Electrokinetic potential and flocculation of bentonite suspensions in solutions of surfactants, polyelectrolytes and their mixtures

Abstract

The effects of cationic and anionic surfactants and polyelectrolytes, as well as their binary mixtures, on the electrokinetic potential and the degree of aggregation/sizes of particle flocs in bentonite suspensions have been studied. The amounts of the above reagents, compositions and ratios of positive and negative charges in the mixtures, charge densities of the polyelectrolytes, and the order of the introduction of the components into systems have been varied. It has been shown that the addition of cationic surfactants or polyelectrolytes and mixtures thereof causes a substantial reduction of the negative ζ-potentials of the particles and reversal of the sign of their charge. The addition of anionic surfactants or polyelectrolytes leads to a twofold increase in the ζ-potential as a result of adsorption of negatively charged surfactant ions/polymer segments on negatively charged clay particles via non-Coulomb forces. In the presence of mixtures of anionic and cationic polyelectrolytes, irrespective of their composition and the order of the introduction of the components into a suspension, the particles acquire a negative ζ-potential, which is characteristic of particles that adsorb an anionic polyelectrolyte alone. Synergistic flocculating action is observed for mixtures of weakly and moderately charged anionic and cationic polyelectrolytes. This effect takes place even for mixtures with a manifold excess of negatively charged segments relative to positively charged ones. The observed regularities have been explained by the structural peculiarities of mixed adsorption layers of polyelectrolytes, i.e., the coexistence of a thin layer of adsorbed cationic polymer chains characterized by a large number of contacts with the surface and an extended layer of an anionic polyelectrolyte (long loops and tails of macromolecules), in which the cationic polymer layer is “drowned.” The electrokinetic potential and the ability of the particles to aggregation (via the formation of polymer bridges) in mixed solutions of anionic and cationic polyelectrolytes are primarily determined by the adsorption value of the anionic polymer.