Molecular Dynamics Simulation of Hydration and Swelling of Mixed-Layer Clays

Swelling of clay minerals is important to a broad class of problems in science and engineering. While the problem has been extensively studied experimentally, past molecular modeling of the phenomenon was focused on pure clays of one type or another. In practice, however, there is a diverse class of mixed-layer clays (MLCs) in sedimentary rock with intermixed stacking sequence of two or more types of distinct layers within a single crystal. In fact, more than 60% of sedimentary rocks in the U.S. contain various types of MLCs. We present the results, to our knowledge, of the first molecular dynamics simulation of hydration energetics and swelling of illite-montmorillonite (I-MMT) MLCs, the most common type of mixed clays. The swelling is studied as a function of the water concentration with three combinations of interlayer cations, namely, Na+ and K+ The hydration energies, the radial distribution functions, and the density profiles in the interlayer region are computed. For regular Na-MMT with layer charge concentrated in the octahedral sheet, weak cation surface interaction results in fully hydrated ions and significant swelling. In the asymmetric interlayer of the MLC, however, the illite sheet with stronger interaction of surface and ions causes adsorption of the cations deep in the ditrigonal cavities of the siloxane surface. Given that the hydration enthalpy of K+ is smaller than that of Na+, its hydration shell is quite unstable compared with that of Na+. Therefore, swelling is inhibited as the ratio K+/Na+ increases. The results demonstrate the significant differences between the hydration and swelling properties of pure clays and the mixed ones, which have important implications in practice, particularly for sequestration of CO2 in sedimentary rock.