Experimental investigations of dispersibility, stability and rheological behaviors of aqueous suspensions of colloidal clays

Abstract

This thesis work reports experimental studies of the dispersibility, stability and rheological behavior of aqueous suspensions of synthetic Laponite and natural Namontmorillonite clays. The primary clay particle, known as platelet, has negative charges on its flat surfaces and pH dependent positive charges on its edge. In dry form, these platelets form stacks, known as tactoids, with positive counterions in the intercalated spaces. In aqueous suspensions, these tactoids exfoliate into single platelets with electric double layers on their surfaces. Due to screened repulsive interactions between the anisotropic electric double layers, the e ective excluded volume is much higher than the geometrical volume of individual platelets. As a result, clay suspensions exhibit spontaneous phase transitions from freely flowing liquids to kinetically arrested, disordered glassy solids at particle volume fractions that lie far below the jamming transition concentration of hard spherical particles. The interaction force between two platelets is a function of the interparticle separation, mutual orientations and thickness of the electric double layers. Therefore, it is possible to tune the interactions between the clay particles externally by controlling the particle and ionic concentrations in the dispersing medium and solution pH. Addition of appropriate amounts of salt leads to attractions between the screened electric double layers, which can result in the formation of volume-spanning network structures known as gels. Kinetically arrested phases, such as glasses and gels, are also characterized by their aging behavior, due to which the dynamics of clay particles slow down with time, leading to changes in their bulk properties. Besides their obvious importance in fundamental research, aqueous suspensions of these clay colloids find wide applications in the pharmaceutical, cosmetics, petroleum, coating, and food industries.