Ferrofluid - Demonstration

Magnetite, Fe₃O₄ is a ferrimagnetic substance containing both Fe²⁺ and Fe³⁺ ions. As in ferromagnetic substances, the unpaired electrons in ferrimagnetic substances are aligned in large domains. However, in ferrimagnetic substances some of the spin sets are opposed to others (this depends on the position of the Fe²⁺ and Fe³⁺ ions in the crystal lattice). Since more of the spins are aligned in one direction than in another, the spins do not cancel. A magnet will attract a ferrimagnetic substance.

In the ferrofluid, tiny particles of magnetite are suspended throughout the liquid medium. In order to prevent the particles from aggregating, they are surrounded by the polar end of long chain fatty acid molecules, to which they are attracted by ion-dipole forces. [See the diagram inverse micelles (Image 19).] The long, nonpolar tails of the molecules are attracted by London forces to the molecules of the oil that serves as the liquid medium, but cannot compete with the polar ends in their attraction for the Fe₃O₄ particles. These structures bear a certain resemblance to the micelles formed by soaps and detergents. However, in the case of soaps, the nonpolar tails are directed toward the interior, where they surround water-insoluble oils, while the polar ends (-COO^- or -SO₃^- groups) are strongly attracted to water molecules by ion-dipole forces.