with several representative clays and clay-related materials in order to better understand and quantify the effects of the substrate composition and structure on the properties of interfacial and nano-confined aqueous fluids. However, simulated system size limitations often impose a considerable quasi-periodicity on the distribution of the tetrahedral and octahedral site substitutions in the T-O-T clay layers. Such quasi-periodicity is then typically propagated further even if larger models are later constructed by simply multiplying the smaller ones, and the effects of this imposed structural ordering on the calculated properties of the interfaces is not obvious. Therefore, we have recently made specific efforts to construct a new set of larger-scale muscovite and montmorillonite models introducing as much structural disorder in the distributions of the Al/Si and Mg/Al substitutions as possible. The effects of such disorder on the local structural and dynamic properties of the interfacial and interlayer water and hydrated ions are then carefully quantified and analyzed in MD simulations for a number of typical monovalent and divalent cations.
The time- and space- averaged properties of the hydrated interfaces and interlayers, such as atomic density profiles, radial distribution functions, and surface diffusion coefficients, are observed to be not particularly sensitive to the specific details of the clay substrate disorder. However, local effects of the disordered distribution of charge-substituted sites in clay layers can be substantial and can influence the strength of the ionic and molecular adsorption at a specific surface site, the structure and dynamics of the interfacial hydrogen bonding network around it, and the molecular mechanisms of its local rearrangement.
Cygan R.T., Liang J.-J., Kalinichev A.G. (2004) Molecular models of hydroxide, oxyhydroxide, and clay phases and the development of a general force field. J. Phys. Chem. B, 108, 1255-1266.