REPOSITORY > RESULTS

Abstract

In my doctoral dissertation, I primarily devoted my attention to the modifying surface
properties of superparamagnetic maghemite nanoparticles. Superparamagnetic maghemite
nanoparticles were synthesized using co-precipitation method from an aqueous solution
of Fe (II) and Fe (III) ions. The size of the nanoparticles was controlled with aging time at
pH = 3. The size of the nanoparticles was varied from 9.5 nm to 14.5 nm.
The surface properties of the maghemite nanoparticles were modified using citric acid,
amorphous silicon dioxide and (3-aminopropyl) triethoxy silane.
Citric acid was adsorbed onto the synthesized nanoparticles, followed by their
dispersion in water. High ζ-potential of the nanoparticles with adsorbed citric acid
prevented agglomeration of the nanoparticles and enabled the preparation of a stable
suspension also known as ferrofluid. Changes of the conditions during the adsorption of
the citric acid onto the maghemite nanoparticles resulted in the changes in their surface
properties. The changes in the surface properties of maghemite nanoparticles exhibited as
a change in the amount of the dispersed nanoparticles in the ferrofluid. Ferrofluid
contained up to 2.5 wt. % of the dispersed nanoparticles.
Maghemite nanoparticles were further coated with a thin layer of an amorphous silicon
dioxide (silica). The coating of nanoparticles with silica was carried out in a mixture of
ethanol and water with the hydrolysis of TEOS at room temperature. A thin layer of silica
on the surface of the nanoparticles provides the presence of a greater number of OH
groups in the whole range of the pH values. I concluded, based on the images taken by the
electron microscope that the silica forms on the surface of the nanoparticles only by the
heterogeneous nucleation.
The presence of the amine groups on the surface of the nanoparticles was assured with
the use of (3-aminopropyl) triethoxy silane (APS). A comparison of APS bonding onto
the bare nanoparticles (MD) and the silica coated nanoparticles was made. In both cases
the bonding of APS took place in the water-ethanol mixture. The surface concentration of
APS, determined with conductometric titration, was found out to be ~4.3 molecules of
APS/nm2 for MD particles and ~1.1 molecules of APS/nm2 for MD-Si particles.
Besides modifying the surface properties of the magnetic nanoparticles, the stability of
the ferrofluid in the presence of the magnetic field was also studied. The stability of the
ferrofluid was observed with rheological measurements.
Up to the magnetic field strength 150 mT, the viscosity of the ferrofluid increased due
to the formation of chain-like agglomerates. At ~200 mT the change in the rheological
character of the ferrofluid occurred. The ferrofluid chanced its character from gel to sol.
At ~200 mT the globular agglomerates become energetically favorable than chain-like
agglomerates. Spherical globular agglomerates in lesser degree hinder the shearing of the
fluid than the elongated chain-like agglomerates. The decrease of the viscosity and the
change of the rheological character of the ferrofluid is a result of the transformation of the
globular agglomerates into the chain-like agglomerates. With further increase of the
magnetic field strength the viscosity of the ferrofluid decreased and at 300 mT the
viscosity of the ferrofluid dropped to the starting value. The settling of the magnetic
particles reduces the hindrance of the shearing, and the viscosity of ferrofluid dropped.