Oleic acid (OA)-coated magnetite (Fe3O4) nanoparticles, denoted Fe3O4@OA, were synthesized by co-precipitation in the presence of varying contents of OA. The Fe3O4@OA nanoparticles were characterized by X-ray diffraction, transmission and scanning electron microscopies, Fourier transform infrared spectroscopy, thermogravimetric–differential thermogravimetric analyses, and vibrating sample magnetometry. Increasing the OA content during preparation resulted in an increase of the OA-coating amount (A O, in units of g of OA/g of Fe3O4) on the Fe3O4 surface, before reaching an equilibrium value. The resulting magnetic nanoparticles were nearly spherical with a size of ∼12–14 nm. OA molecules formed a single layer coating on the Fe3O4 surface. The A O and area occupied by a single OA molecule at saturation coating were estimated to be 0.11 g g–1 (1.22 mg m–2) and 0.37 nm2, respectively. The Fe3O4@OA nanoparticles were applied in the demulsification of a cyclohexane-in-water nanoemulsion, under an external magnetic field. The effects of A O, demulsifier dosage, pH, and electrolytes on the demulsification efficiency (E D) were investigated. The E D increased and then decreased with increasing A O, which was attributed to a change in wettability of the magnetic nanoparticles. A maximum E D of ∼98% was observed at a ∼90° contact angle between water and the magnetic nanoparticles. The E D was independent of pH and electrolyte (NaCl or CaCl2) concentration, under the studied conditions. The magnetic demulsifier exhibited excellent stability after reuse over 6 cycles. Fe3O4@OA nanoparticles are effective for oil–water multiphase separation and treating oily wastewater.

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