Electrostatically Driven Structure Formation in Clay Dispersions

Sammanfattning: Clay minerals consist of clay platelets, which are anisotropic in both size and charge, and these can form a lamellar structure depending on the size, making it a seemingly perfect model system for an electrical double layer. The situation is however, from a structural point, slightly less ideal. Clay is a challenging system since it has a polydisperse distribution of anisotropic clay platelets and it is normally not a homogeneous lamellar material, it is rather described as a disordered structure of stacks of platelets, also denoted tactoids. The stability in saline solution depends strongly on the surface charge density of the platelets, and on the counterion valency. By altering the physiochemical properties, tactoid formation can be promoted, where a transition from a repulsive to an attractive system is obtained. In addition, the overcharging effect may result in tactoidal dissolution, where the tactoids dissolve into smaller sized tactoids. The aim of this thesis has been to get a better understanding of the electrostatically driven structure formation in clay dispersions and to gain a deeper molecular knowledge of the interactions between the clay platelets, as well as the interactions of the clay platelets in the presence of other macromolecules. The research has focused on extensive studies of the tactoid formation and the tactoidal behaviour of the natural clay mineral montmorillonite and the synthetic clay mineral Laponite® by varying the valency of the counterions, the temperature, and the solvent properties, as well as with addition of multivalent salt and positively charged peptides and polyelectrolytes. It was found that it is possible to capture the behaviour of tactoid formation, tactoidal growth, and tactoidal dissolution for clay systems where the interactions are dominated by electrostatics by utilising coarse-grained molecular dynamics simulations based on the continuum model in combination with a number of different experimental techniques.