Behaviour of SDS micelles bound to mixtures of divalent and trivalent cations during ultrafiltration

SDS micelles with bound mixtures of Al3+ and Zn2+ cations may flocculate or remain colloidal. When the micellar solution does not undergo flocculation, the pattern for the rates of flux for the micellar solutions is similar to unbound SDS micelles and even lower, remaining mostly constant. However, when part of the original micellar surfactant has flocculated, the permeability of the membrane for the remaining colloidal solution is lower than in the case of unbound SDS micelles, even at much lower surfactant concentrations. This may be explained by the fact that the residual colloidal material in partly flocculated solutions is composed of larger colloidal aggregates. True micellar SDS solutions bound to polyvalent cations may be more easily retained within UF systems. The apparent charge inversion observed in some Al3+/Zn2+/SDS compositions is shown not to be as a true charge inversion using Perel and Shklovskii strongly correlated liquid theory. It is shown that the phenomenon can be explained with the compression of the diffuse layer around the micelle and the build-up of a rheological barrier around the micelles, resulting in the capture of part of the bulk solution.