High concentration aqueous magnetic fluids: structure, colloidal stability, magnetic and flow properties

Abstract

This paper is an in-depth analysis devoted to two basic types of water based magnetic fluids (MFs), containing magnetite nanoparticles with electrostatic and with electro-steric stabilization, both obtained by chemical coprecipitation synthesis under atmospheric conditions. The two sets of magnetic fluid samples, one with citric acid (MF/CA) and the other with oleic acid (MF/OA) coated magnetic nanoparticles, respectively, achieved saturation magnetization values of 78.20 kA m-1 for the electrostatically and 48.73 kA m-1 for the electro-sterically stabilized aqueous ferrofluids which are among the highest reported to date. A comprehensive comparative analysis combining electron microscopy, X-ray photoelectron spectroscopy, attenuated total reflectance Fourier transform infrared spectroscopy, vibrating sample magnetometry, small-angle X-ray and neutron scattering, dynamic light scattering and magneto-rheometry revealed similarities and essential differences on the microscopic and macroscopic level between the two kinds of water-based ferrofluids. While the saturation magnetization values are quite different, the hydrodynamic volume fractions of the highest concentration MF/CA and MF/OA samples are practically the same, due to the significantly different thicknesses of the particles' coating layers. The results of volume fraction dependent structure analyses over a large concentration range by small-angle X-ray and neutron scattering, correlated with magneto-rheological investigations for the electrostatically stabilized MFs, demonstrate formation of short chains of magnetic nanoparticles which are relatively stable against coagulation with increasing concentration, while for MFs with electro-steric stabilization, magnetic field and shear rate dependent loosely bound structures are observed. These particle structures in MF/OA samples manifest themselves already at low volume fraction values, which can be attributed mainly to magnetic interactions of larger size particles, besides non-magnetic interactions mediated by excess surfactant.

Magnetic Nanoparticle Systems for Nanomedicine—A Materials Science Perspective

“…A comprehensive comparative study by small-angle neutron and X-ray scattering (SAXS and SANS) of water-based magnetic fluids with two different stabilization mechanisms-electrostatic (with citric acid (CA) Figure 16) and electro-steric (with oleic acid (OA) double layer; Figure 17)-over a large concentration range up to 30% hydrodynamic volume fraction, identified important differences on the microscopic level for these colloidal systems, as evidenced by the scattering curves in Figures 16 and 17. The electrostatic stabilization ensured high colloidal stability up to the highest magnetization 78.20 kA/m, while the electro-steric stabilized samples already show relatively large agglomerates at reduced volume fraction values [85].…”

“…The steric stabilizing layer (usually a chemisorbed primary and a physisorbed secondary layer of surfactant molecules) has a much greater thickness than the electrostatic one, therefore the hydrodynamic volume fraction at the same magnetic volume fraction is much higher (approx. 7-8 times) for electro-steric (e.g., oleic acid double layer) than for electrostatic stabilized aqueous ferrofluids [85]. The significantly reduced interparticle distance produces colloidal stability issues that involve nanoparticle size and magnetic moment, dipolar interactions, excess surfactant, and agglomerate formation.…”

“…XPS allows for determining the oxidation states of iron and to quantify Fe 2+ and Fe 3+ ions in iron oxide nanoparticles. These oxidation states of iron can be determined by Fe2p spectrum employing chemical shift and multiplet splitting and the characteristic satellites [85,[118][119][120][121][122].…”

“…To illustrate the significance of the stabilization mechanism applied-electrostatic or electrosteric-the zeta potentials and hydrodynamic sizes were measured for citrate and oleate stabilized aqueous ferrofluid samples and are given in Figure 8 to show the characteristic pH-dependence due to the different dissociation behaviors of the acidic groups on the coating molecules [85]. The MF/CA sample loses colloidal stability below pH6 (Figure 8a) due to the difference in the charged state of citrated and oleic acid double layer coated MNPs, while the MF/OA ferrofluid below pH5 (Figure 8).…”

“…(b) pH dependence of the Z-average particle diameter (blue) and the zeta potential (red) for the oleic acid double layer stabilized (MF/OA) sample. Reprinted with permission from Reference [85].…”