Surface analysis of magnetite nanoparticles in cyclohexane solutions of oleic acid and oleylamine

The surface of magnetite (Fe3O4, d=6.3±0.7 nm) nanoparticles dispersed in cyclohexane was studied in the presence of oleic acid and oleylamine using in situ FTIR spectroscopy. Equimolar mixtures of these surfactants are widely used in the chemical synthesis of nanoparticles with a low polydispersity. Here, the IR spectra indicate that oleic acid molecules adsorb to the magnetite surface as a carboxylate. Measurements as a function of surfactant concentration yield an adsorption isotherm, with about two surfactant molecules adsorbed per nm2 of magnetite at 1 mM surfactant concentration and about 3.5 molecules per nm2 at 310 mM, of the order expected for full monolayer coverage. No spectral indication is found of oleylamine molecules at the surface of magnetite. In solution, however, almost every oleylamine molecule combines with an oleic acid molecule to form an acid–base complex, with an association constant of 3.5×104dm3mol−1.