Neutron investigations of magnetite-oleic acid-benzene ferrofluids stability with the excess of surfactant

Abstract

Magnetite-oleic acid-benzene magnetic fluid with the excess of oleic acid was studied by small-angle neutron scattering. Pure solutions of oleic acid in benzene were also investigated to reveal a possible micelle formation of molecules. It was shown that oleic acid does not form micelles in the investigated range of concentration. The results of experiment indicate that the surfactant excess (smaller than 25% volume fraction) does not lead to any observable aggregation of the magnetic particles. At the same time a bigger attraction between free molecules of surfactant in the ferrofluids in comparison with solutions of oleic acid in benzene is observed. A dramatic breaking of the stability occurres and it leads to the aggregation of magnetic particles if the excess of surfactant is bigger than 25%.