Study on the properties and stability of ionic liquid-based ferrofluids
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Study on the properties and stability of ionic liquid-based ferrofluids Wei Huang & Xiaolei Wang
Received: 17 July 2012 / Revised: 12 August 2012 / Accepted: 12 August 2012 # Springer-Verlag 2012
Abstract Ionic liquid (IL)-based ferrofluids have been prepared dispersing both bare and sterically stabilized CoFe2O4 nanoparticles. The precipitated particles were characterized by X-ray diffraction, scanning electronic microscopy, transmission electron microscopy, Fourier transform infrared, and vibrating sample magnetometry studies. The waterabsorbing property of ferrofluids at ambient temperature was estimated by weight and viscosity measurements. Colloidal dispersion stability of the ferrofluids was evaluated by particle suspension percentage. Experimental results indicate that interparticle electrostatic repulsion is not effective in stabilizing bare magnetic particles in IL. There is no significant increase on the dispersion stability when the particles were coated with a monolayer of oleic acid. The reason could be caused by the incompatibility between the nonpolar tail of surfactant and carrier liquid. When excess oleic acid was added into IL, stable magnetic colloid was achieved by a steric stabilization layer coated to be compatible with the IL. Keywords Ferrofluids . Ionic liquid . Suspension stability . Nanoparticles
Introduction Ferrofluids (FFs) are stable colloidal dispersions of nanosized ferro- or ferrimagnetic particles in a carrier liquid [1]. Brownian motion keeps the particles from settling under gravity, and a surfactant is placed around each particle to W. Huang (*) : X. Wang College of Mechanical and Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016, People’s Republic of China e-mail: [email protected]
provide short-range steric repulsion between particles to prevent particle agglomeration in the presence of nonuniform magnetic fields [2]. Owing to their unique physical and chemical properties, FFs have attracted wide interest since their inception in the late 1960s. The peculiarity of FFs is the combination of normal liquid behavior with a magnetic control of their flow [3]. Thus, for the first time in history, it became possible to exert a magnetic force on a fluid to control its position. At the same time, the coexistence in such colloid systems of the magnetism and fluidity allowed them to be referred as unique physical objects [4]. Up to now, many successful applications of FFs appeared, and the three main application areas are focused on sealing, damping, and heat transfer [5]. As dispersion medium, carrier liquid is the major component of FFs. A choice of a carrier liquid, on which base a FF is prepared, is specified by a purpose and place of its application. So far, most of the FFs used in different fields of research and technology have been prepared in certain carrier liquids such as water, kerosene, or different oils [6]. However, the use of these carrier liquids may limit the potential applications of
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