title: Liquid-liquid phase-transfer of magnetic nanoparticles in organic solvents.
abstract: We report a novel route for the preparation of well-defined colloidal dispersions of magnetic nanoparticles stabilized by steric repulsion in organic solvents. The usual methods standardly lead to the surfaction of multiparticle aggregates, incompatible with our long-term aim of studying and modeling the influence of magnetic dipolar interactions in colloidal dispersions which are free of aggregates, all other interactions being perfectly defined. A new and reproducible method based on a surfactant-mediated liquid-liquid phase transfer of individually dispersed gamma-Fe(2)O(3) nanoparticles from an aqueous colloidal dispersion to an organic phase is developed. The choice of the reagent and the preparation techniques is discussed. Among several solvent/surfactant pairs, the cyclohexane/dimethyldidodecylammonium bromide (DDAB) system is found to fulfill the colloidal stability criterion: aggregation does not appear, even upon aging. A complete transfer of isolated particles is observed above a threshold in DDAB concentration. The nanoparticle surface is then fully covered with adsorbed DDAB molecules, each surfactant head occupying a surface of 0.57+/-0.05 nm(2). The volume fraction of the cyclohexane-based organosols is easily tunable up to a volume fraction of 12% by modifying the volume ratio of the organic and of the aqueous phases during the liquid-liquid phase transfer.
authors.affiliation: Laboratoire LI2C, Université Pierre et Marie Curie, UMR CNRS 7612, 4, place Jussieu, case 51, 75252 Paris Cedex 5, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Mériguet
authors.forename: Emmanuelle
authors.initials: E
authors.lastname: Dubois
authors.forename: Régine
authors.initials: R
authors.lastname: Perzynski