Self-association and reverse hemimicelle formation at solid-water interfaces in dilute surfactant solutions

Using monodisperse anatase and hematite particles, such classical technique s as colloid stability, zeta potential and wettability measurements along w ith fluorescence spectroscopy show that anionic alkyl sulfate and sulfonate surfactants first associate into hemimicelles at the interface and cause t he particles to begin to coagulate and to become hydrophobic. Maximum coagu lation and hydrophobicity occur under conditions where the zeta potential i s zero. At higher surfactant concentrations, the particles redisperse and r egain their hydrophilicity. Upon reversal of the zeta potential, the adsorb ed surfactant ions appear to form hemimicelles with reverse orientation or bilayers, at surfactant concentrations significantly lower than the critica l micelle concentration. The results of these experiments correlate well wi th spectroscopic and adsorption measurements. (C) 1999 Elsevier Science B.V . All rights reserved.