Simultaneous adsorption of CTAB surfactant and different size magnetite nanoparticles (NPs) on the interfacial tension of n-hexane–water was investigated. Different methods were used to characterize the NPs and aqueous suspensions. Interfacial tension measurements were performed at 293.2 K and ambient pressure of 83.5 kPa. Results indicate that CTAB, with concentrations up to 7.50 × 10−4 mol dm−3, was positively adsorbed whereas no significant adsorption was relevant for NPs alone within (10−4–10−2) wt%. It was while CTAB adsorption was significantly improved in the presence of NPs and more effectivewith the small size particles. Accordingly, either alkaline or acidic solutionswere more effective than neutral pH due to the provided attractive or repulsion interactions. The provided data were nicely reproduced by the well-known Frumkin adsorption isotherm. Accordingly, the saturated interface excess was decreased under alkaline or acidic conditions; whereas adsorption equilibriumconstant showed a slight decreasewith pH up to neutral pH and then sharply increased. With respect to the size of particles, it was revealed that higher saturated interface excess and lower adsorption equilibrium constant were appropriate to the large size particles.
