THERMODYNAMIC STUDY OF ADSORPTION OF ANIONIC NONIONIC SURFACTANT MIXTURES AT THE ALUMINA WATER INTERFACE

Mechanisms of adsorption of an anionic surfactant, sodium octylbenzenesulfonate (C8phiS), and a nonionic surfactant, dodecyloxyheptaethoxyethylalcohol (C12EO8), and their mixtures on alumina were investigated by adsorption and microcalorimetric studies. Adsorption of anionic surfactant alone on alumina was initially highly exothermic due to the electrostatic interaction with the substrate. Further adsorption leading to solloid (hemimicelle) formation is mainly an entropy driven process. The entropy effect was found to be more pronounced for the adsorption of anionic-nonionic surfactant mixtures than that for anionic C8phiS alone. High surface activity of the nonionic C12EO8 and its hydrophobic interaction with adsorbed C8phiS is proposed to be the main mechanism for the marked entropy effect for mixture adsorption.