Petrenko V. Neutron investigations of the influence of the magnetic fluid structure to its stability

The thesis concerns the research of the factors, which determine the stability of ferrofluids by small-angle neutron scattering (SANS). The main attention attends to the study of magnetic fluid stability at the surfactant excess and also investigation of the behavior of acids molecules, used for ferrofluids stabilization, in the bulk of carrier for elucidation of possible reasons of different stabilization efficiency. It was obtained that repulsion dominates for solutions of oleic (OA) and myristic (MA) acids in benzene, however a significant attraction between acids molecules in solutions is observed. For stearic acid the attractive component results in the effective total attraction, which results in formation of big aggregates with size of about 10 nm at the volume fraction of stearic acid higher than 5%. It was shown that attraction shifts the transition from isotropic into the liquid crystalline phase towards smaller acid concentrations in solutions of saturated mono-carboxylic acids. It was obtained that differences in the stabilization efficiency of oleic and myristic acids are related to their behavior on the magnetite surface. It was obtained that dodecyl benzenesulfonic acid forms micelles in heavy water starting from the concentration ? = 0.5%. The structure parameters of magnetite-oleic acid-benzene ferrofluid are determined by mathematical approximation of SANS data. The new model for approximation of SANS data from ferrofluids with excess of surfactant is proposed. The structure parameters of ferrofluids with excess of oleic acid are determined by approximation of the experimental SANS data with the new proposed model. It was obtained that excess of surfactant higher than 25 vol. % results to a sharp break in the stability of ferrofluids followed by coagulation and precipitation. A significant increase in the attraction is observed for oleic acid molecules in the ferrofluids in comparison with the solutions in benzene, which is related to the loss of magnetic fluid stability at high excess ofacid. The cluster formation in water based ferrofluids was detected by SANS. The investigations of water based magnetic fluids showed that the use of monocarboxylic acids for such kind of ferrofluids results to the most stable magnetic fluids.