Evaluation of the Tactoid Formation in Clay Systems

Sammanfattning: Clay minerals consist of anisotropic nanoplatelets where the majority of the clay minerals are highly charged, and thus, swell in aqueous solution. The structure and swelling depend on the type of clay mineral, and also on the ionic composition, and temperature. The natural clay mineral montmorillonite have been studied both theoretically and experimentally. Theoretically, coarse-grained models have been used to represent the clay platelets where Monte Carlo (MC), and molecular dynamics (MD) simulations have been utilized to study the electrostatic interactions between the platelets. Experimentally, the structure has been studied with small angle X-ray scattering (SAXS), and the swelling has been studied by swelling pressure measurements in a test cell.The research in this thesis has been focused on the tactoid formation, i.e. platelets aggregating in a face-to-face configuration with an equidistant separation, as a function of the divalent counterion charge ratio, and as an effect of the relative permittivity of the solvent. It was found that it was possible to tune the electrostatic interactions to obtain a transition from a repulsive to an attractive system when increasing the divalent counterion charge ratio or by decreasing the relative permittivity of the solvent. Moreover, the temperature response was investigated, where it was found that the electrostatic interactions alone can give a positive, negative, or constant osmotic pressure response with temperature.