Interaction forces, heteroaggregation, and deposition involving charged colloidal particles.

ABSTRACT

Force profiles as well as aggregation and deposition rates are studied for asymmetrically charged particles and surfaces in aqueous electrolytes theoretically. Interactions are calculated within the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, whereby the electrostatic part is modeled at Poisson-Boltzmann (PB) level. Unequally charged surfaces are examined, from the symmetric system, where both surfaces are equally charged, to fully asymmetric systems, where the surfaces are oppositely charged. Charged-neutral systems, where one surface is charged and the other is neutral, emerge as an essential scenario. In this case, the choice of boundary conditions used for solving the PB equation is crucial, whereby constant charge and constant potential boundary conditions lead to either fully repulsive or fully attractive forces. Consequently, charge regulation has a major influence on particle aggregation and deposition rates too. In the charge-neutral case, substantial shifts in the critical coagulation concentration (CCC) are observed when the regulation properties are changed. In the presence of multivalent ions, these systems behave similarly to the symmetrically charged ones. The CCC decreases with the square of the valence in weakly charged systems, while unrealistically high charge densities are needed to recover the classical Schulze-Hardy limit, which predicts a sixth power dependence on valence.