ADSORPTION-ISOTHERM OF SODIUM OCTYLBENZENESULFONATE ON IRON-OXIDE PARTICLES IN AQUEOUS-SOLUTIONS

The adsorption isotherm of sodium octylbenzene sulfonate has been dete rmined at two pH values on dispersed maghemite particles in water. The isotherms present an adsorption maximum in the region of the surfacta nt critical micelle concentration. Additional adsorption experiments a t the alumina/water interface as well as at the air/water interface in the presence of ferric ions suggest that the adsorption maximum in th e case of the iron oxide particles is due to the presence of very smal l concentrations of ferric ions in the aqueous solution. These ions fo rm surface active complexes with sodium octylbenzenesulfonate. These c omplexes are adsolubilized in the surfactant layers on the particles b elow the critical micelle concentration. They are desorbed from the su rface and transferred into free micelles above the critical micelle co ncentration. These two phenomena are shown to be responsible for the a dsorption maximum observed. The presence of ferric ions induces a surf ace tension minimum of the anionic surfactant at the air/water interfa ce. This effect is discussed in relation to the adsorption maximum at the solid/water interface, (C) 1998 Academic Press.