Surface charging, polyanionic coating and colloid stability of magnetite nanoparticles

Abstract

a b s t r a c tThe formation of small and large molecular polyanionic coating on the surface of magnetite (Fe 3 O 4 ) nanoparticles and their role in the stability enhancement of aqueous magnetic fluids are compared. Magnetite was synthesized and stabilized with citric (CA) and humic (HA) acids. The macromolecular HA, a notable fraction of the natural organic matter (NOM), contains mainly carboxylic groups similarly to the CA, and both acids are able to form surface complexes on the ≡Fe-OH sites of iron oxides. The pHdependent charge state of particles and their aggregation were quantified, and the enhanced salt tolerance of stabilized systems was studied. The dynamic light scattering (DLS) method was used to characterize colloidal stability under different conditions. The average particle size and electrophoretic mobility were measured, and the electrolyte tolerance was tested in coagulation kinetic measurements. The colloidal stability of magnetite dispersions depends sensitively on the pH and the concentration of organic acids present. The trace amount of HA neutralizes the positive charges on magnetite surface under acidic condition only in part, and so it promotes the aggregation between the particles having both positive sites and negative humate patches on the surface. Above the adsorption saturation, the surface becomes completely covered causing the reversal of charge sign and overcharging of nanoparticles. The magnetite nanoparticles become stabilized in a way of combined steric and electrostatic effects. The thicker layer of macromolecular HA provides better electrosteric stability than that of CA coating. However, in the presence of either CA or HA, the dissolution of magnetite is enhanced due to the complexation of iron ions in the aqueous medium.

Adsorption of organic acids on magnetite nanoparticles, pH-dependent colloidal stability and salt tolerance

“…This is explained [22] by either the cooperative interactions occurring between the many functional groups of the macromolecules and the surface sites of the MNPs or the intrinsic high affinity of the individual carboxyl groups to the surface sites. We have proven the formation of direct metal-carboxylate surface complexes in the case of CA, PAM and HA [6,8,15,16], and only H-bonding in the case of PAA [9]. The results suggest that surface Fe-carboxylate complex bonds can form when the geometric arrangement of the neighboring carboxyl groups matches the distance between surface Fe-OH sites.…”

“…Besides our several years' experience with aqueous solutions of humic acids, their interaction with magnetite (Fe3O4, magnetic iron oxide) has been also studied [6,7]. The CA, PAA and PAM interactions with magnetite nanoparticles have been recently investigated [8,9,16]. Besides their environmental relevance, these are important from biomedical application point of view.…”

“…The thicker coating provides better stability [14]. Magnetite nanoparticles were stabilized with CA as a well-known complexant of Fe-OH surface sites [8,15], natural polycarboxylic acid HA [6,7,8] and artificial polymers polyacrylic acid (PAA) and poly(acrylic-co-maleic) acid (PAM) [9,16].…”

“…We compared the effect of the adsorption of the different organic acids on the pH-dependent charging, salt tolerance and so the colloidal stability of magnetite nanoparticles. Some of the results used here can be found in our earlier publications [6][7][8][9]16], together with the detailed discussions on the mechanisms of adsorption. Therefore, we do not intend to go into those details, but we use our earlier conclusions to support the findings of the present comparative study on the stabilizing efficiencies of small and large molecular organic acids.…”

Novel ferromagnetic nanoparticle composited PACls and their coagulation characteristics

“…The isoelectric point (IEP) of FSCSP is at pH 5.2 while that of SI is at pH 6.6. It is worth noting that magnetite can produce charges in the protonation and deprotonation reactions of^FeeOH surface sites as expected for an amorphous solid (Hajdú et al, 2009). Therefore, both particles can be positively charged in composited coagulants since the pH value of PACl2.0 is around 4.02, lower than IEPs of SI and FSCSP (Table 1) …”