Ferrofluids - MRSEC Education Group

Ferrofluids containing magnetite can be prepared by combining the appropriate amounts of an Fe(II) salt and an Fe(III) salt in basic solution, a combination that causes the mixed valence oxide, Fe3O4, to precipitate from solution: 2 FeCl3 + FeCl2 + 8 NH3 + 4H2O –> Fe3O4 + 8 NH4Cl

However, the particles of magnetite must remain small in order to remain suspended in the liquid medium. To keep them small, magnetic and van der Waals interactions must be overcome to prevent the particles from agglomerating. Thermal motion of magnetite particles smaller than ~10 nm in diameter is sufficient to prevent agglomeration due to magnetic interactions.

The van der Waals attraction between two particles is strongest when the particles approach each other at close distances. Therefore, one method of preventing agglomeration due to van der Waals and magnetic forces is to keep the particles well separated. This separation can be accomplished by adding a surfactant to the liquid medium. The surfactants can generate either steric or electrostatic repulsions between the magnetic particles. For example, cis- oleic acid can be used for oil-based ferrofluids as a surfactant that produces steric repulsions. The surfactant is a long-chain hydrocarbon with a polar head that is attracted to the surface of the magnetite particle; thus a surfactant coating is formed on the surface. The long chains of the tails act as a repellent cushion and prevent the close approach of other magnetite particles (Figure 1).

A. 2 FeCl3 + FeCl2 + 8 NH3 + 4H2O –> Fe3O4 + 8 NH4Cl

B. Add cis-oleic acid, CH3(CH2)7CH=CH(CH2)7COOH, in oil.

C. Remove water:

Figure 1. A preparation of ferrofluid: (A) synthetic conditions for production of Fe3O4(s); (B) addition of surfactant; and (C) removal of water to give small particles of Fe3O4 stabilized by the interaction of the polar ends of the oleic acid molecules with the magnetite particles, and by the interaction of the nonpolar ends of the oleic acid molecules with the oil serving as the liquid medium.

Ionic surfactants such as tetramethylammonium hydroxide can be used as a surfactant that produces electrostatic repulsion in an aqueous medium. The hydroxide ions are attracted to the surface of each magnetite particle, forming a negatively charged layer at the magnetite surface. The tetramethyl ammonium cations are attracted to the negatively charged layer, forming a positive layer. When magnetite particles approach each other the repulsions between their positively-charged layers keeps them from getting too close. (Figure 2).

A. 2 FeCl3 + FeCl2 + 8 NH3 + 4H2O –> Fe3O4 + 8 NH4Cl

B. Replace excess NH4OH on Fe3O4 surface with N(CH3)4OH Figure 2. A preparation of ferrofluids: (A) the synthetic conditions for production of Fe3O4(s); and (B) addition of surfactant to give small particle of Fe3O4 (shaded circles) stabilized by interaction of the hydroxide anions with the magnetite and by the interactions of the tetramethylammonium cations with the water serving as the medium.

Some ferrofluids are so attracted to magnetic fields that they will stand up along magnetic field lines, forming an array of spikes (Figure 3). Figure 3. Top and side views of the magnetic spiking phenomenon observed when a cow magnet is placed beneath a Petri dish containing a ferrofluid. The ferrofluid aligns with the magnetic field lines of the magnet to produce the spikes.