Magnetically responsive and interfacially active composite nanoparticles comprising an inner magnetic core and an outer layer of interfacially active materials were prepared by first priming iron oxide (Fe3O4) nanoparticles with sodium carboxymethyl cellulose (CMC), followed by the adsorption of ethyl cellulose (EC) on the CMC-primed nanoparticles. In contrast to the previous preparation methods, synthesizing magnetically responsive and interfacially active composite nanoparticles by sequential adsorption of contrasting cellulosic materials without unnecessary derivatization reactions is much simpler and more energy efficient while generating less waste. The resulting composite nanoparticles are interfacially active and thus can be effectively partitioned at the oil-water interface. Once the nanoparticles attached to the interface, multiphase materials will be magnetically tagged as desired and effectively manipulated or isolated by an applied magnetic field. The sequential adsorption of CMC and EC on iron oxide surface was investigated using a quartz crystal microbalance with dissipation monitoring (QCM-D). The adsorption of CMC on bare iron oxide surface can not only enhance the subsequent adsorption of EC on CMC-primed iron oxide surface, but also drastically improved colloidal stability of iron oxide dispersions. Quick phase separation of emulsions, oily rag layers, and sludge was achieved by applying the magnetically responsive and interfacially active composite nanoparticles.