A magnetic nanoparticle complex includes a magnetic nanoparticle; and a ligand associated with the magnetic nanoparticle, the ligand including a functional group capable of combining with an acid component or a conjugate base of the acid component, in an oil. A method for preparing a magnetic nanopartide complex, includes preparing a pre-ligand having at least one amino group and at least one carbamate group or dithiocarbamate group; associating the pre-ligand with a magnetic nanoparticle to form a magnetic nanoparticle-ligand complex; and modifying the ligand to form a modified ligand having a functional group capable of combining with an acid component in an oil or a conjugate base of the acid component.