Structural Organization of Magnetic Fluids Stabilized with Fatty Acids

Abstract

Magnetic colloids which exist in a nonflowing pasty form at room temperature and are transformed into a liquid state during heating are described for the first time. Magnetic fluids (MFs) are obtained in a hydrocarbon medium after introduction of a nanosize magnetite into kerosene in the presence of stabilizers. Fatty acids with different lengths of a hydrocarbon fragment, from C₁₂ to C₂₂, were used as stabilizers. Magnetic granulometry data showed that the average diameter of particles is 8.3 nm. The MF samples are studied by differential scanning calorimetry. The geometric parameters of adsorption layers are calculated taking into account the geometric parameters of nanoparticles and stabilizer molecules. The models of liquid and solid states of the adsorption and overlapping layers of two particles are constructed from calorimetric measurements, calculation data, and observations of the phase state of MFs. The experimental and theoretical data prove that transition of the adsorption layer from the liquid phase to the solid one for particles ~8 nm in size is possible if the length of the hydrocarbon moiety in the stabilizer is C₁₄ and higher. The length of the stabilizer molecule and its melting point are the main parameters which determine the structure and properties of MFs. Moreover, when the length of the hydrocarbon fragment in the stabilizer is C₁₂ and higher, magnetic and van der Waals interactions between solid particles may be neglected.