Flocculation of clay colloids induced by model polyelectrolytes - effects of relative charge density and size

Flocculation and its tuning is of utmost importance in the optimization of the several industrial protocols, in areas such as purification of waste-water or civil engineering. We study polyelectrolyte-induced flocculation of clay colloids on a model system, consisting of purified clay colloids of well-defined size fractions and ionene polyelectrolytes presenting regular and tuneable chain charge density. To characterize ionene-induced clay flocculation, we turn to the combination of light absorbance (turbidity) and zeta potential measurements, as well as adsorption isotherms. Our model system allows us to identify the exact ratio of positive and negative charge in clay-ionene mixtures, the (c+/c-) ratio. For all samples studied, the onset of efficient flocculation occurs consistently at c+/c- ratios significantly below 1, indicating the formation of highly ionene-deficient aggregates. At the same time, zeta potential measurements indicate an apparent zero charge on such aggregates. Thus, zeta potential values cannot inform us on the stoichiometry inside the clay-ionene aggregates. The early onset of flocculation in clay-ionene mixtures is reminiscent of the behaviour of multivalent salts and contrasts with that of monovalent salts, where a large excess of ions is necessary to achieve flocculation. Clear differences in the flocculation behaviour are visible as a function of the ionene charge density, which governs the conformation of the ionene chains on the clay surface.