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Abstract

The dissertation presents a study of the colloidal stabilization of ferrimagnetic barium hexaferrite nanoplatelets in various isotropic and nematic media for the preparation of ferromagnetic fluids. Colloidal stability is the most important requirement to obtain a ferromagnetic liquid of barium hexaferrite nanoplatelets that exhibits spontaneous magnetization in the absence of an external magnetic field. The work is divided into three parts.
In the first part, the focus was on the study of the mechanisms for the colloidal stabilization of barium hexaferrite nanoplatelets, the surface of which was modified with a double layer of the surfactant dodecylbenzenesulfonic acid. For the nanoplatelets dispersed in alcohols, we measured the zeta potential, determined the concentration of the free surfactant in the suspensions, and calculated the Debye length. The results of the measurements formed the basis for calculating the electrostatic interaction between the nanoplatelets in four different alcohols: tert-butanol, 1-hexanol, 1-butanol, and 2-propanol. The comparison of the electrostatic interactions between the nanoplatelets dispersed in alcohols with different polarity allowed to determine the optimal alcohol to obtain a ferromagnetic nematic ferrofluid. 1-Butanol proved to be the best choice. The results obtained in the first part formed the basis for the continuation, i.e., the second part of the dissertation.
In the second part, we focused on the preparation of 1-butanol ferromagnetic nematic suspensions of barium hexaferrite nanoplatelets with a dodecylbenzenesulfonic acid double layer and the determination of the nanoplatelets threshold volume fraction at which the phase transition from the isotropic to the nematic phase occurs. The phase transition was determined by diluting the nematic suspension to the isotropic phase. The exact threshold volume fraction of the nanoplatelets was determined by polarization optical microscopy. Based on the birefringence of the sample, it is possible to distinguish between the isotropic (dark color) and nematic (lighter color, presence of magnetic domains) phases. We determined the threshold volume fraction for suspensions with different equivalent diameter (i.e., a disk diameter with an area equal to the basal surface of the platelet) distribution and magnetization of nanoplatelets, or concentration of dissolved dodecylbenzenesulfonic acid. In this way, we were able to determine the effect of each parameter (without changing the other two) on the threshold volume fraction. We found that the threshold volume fraction shifts to lower values when the average equivalent diameter is larger, when the magnetization of the nanoplatelets is larger, and when the concentration of dissolved dodecylbenzenesulfonic acid in the suspension is increased.
In the last part of the dissertation, we studied the colloidal stabilization of the nanoplatelets with different surface modifications in some other solvents and in the isotropic and nematic phases of a liquid crystal. The first requirement for the colloidal stabilization of nanoplatelets is a solvent that wets their surface. At the same time, sufficient repulsion between nanoplatelets is required to prevent magnetic dipole-dipole interactions from prevailing. Only smaller nanoplatelets (< 35 nm) with lower magnetization remained colloidally stable in nonpolar solvents such as toluene and
chloroform, with negligible electrostatic repulsion that enabled the stability of larger nanoplatelets with higher magnetization in alcohols. Favorable interaction between the modified nanoplatelet surface and the liquid crystal is essential for colloidal stabilization in liquid crystals. In our study, oleic acid, ricinoleic acid, and a polyether chain ligand were found to be the only surface ligands for stabilization in the nematic phase of the liquid crystal. Ricinoleic acid and oleic acid enabled the preparation of a suspension in a liquid crystal with a sufficiently high concentration of the nanoplatelets to form magnetic domains.
Colloidal stabilization of the barium hexaferrite nanoplatelets was ensured in various media with sufficient electrostatic or (electro)steric repulsion. Ferromagnetic ferrofluids can be prepared in isotropic media with electrostatic repulsion (in the case of dodecylbenzenesulfonic acid and ligand with polyether chain) and in nematic medium with steric effect and suitable nematic-mediated elastic interactions (in the case of oleic and ricinoleic acids and ligand with polyether chain).