Swelling and Microstructure of Nanoplatelet Systems

Sammanfattning: Many clay minerals consist of charged nanoplatelets that swell in an aqueous solution. The swelling and the microstructure depend on the type of clay mineral but are also dependent on, for example, the ionic composition and the temperature of the solution. The synthetic clay mineral Laponite and the natural clay mineral montmorillonite have been studied experimentally and theoretically. The swelling has been studied by swelling pressure measurements in a test cell and the microstructure has been studied by small angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryo-TEM). Theoretically, Metropolis Monte Carlo (MC) simulations and molecular dynamics (MD) simulations were used to study the electrostatic interactions between the platelets and an adsorbing polymer was also included. Coarse-grained models have been used to represent the platelets, the ions, and the polymers. This thesis can be divided into three parts:In the first part, the tactoid formation (platelets aggregating face-to-face with an equidistant separation) and the microstructure of negatively charged platelets were investigated by simulations. Tactoid formation was promoted by increasing the platelet surface charge density, the platelet size, the ion valency, and the salt concentration. With enough added salt, an isodesmic model was suggested that gives a monotonically decaying distribution of aggregation numbers.In the second part, the tactoid formation and the microstructure were investigated with respect to the platelet size for flocculated Laponite and montmorillonite at elevated salt concentrations, with and without the addition of the polymer polyethylene glycol (PEG). The smaller Laponite platelets give rise to a more disordered microstructure compared to the larger montmorillonite platelets. The number of platelets per tactoid increased with the addition of PEG. The simulations suggested that the role of the salt is to screen the repulsive interactions between the platelets, and the role of the polymer is to bridge between the platelets.In the third part, the temperature response of Na/Ca montmorillonite was investigated. It was found that the swelling pressure increased with increased temperature if sodium is the dominating counterion whereas the opposite was found if calcium is the dominating counterion. The simulations predicted this behavior and it was shown that the results could be explained by a single equation.