In the Laboratory JChemEd.chem.wisc.edu • Vol. 76 No. 7 July 1 9 9 9 • Jo urna l o f Chemic a l Educ a tio n 943 Preparation and Properties of an Aqueous Ferrofluid W Patricia Berger † Department o f Chemistry, So uthern O reg o n University, Ashland, O R 97520 Nicholas B. Adelman, Katie J. Beckman, Dean J. Campbell, †† and Arthur B. Ellis* Department of Chemistry, University of W isconsin-Madison, Madison, W I 53706; * ellis@chem.wisc.edu George C. Lisensky Department of Chemistry, Beloit College, Beloit, W I 53511 Introduction Imagine the production and applications of a liquid that can be controlled by a magnetic field. Creating a strongly magnetic liquid is not as easy as melting a strongly magnetic solid, since magnetic solids lose much of their magnetism above what is known as the Curie temperature, as thermal energy overwhelms the tendency of their electrons to align in magnetic domains (regions of similarly oriented electron spins). T he Curie temperature is well below the melting point for known magnetic materials (1–3 ). Ferrofluids, which are colloidal suspensions of magnetic material in a liquid me- dium, are an example of a liquid that responds to an exter- nal magnetic field. The coupling of liquid and magnetic be- havior means that the liquid’s location may be manipulated by an applied magnetic field. Ferrofluids were first developed and classified in the 1960s by Stephen Pappell at NASA as a method for controlling fluids in space (4 ). NASA initially used them as rotating shaft seals in satellites, and they now serve the same purpose in a wide variety of machines, ranging from centrifuges to computer hard disk drives (1, 2 ). They are incorporated into the voice coil gap of loudspeakers for damping undesired vibrations and for cooling. Ferrofluids have also been used in the separation of metals from ores by taking advantage of a density change that appears in the fluid under application of a magnetic field. O ne South African company has even been utilizing ferrofluids to separate diamonds from beach sand (5 ). In medicine, a ferrofluidic actuator has been proposed for an implantable artificial heart ( 1). This actuator would be driven simply by applying an external magnetic field. It is possible to attach drugs to the surface of the magnetic particles and use magnetic fields to hold the drug at the site where it is needed ( 3). Aqueous magnetic fluids have successfully oriented biological assemblies such as the tobacco mosaic virus, enabling information concerning the helical structure of the virus to be obtained ( 6 ). Recently, ferrofluids have been utilized in conjunction with microcontact printing and capillary filling to fabricate patterned structures of magnetic materials on the micron scale (7 ). The ability to produce patterns of ultrafine magnetic particles has important technological applications, since the information density on tapes, for example, is inversely proportional to the size of the particles. Research has been conducted exploring the use of ferrofluids as magnetic inks for ink jet printing (2). Magnetic inks are currently used in printing United States paper currency, as can be demonstrated by the attraction of a genuine dollar bill to a strong magnet (Fig. 1) ( 8). Background T here are two major steps in synthesizing a ferrofluid. The first is to make the magnetic nanoparticles (~100 Å diameter) that will be dispersed in the colloidal suspension. T hese particles must be chemically stable in the liquid carrier. T he magnetic particles in ferrofluid are generally magnetite, Fe 3 O 4 , although other magnetic particles have been used. T he second synthetic step is the dispersion of the magnetic particles into a carrier liquid by utilizing a surfactant to create a colloidal suspension. Surfactants are dispersion agents for particles in a liquid that work by adhering to the particles and creating a net repulsion between them (steric and/or coulombic), raising the energy required for the particles to agglomerate, and stabilizing the colloid (Fig. 2) (3). Aqueous-, oil-, and liquid-metal-based (mercury; gallium alloys) ferrofluids have been developed with the proper choice of surfactant (1 ). The magnetic properties of magnetite that make it a desirable component of ferrofluids are derived from its crystal structure. Magnetite crystallizes in the inverse spinel structure above 120 K (9). T he inverse spinel structure consists of oxide ions in a cubic close-packed arrangement. Iron(II) ions occupy 1/4 of the octahedral holes, and the iron(III) ions are equally † Current address: Procter & Gamble de Mexico—PDD, poniente 146 #850, Colonia Industrial Vallejo, Mexico DF— 02300, Mexico. †† Current address: Department o f Chemistry, Bradley University, Pe o ria , IL 61625. Figure 1. Magnetic inks are printed onto paper money for identification purposes. W hen a stro ng ma g ne t is b ro ug ht ne a r a do llar bill (A), the bill is attracted to the magnet (B).