Stable ferrofluid compositions and method of making same

A stable ferrofluid composition which comprises a colloidal dispersion of finely-divided magnetite particles, having a particle size of less than about 800 Angstroms and comprising from about 1% to 20% by volume of the ferrofluid composition, in a liquid silicone-oil carrier and a dispersing amount of a surfactant, which surfactant comprises a silicone-oil surfactant wherein the ratio of surfactant to magnetite particles comprises from 2:1 to 20:1 volume, and which surfactant contains a functional group which comprises a carboxylic amino, mercapto, aldehyde or alcohol group reactive with the surface of the magnetite particles and which forms a chemical bond with the surface of the magnetite particles and a tail group soluble in the liquid carrier, to provide a stable ferrofluid composition.

In a method of preparing a stable ferrofluid composition, which method comprises dispersing finely-divided magnetic particles in a liquid silicone-oil carrier with a dispersing amount of a surfactant to form a colloidal dispersion of a ferrofluid composition, the improvement which comprises

• employing as the surfactant a silicone-oil surfactant having a functional group reactive with the surface of the magnetic particles during the dispersion, to form a chemical bond with the magnetic particles and to provide a stable silicone-oil ferrofluid composition.