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

Abstract

Using monodisperse anatase and hematite particles, such classical techniques as colloid stability, ζ potential and wettability measurements along with fluorescence spectroscopy show that anionic alkyl sulfate and sulfonate surfactants first associate into hemimicelles at the interface and cause the particles to begin to coagulate and to become hydrophobic. Maximum coagulation and hydrophobicity occur under conditions where the ζ potential is zero. At higher surfactant concentrations, the particles redisperse and regain their hydrophilicity. Upon reversal of the ζ potential, the adsorbed surfactant ions appear to form hemimicelles with reverse orientation or bilayers, at surfactant concentrations significantly lower than the critical micelle concentration. The results of these experiments correlate well with spectroscopic and adsorption measurements.