Two types of electrokinetic behavior of solid particles in the presence of anionic surfactants

The electrokinetic curves of metal oxides obtained at moderate concentrations of sodium dodecylsulfate show a maximum at pH close to the pristine IEP of the oxide (IEP in the absence of surfactant). This behavior of SDS is very different from most other anions, in which the electrokinetic curves did not show a maximum. We hypothesize that the unusual behavior of SDS is due to its micellization in solution and on the surface.

The ζ potentials of alumina, titania, and hematite containing sodium tetradecyl- and hexadecylsulfate at different concentrations have been studied. These surfactants do not form micelles in solution.

In dispersions of hematite, titania, and alumina stabilized with SDS, moderate concentrations of the surfactant induced a reversal of sign of the ζ potential at acidic pH (<4) while the effect of the surfactant at neutral and basic pH (>5) was less significant. Hematite dispersions stabilized with sodium tetradecyl- and hexadecylsulfate showed the electrokinetic behavior typical for most anions (no maximum in ζ potential) rather than that characteristic for SDS. The increase in the chain length from 12 to 16 C atoms induces depression and then disappearance of a maximum in the ζ potential vs. pH curves.