Suspensions of Iron Oxide Nanoparticles Stabilized by Anionic Surfactants

Two common anionic surfactants, sodium oleate(SO) and sodium dodecyl benzene sulfonate (SDBS)were used to re-suspend iron oxide nanoparticles in aqueoussolutions. At certain SO concentrations, the SO formulationsproduced highly stable suspensions. In contrast,SDBS-stabilized nanoparticles exhibited poor stability at allconcentrations. The adsorption isotherm of SO on ironoxide nanoparticles revealed that stable suspensions wereobtained when the equilibrium SO concentration (afteradsorption) reached its critical micelle concentration(CMC). At this ‘‘optimal’’ condition, the maximum SOadsorption was reached, and the zeta-potential of the particleswas highly negative (* -50 mV). According to theSO isotherm, this optimal formulation coincided with theformation of a highly compact SO bilayer. The SDBS isotherm,on the other hand, revealed that SDBS is not stronglyadsorbed on the surface of iron oxide nanoparticles and thatis likely that a patchy, loosely packed bilayer, is formed onthe surface of the iron oxide nanoparticles when the equilibriumSDBS concentration reaches its CMC. The DLVOtheory confirmed the connection between formulationconditions and the corresponding stability. This worksconfirmed that the formation of a surfactant bilayer is animportant element in producing stable nanoparticle suspensionswith anionic surfactants. It was also confirmed thatfor anionic surfactants, electrostatic repulsions are animportant factor in establishing an energy barrier againstflocculation. This work also introduced two more elementsinto the design of nanoparticle suspensions. The first elementis that, in order to ensure the best possible dispersion,the surfactant concentration in solution at equilibrium withthe adsorbed surfactant should be close or slightly above itsCMC. The second element is that the molecular structure ofthe surfactant should facilitate the formation of closelypacked bilayers.