On the effective length of erucic and oleic acid molecules as part of the protective shell of magnetite colloidal particles

Purpose . Experimental evaluation of the thickness of the protective shell thickness of magnetite colloidal particles representing a single molecular layer of oleic or erucic fatty acids. Methods . Experimental data were obtained by independent measurement of three volumetric concentrations (magnetic, solid and hydrodynamic) of magnetite particles in magnetic liquid using magneto-granulometric analysis, density measurement and titration of pure carrier liquid during peptization of dry sediment of particles. The experimental data were processed by numerical solution of the system of nonlinear equations describing the additivity of volume, mass and magnetic contributions of stabilized colloidal particles and carrier liquid to the volume, density and saturation magnetization of polydisperse magnetic liquid, respectively. Results . Four samples of magnetic liquid differing in the disperse composition of magnetic nanoparticles, three of which were stabilized by oleic and the fourth by erucic fatty acids (surfactants), were experimentally investigated. The samples were prepared by titration of pure carrier and peptization of dry sediment of colloidal particles prewashed from free surfactant molecules. For each sample, the density and magnetization curve were measured and magneto-granulometric analysis was performed. The data obtained by different methods agree with each other provided that the thickness of the nonmagnetic layer on the surface of magnetite particles is equal to 0.76 nm and the effective length of the oleic and erucic acid molecules is equal to their maximum possible values in the fully straightened state. Conclusion . At stabilization of magnetite particles by unsaturated fatty acids, the double bond, providing bending of these molecules in the basic energy state, does not play an essential role, since the thickness of the protective shell, equal to the effective length of surfactant molecules, is equal to their maximum possible length in the straightened state.