Synchrotron X-ray Scattering and Monte Carlo Simulations of Structure and Forces in Silicate Nanoplatelet Dispersions

Abstract: Clays are the world’s most widely used natural material, however, little is known regarding the microstructure as well as the forces involved in clay-water interactions, and their influence on the swelling properties. The utilization of clay platelets is nowadays a key in a number of biological and industrial applications e.g. nuclear waste management. Bentonites from different natural sources and pure Na/Ca montmorillonite platelets have been studied experimentally and theoretically. Small angle X-ray scattering (SAXS), dynamic light scattering (DLS), nuclear magnetic resonance (NMR), X-ray absorption spectroscopy (XAS), transmission electron microscopy (TEM) and Cryogenic-TEM were applied to provide direct information about the structure of dry clays as well as clay platelets in equilibrium with a bulk solution of given ionic composition, temperature and pH. Monte Carlo (MC) simulations have been used to predict the osmotic pressure of montmorillonite dispersions. The swelling behavior is mainly regulated by counterion valency and surface charge density. Divalent counterions and high surface charge density lead to a limited swelling, while monovalent counterions favor a large swelling. This thesis has also investigated the aggregation of nanoplatelets in clay-water systems, in order to understand the effect of platelet size on the structure and swelling behavior. A new twist on aggregation phenomenon is that, really small platelets (~ 20 nm) do not form a tactoid, whereas larger platelets give rise to larger tactoids. The platelet size controls the aggregation and microstructure of silicate platelets into tactoids following an empirical relation as: N ≃ δ + α Deff, where N is the number of platelets per tactoid, Deff is the effective diameter of platelets, δ and α are constants.