Investigation of the Molecular Surface Coating on the Stability of Insulating Magnetic Oils Wesley R. Viali, † Gustavo B. Alcantara, ‡ Patricia P. C. Sartoratto, † Maria A. G. Soler,* ,‡ Ewa Mosiniewicz-Szablewska, § Bartlomiej Andrzejewski, | and Paulo C. Morais ‡ Instituto de Quimica, UniVersidade Federal de Goias, Goiania GO 74001-970, Brazil, Instituto de Fisica, UniVersidade de Brasilia, Brasilia DF 70910-900, Brazil, Institute of Physics, Polish Academy of Sciences, Al. Lotnikow 32/46, 02-668 Warsaw, Poland, and Institute of Molecular Physics, Polish Academy of Sciences, Smoluchowskiego 17, 60-179 Poznan, Poland ReceiVed: September 9, 2009; ReVised Manuscript ReceiVed: NoVember 14, 2009 Surface functionalization of a series of nanosized iron oxide particles (average diameter around 6 nm) with oleic acid was realized in this study. The aim is to suspend the surface-functionalized nanoparticulated materials in insulating mineral oil and evaluate their colloidal stability as a function of time. Nanoparticulated samples presenting stoichiometry close to maghemite were obtained by oxidation of a freshly precipitated magnetite sample. Systematic variations observed in the Fe 3+ /Fe 2+ ratio, average particle size, and lattice constant were attributed to differences in oxidation route and oxidation condition employed. Morphological, compositional, thermal, optical and magnetic characterization techniques were used in the investigation of native (P, PN1, PN2, POX1, POX3, and POX7) and surface-functionalized (POA, PN1OA, PN2OA, POX1OA, POX3OA, and POX7OA) samples. While suspending the oleic-acid-coated nanosized iron oxide particles in insulating mineral oil, the best colloidal stability was achieved at the oxidation profile of Fe 3+ /Fe 2+ ) 40 (5.9 nm average core diameter), leading to a surface grafting coefficient of about 75% of a full monolayer coating of chemisorbed species only and resulting in the smallest observed hydrodynamic radius (8.1 nm). Within the range of our investigation, our findings reveal the characteristics and the chemical protocol used to produce a magnetic fluid sample embodying long-term colloidal stability, thus representing a very much promising material for application as a refrigerating fluid in power transformers and related devices. Introduction Over the past two decades, nanosized materials, for instance, superparamagnetic iron oxide (SPIO), have received a great deal of attention focusing on topics as different as insulating magnetic oils for transformers 1-4 and drug delivery systems for cancer therapy, 5 particularly due to the unusual and enhanced properties of such materials, 6,7 opening up perspectives for a wide variety of industrial and medical applications. 8-10 Nanosized particles consisting of iron-based cubic ferrites, more specifically, magnetite (Fe 3 O 4 ) and maghemite (γ-Fe 2 O 3 ), represent typical SPIO materials, embodying improved chemical stability and enhanced biological compatibility. SPIO-based magnetic fluids (MFs) have long been used in several industrial applications and more recently exploited as a very promising material platform for biomedical applications 11 while providing simple routes for incorporation of nanosized magnetic particles in different hosting templates. 12-15 However, each particular ap- plication demands very specific characteristics of the SPIO-based material and the corresponding surface-functionalization, taking into account the chemical and structural stability of the nanoparticle core as well as the physicochemical and biological properties of the molecular layer adsorbed onto the nanoparticle surface, the latter playing a key role on the MF colloidal stability. Two basic mechanisms have been used to promote colloidal stability while preparing magnetic fluids: the steric and the electrostatic repulsion mechanisms aimed to overcome van der Waals and magnetic dipole attractions. 16 Stable MF samples can be prepared using several routes. 17 An approach for the production of nonaqueous high-quality MF samples is the thermal decomposition of iron precursors in the presence of hot organic surfactants. 18 Willis et al. reported the synthesis of monodisperse maghemite (diameter around 11 nm) from tri- octylamine, oleic acid, and Fe(CO) 5 using a high-temperature decomposition reaction. 19 The oleic acid (OA) promotes the nanoparticle surface passivation and renders the nanosized particle stable in a variety of organic media. By monitoring the surfactant chemical structure during the nanoparticle synthesis, the authors found that the OA coating is chemically transformed during the high-temperature synthesis process, leading the formation of high-quality nanosized maghemite particles. 19 In another approach, Pascal et al. developed an electrochemical synthesis route for nanosized maghemite in organic medium. 20 The authors claim that the particle diameter could be controlled between 3 and 8 nm by adjusting the imposed current density while adsorption of long-chain tetraalkylammonium salts prevent particle aggregation in organic media. 20 Sahoo et al. have produced surface-coated magnetite particles (6-8 nm average diameter) using different surfactants (OA, lauric acid, dodecyl phosphonate, hexadecyl phosphonate, and dihexadecyl phos- phonate) in order to stabilize the nanosized particles and disperse them in organic solvents. 21 In contrast, water-based MF samples can be produced by surface dressing the nanosized particle using molecular coatings presenting ionizable functional groups facing the bulk solvent, thus adding an extra component to the colloidal stability of the MF sample through the electrostatic repulsion. Among the huge variety of chemical routes used to synthesize * To whom correspondence should be addressed. Tel: +55-61-3307- 2900. Fax: +55-61-3307-2363. E-mail: soler@unb.br. † Universidade Federal de Goias. ‡ Universidade de Brasilia. § Institute of Physics, Polish Academy of Sciences. | Institute of Molecular Physics, Polish Academy of Sciences. J. Phys. Chem. C 2010, 114, 179–188 179 10.1021/jp908732b  2010 American Chemical Society Published on Web 12/15/2009