Romanian Reports in Physics, Vol. 58, No. 3, P. 281–286, 2006

MAGNETITE IN AQUEOUS MEDIUM: COATING ITS SURFACE AND SURFACE COATED WITH IT

E. TOMBÁCZ, A. MAJZIK, ZS. HORVÁT, E. ILLÉS

University of Szeged, Department of Colloid Chemistry, H-6720 Szeged, Aradi Vt.1. Hungary E-mail: tombacz@chem.u-szeged.hu

(Received March 14, 2006)

Abstract. Nanoparticles form in hydrolysis using either diluted or concentrated solutions (Weimarn rule). Particle surface must be coated by electrostatic, steric or combined stabilization layers to prevent coagulation. Sticking magnetite to solid support hinders aggregation, too. Novel nanocomposites can be prepared by heterocoagulation and enhanced precipitation of magnetite on montmorillonite lamellae.

Key words: magnetite, alkaline hydrolysis, colloidal stability, surface charging, electric double layer, coagulation, montmorillonite, heterocoagulation, surface precipitation, heterogeneous nucleation.

The common way of magnetite synthesis is the alkaline hydrolysis of iron(II)- and iron(III)-salts. The size of formed particles depends on the relative oversaturation of solution; formation of nanoparticles is expected at very low and very high concentration according to the Weimarn rule. Single magnetic domains with size below ~10 nm develop under appropriate hydrolysis conditions. Size tailoring of magnetite particles over a large range at nanometric scale (1.5-12.5 nm) can be obtained by controlling pH and ionic strength in the coprecipitation medium [1]. The smaller the particles, the less stable systems form in colloidal point of view. Spontaneous processes known as ageing, take place in time in any colloidal dispersion, which involve i) an increase in the primary size due to the size dependent solubility of solid particles in the liquid (ln c(r)/c∞ = 2VS γ/RTr in e.g., [2]), ii) the aggregation, i.e., the adhesion of colloidal particles because of Van der Waals attraction [3] in general and magnetic contribution in the case of magnetite [4], and iii) the chemical transformation of solid Fe3O4 (the surface Fe(II) cations react with the adsorbed oxygen to form a rim of maghemite Fe2O3 [5]).

Coating of particle surface can effectively prevent the adhesion of colliding particles during thermal motion. Covering particles with adsorption layer usually results in enhanced resistance against the particle aggregation. In aqueous medium, electrostatic, steric and combined stabilization layers can develop [6]. The thicker

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coating provides better stability, especially in the case of magnetic fluids, since the spacing (typically 2-3 nm) between magnetic domains is important, if magnetic field is applied [7]. Depending on the purposes there are several choices of coating, and so several particular systems can be prepared from the water based magnetic fluid to the biocompatible or specifically functionalised magnetite particles for biomedical applications (products of Liquids Research Ltd. with e.g., dextran, albumin, transferrin, polyethyleneimine, chitosan, biotin, insulin coatings are commercially available).

Sticking of nanosized magnets to solid support can also prevent the particle aggregation in principle. This may result in novel composite materials with a chance to utilize some in future. It is possible to prepare magnetic particles with extremely asymmetric geometry sticking magnetite nanoparticles to the thin (~1 nm) micronsized clay lamellae. The question is how to prepare this kind of composite material. There are two ways: one is the heterocoagulation of oppositely charged montmorillonite and magnetite particles under appropriate conditions [8], and the other is the enhanced precipitation of magnetite on the surface of montmorillonite lamellae due to heterogeneous nucleation. Surface coating in both processes is random, however, surface density of magnetic domains on the plate of montmorillonite can be tuned, and so the asymmetry in magnetic properties is probably variable.

Some of our recent results were presented on the workshop. Synthesis: Magnetite nanoparticles were prepared by alkaline hydrolysis of

the most concentrated mixed solution of iron(II)- and iron(III)-salts. The details of preparation and purification are given in our recent paper [9]. The size of primary particles was about ~8 nm as shown in TEM picture inserted in Fig. 1.

Surface charge formation was measured by potentiometric acid-base titration [10]. The pH-dependent net proton surface excess amount (∆nσ) of magnetite can be seen in Fig. 1. Magnetite is an amphoteric solid, which can develop charges in the protonation (Fe-OH + H+ ⇔ Fe-OH2

+) and deprotonation (Fe-OH ⇔ Fe-O- + H+) reactions of Fe-OH sites on surface. These surface reactions can be interpreted as the specific adsorption of H+- and OH--ions at the hydrated solid/water interface [2]. The net proton surface excess amount (∆nσ= nσH+ - nσOH-) is proportional to the surface charge density (σ0,H = F ∆nσ/aS, F Faraday constant, aS specific surface area). Stabilizing electric double layer develops, and the surface charge density increases with increasing ionic strength in both acidic and alkaline media. For example, the amount of positive charges on surface rises from 0.16 to 0.37 C/m2 at pH~4 as NaCl concentration goes from 0.01 M up to 1 M. The point of zero charge (PZC) could be determined as the intersection point of the ∆nσ vs. pH curves at different ionic strengths, where surface charge density is also zero σ0,H = 0. The PZC of magnetite seems to be at pH 7.9 ± 0.1, and this value falls in the range given in the literature [5].

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Fig. 1 – pH-dependent surface charge development of magnetite at different ionic strengths. The points were calculated from the material balance of H+/OH- in the course of titration [10]. The continuous lines were numerically fitted using SCM model and FITEQL program (constant capacity model, C = 1.6 F/m2). The calculated

equilibrium constants are log Kpr.= 6.6 ± 0.1 and log Kdepr. = -9.1 ± 0.1.

The electrostatic field developing around magnetite particles under acidic and alkaline conditions far from the pH of PZC can prevent particle aggregation due to the repulsion of overlapping electric double layers of approaching particles with similar charges [2,6]. Therefore, a simple electrostatic stabilization occurs and may provide the colloidal stability of magnetite sols. However, this kind of stabilization shows high pH sensitivity besides the poor electrolyte resistance. The pH-dependent charge state of particles and the aggregation taking place spontaneously in magnetite sols at pHs near to PZC can be seen in Fig. 2. The zeta-potential measured by means of laser Doppler electrophoresis decreases significantly over the whole range of pH (Fig. 2 open circles) showing charge reversal at pH~8 identified as isoelectric point (IEP) in good agreement with PZC in pure magnetite sols. The pH-dependent particle aggregation was measured by dynamic light scattering. In electric double layer stabilized systems far from the PZC, the average hydrodynamic radius was about 150 nm at low salt concentration. In the absence of electrostatic stabilization near the pH of PZC, large aggregates form even at low 0.002 M ionic strength (Fig. 2 gray circles).

The coating of magnetite particles by a large variety of complexing agents provides enhanced stability due to steric hindrance or combined electrostatic and steric (electrosteric) stabilization. Synthetic and natural polyacids (e.g., PMAA – poly(methacrylic acid), DMSA – dimercaptosuccinic acid, citric and tartaric acids) are the most frequently used coating agents [11,12].

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Fig. 2 – pH-dependent charge state (continuous lines) and aggregation (dashed lines) of magnetite particles in pure electrolyte solution, and in the presence of increasing amount of humic acid (natural

polyanionic complexant) at 0.002 M ionic strength. These polianionic compounds modify the surface charge properties of

magnetite entirely or in a certain degree depending on the amount adsorbed. We chose a natural polyacid, humic acid (HA) to improve the pH and electrolyte dependent colloidal stability of magnetite, since HA has good chemical stability and high affinity to form complex on the surface sites of magnetite [9,10]. In the presence of trace amount of HA the negatively charged macroions neutralize surface sites charged positively at pH below the PZC of magnetite. Therefore, in acidic systems trace HA amounts (~2 mg/L, triangles in Fig. 2.) promote the coagulation of iron oxide particles by reducing the positive charge density to a certain extent, and creating negatively charged patches on the oxide surface. The gradual charge reversal of magnetite particles is obvious from the successive shift of zeta potential curves to the negative region with increasing humic acid concentration. Relatively small amount of polyanionic humic acid can entirely alter the charge state of magnetite particles, and prevent the aggregation of magnetite particles over the whole range of pH (~20 mg/L belongs to the lowest curves of both the zeta potential and the average particle size, diamonds in Fig. 2). The noteworthy stabilization effect of HA and the enhanced resistance of magnetite sols against electrolytes were proved by coagulation kinetics measurements [9].

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The stabilizing effect of multifunctional complexants such as humic acid can be summarized. If their enough amounts are present, i.e., at/above the adsorption coverage, the polyanionic coating layer stabilizes particles in a way of combined steric and electrostatic effects; therefore, the colloidal stability is significantly improved. The coated magnetite nanoparticles form stable colloidal dispersion, particle aggregation and sedimentation do not occur in these systems in a wide range of pH and their salt tolerance is better than that of pure magnetite sols.

The bonding of magnetite nanoparticles with Coulombic attraction or chemical bonds to an appropriate solid support is a new way to prepare novel composite materials having chance on utilization. We attempted to fix magnetite particles on the extremely asymmetric clay lamellae in both ways mentioned above. Clays are finely divided crystalline aluminosilicates, among them montmorillonite was chosen, in which an octahedral alumina sheet shares oxygen atoms with two silica sheets resulting in very thin (~1 nm) micronsized lamellae with permanent negative layer charge due to isomorphic substitution [13].

In the heterocoagulation process, the variable charge state of interacting particles was utilized. Since magnetite particles have pH-dependent surface charge (Fig. 1) and montmorillonite lamellae hold permanent charges, their oppositely charged state should exist under appropriate conditions. Heterocoagulation between the positively charged magnetite and the negatively charged montmorillonite particles takes place at pHs below the PZC of oxide above a heterocoagulation threshold of electrolyte concentration, advantageously above 0.01 M at pH~4 as proved before [8]. Coulombic attraction was supposed between the oppositely charged particles, however, a balance between positive and negative charges could not be accounted probably due to geometric reason. A large charge excess of montmorillonite exists in the strongest heterocoagulated network formed randomly from the huge lamellae and small spheres at 1:15 mass ratio of magnetite to montmorillonite proved by rheology. As the coverage of montmorillonite plates with magnetite particles increased, the particle network collapses gradually, the mixed systems become stabilized, e.g., that with 1:5 mass ratio significantly even at 0.01 M NaCl, which is the heterocoagulation threshold of the mixed system. The colloidal behavior of magnetite coated montmorillonite suspensions becomes similar to that of pure magnetite sols.

The heterogeneous nucleation on lamellae being present in the hydrolysis of iron(II)- and iron(III)-salts results in the enhanced precipitation of magnetite particles on the montmorillonite surface. The driving force of surface precipitation is the surface accumulation of iron-ions due to the adsorption on the lattice charge defect sites of montmorillonite [2]. We supposed that local concentration of hydrolyzing ions at the solid/water interface is higher than that in the aqueous bulk phase, nuclei have to form on the basal plane of montmorillonite, therefore the magnetite hydrolysis has to be enhanced on lamellae. Preliminary results based on this hypothesis were presented. The mass ratio of magnetite to montmorillonite was varied from 1:3 to 1:0.5 in the hydrolysis process to modify the surface coverage. TEM pictures of some nanocomposites are shown in Fig. 3.

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Fig.3 – TEM micrographs of magnetite (black spots) coated lamellae with low and high surface coverage.

Magnetite coating on montmorillonite lamellae in both processes is random,

however, surface density of magnetic domains on the plate of montmorillonite can be tuned, and so the asymmetry in magnetic properties is probably variable.

REFERENCES

1. L. Vayssieres, C. Chaneac, E. Tronc, J.P. Jolivet: Size Tailoring of Magnetite Particles Formed by Aqueous Precipitation: An Example of Thermodynamic Stability of Nanometric Oxide Particles, J. Colloid Interface Sci. 205, 205–212. 1998.

2. E. Tombácz: Adsorption from Electrolyte Solutions, Ch.12. In: J. Tóth (Ed.) Adsorption: Theory, Modeling, and Analysis, Marcel Dekker, New York, 2002. pp. 711–742.

3. E.J.W. Verwey, J.TH. Overbeek: Theory of the Stability of Lyophobic Colloids, Elsevier, New York, 1947. 4. R.E. Rosenweig: Ferrohydrodynamics, Cambridge Univ. Press, Cambridge, 1985. 5. R.M. Cornell, U. Schwertmann: The iron oxides, VCH, Weimheim, 1996. 6. R.J. Hunter: Foundations of Colloid Science, Vol. I, Clarendon Press, Oxford, 1987. 7. S. Odenbach: Ferrofluids – magnetically controlled suspensions, Colloids Surfaces A, 217, 171–178,

2003. 8. E.Tombácz, CS.Csanaky, E.Illés: Polydisperse fractal aggregate formation in clay and iron oxide

suspensions, pH and ionic strength dependence, Colloid Polym. Sci. 279, 484–492, 2001. 9. E. ILLÉS, E. TOMBÁCZ: The effect of humic acid adsorption on pH-dependent surface charging and

aggregation of magnetite nanoparticles, J. Colloid Interface Sci. (YJCIS_11563 in press), 2005. 10. E. Illés, E. Tombácz: The role of variable surface charge and surface complexation in the

adsorption of humic acid on magnetite, Colloids Surfaces A, 230, 99–109, 2003. 11. G.D. Mendenhall, Y. Geng, J. Hwang: Optimization of long-term stability of magnetite fluids from

magnetite and synthetic polyelectrolytes, J. Colloid Interface Sci. 184, 519–526, 1996. 12. N. Fauconnier, A. Bée, J. Roger, J.N. Pons: Synthesis of aqueous magnetic liquids by surface

complexation of maghemite nanoparticles, J. Molecular Liquids, 83, 233–242, 1999. 13. E. Tombácz, M. Szekeres: Colloidal behavior of aqueous montmorillonite suspensions: the

specific role of pH in the presence of indifferent electrolytes. Appl. Clay Sci. 27, 75-94, 2004.

Magn : Montn 1:3 Magn : Montm 1:1.6