The colloidal stability of oleic- and ricinoleic-acid-coated nanoparticles in organic solvents with dielectric constants epsilon(r) ranging from 2.0 to 9.8 was studied. Although the acids are structurally similar, there is an OH group in the ricinoleic acid's tail, a marked improvement in the colloidal stability of the ricinoleic-acid-coated magnetic nanoparticles in moderately polar organic solvents and monomer methyl methacrylate was observed as a result. The bonding of both acids provokes a significant change in the surface properties of the iron-oxide nanoparticles. A clear shift from a strong electron-donor to a weak electron-donor was confirmed with the bonding of the oleic acid. The effect of ricinoleic acid bonding is even more dramatic: a clear shift toward a weak electron-acceptor is evident. A detailed analysis of the total energy of interaction, including the vOCG theory, between two particles was used to describe the different behaviors of the coated nanoparticles. In the case of the oleic acid nanoparticles in an apolar medium, such as decane, a small net attraction of similar to 0.84 k(B)T, which is insufficient to cause nanoparticles agglomeration, exists. In polar media the net attraction is larger than 1.5 k(B)T, resulting in precipitation of the oleic-acid-coated nanoparticles. The same findings apply to the ricinoleic-acid-coated nanoparticles, but only when dispersed in the apolar medium. In the polar medium an additional repulsion due to polar solvation forces exists, resulting in a decrease of the net attraction to as low as similar to 0.14 k(B)T. (C) 2010 Elsevier Inc. All rights reserved.