COLLOIDAL STABILITY OF MAGNETITE NANOPARTICLES

IN AQUEOUS MEDIUM

E. Tombacz, E. Illes.

University of Szeged – Department of Colloidal Chemistry – Hungary

Formation of similarly charged electric double layer and / or of adsorption layers on the particle surface enhances the stability of lyophobic colloids in aqueous medium. In metal oxide suspensions drugs development occurs by direct proton transfer to surface hydroxyls. Positive and negative charges can develop due to the protonation and deprotonation reactions on the Me-OH groups commonalty below or above the pH of the point of zero charge (PZC), respectively. In any kind of oxide suspensions coagulation occurs around PZC, while those are stable far from it, but an increase in salt concentration induces homocoagulation of particles. Adsorption layers can significantly enhance the colloidal stability of oxide particles, where either steric (e.g. macromolecular coating) or combined steric and electrostatic (e. g. surface complexation by polyionic ligands) stabilization has develop. In principle, the most effective stabilizing agents are the polyanionic organic complexants. However, the effect of any complexants is specific, since a chemical reaction, the surface complexation takes place at an electriс field interface. This specific interaction between the active sites of surface and organic ligands exhibits both a chemical and an electrostatic feature, which is controlled by the chemical characters of the interacting partners and by the solution conditions.

In this work we present a summary of the pH- and ionic strength dependent surface charge formation, particle charge characterization of magnetite, and its surface modification by polyanions. Characterization of colloidal stability and its relation to the surface charging will be also interspersed.

Synthetic magnetite nanoparticles were prepared by alkaline hydrolysis of Fe (II) and Fe (III). Surface charge formation was measured by potentiometric method. The intrinsic equilibrium constants of protonation and deprotonation reactions were log K = 6.6 ± 0.1 and – 9.1± 0.1, respectively, and the point of zero charge (PZC) was at pH 7.9 ± 0.1. The zeta-potential measured systems of laser Doppler electrophoresis decreases significantly over the whole range of pH showing charge reversal at pH 8 identified as isoelectric point (IEP) in good agreement with the pH of PZC in the absence of organic complexants. The pH-dependent particle aggregation and coagulation kinetics at constant pHs were measured by dynamic light scattering. In well-stabilized systems for from the PZC, at low salt concentration the average hydrodynamic radius was about 100 nm. Is the absence of electrostatic stabilization near the pH of PZC, large aggregates form even at love ionic strength. Rheological investigation of dense (1.09 g/ml) stable and coagulated systems at pH 4 and 8 showed essential difference. Liquid-like Newtonian behavior was characteristic of this well-stabilized system at pH 4, while appearance of plastic feature referred to the formation a particle network at pH 8. The polyanionic organic complexants (advantageously with carboxylates or biacetate forming groups) modify the surface charge properties of magnetite. The trace amounts of negatively charged macroions neutralize surface charged positively at pH below the PZC of magnetite. Therefore, in acidic systems these trace amounts promote the coagulation of magnetic particles by reducing the positive charge density and creating negatively charged patches on oxide surface. However, above the adsorption saturation, the most reactive surface sites become hidden, the polyanionic coating stabilizes particles in a way of combined steric and electrostatic effects therefore, the colloidal stability is significantly improved. The coated magnetite nanoparticles force stable colloidal dispersion, particle aggregation and sedimentation do not occur in these systems in a wide range of pH and their salt salt tolerance is better than that of pure magnetite sols.

Acknowledgement: OTKA T034755 supported this work. Authors are thankful for measurements in the Laboratory of Magnetic Fluids, CFATR, Timisoara, Romania.