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Molecular modeling of interlayer swelling in na-montmorillonite

Expansive clays swell dramatically upon wetting and shrink upon drying causing enormous distress to infrastructure around the world. When swelling is restrained, these soils exert tremendous pressures on the structure causing damage. The small size of clay particles that result in small pore sizes, large surface area to weight ratio and more importantly the breakdown of particles due to interlayer swelling, result in very low hydraulic conductivity and thus make these clays great barrier materials for applications in landfills, nuclear waste disposal etc. In the last 15 years, these clays and particularly clays containing montmorillonite clay minerals are used as nanoparticles. These clay nanoparticles are used in nanocomposites for structural, biomedical and drug delivery applications among many other applications. The interlayer swelling is a key contributor to the macroscale mechanical and fluid flow properties exhibited by these clays. A comprehensive modeling study of Na-montmorillonite clay interlayer to understand key mechanisms during hydration is conducted. The modeling studies are complimented by experiments. Atomic models of Na-montmorillonite models are constructed. Force-field parameters for the clay are found and validated. Solvation simulations are conducted by placing the clay model in a solvation box and conducting simulations for long duration of time. Flow of water molecules into the interlayer are tracked, and change in interlayer spacing is observed. Detailed calculations of molecular interactions in the clay interlayer and conformational changes to water molecules are made, and the key mechanisms that initiate swelling are expounded. Next, simulations of clay with varying magnitudes of hydration are conducted and related to changes in binding interaction energies. The results provide a detailed picture of the evolution of binding interactions while swelling and for the first time describe the role of “bound” water in maintaining the stability of clay interlayer at large interlayer spacing.

Author(s):

Dinesh Katti    
North Dakota State University
United States

Kalpana Katti    
North Dakota State University
United States