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Expressions for the magnetic, electrostatic and van der Waals interactions between isolated magnetic spheres which are coated with an inert material and immersed in an aqueous electrolyte solution are obtained and used to study the stability of a colloid of spheres in an electrolyte. Use is made of a simplified version of the theory of colloid stability of Derjaguin, Landau, Verwey, and Overbeek. We find that the colloidal dispersion becomes more stable as 1) the electrolyte concentration is decreased, 2) the radius of the magnetic spheres is decreased, or 3) the thickness of the inert layer is increased. In order to obtain stability with uncoated spheres, the spheres should have radii of about 5 nm. Such radii are typical of ferrofluids.
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