Ferrofluids

Finally, selective separation and dewatering of one suspended substance in a slurry containing different minerals or precipitates is possible by selectively adsorbing a magnetic material (usually hydrophobic) onto a soHd that is also naturally or chemically conditioned to a hydrophobic state. This process (Murex) was used on both sulfide ores and some oxides (145). More recently, hydrocarbon-based ferrofluids were tested and shown to selectively adsorb on coal from slurries of coal and mineral matter, allowing magnetic recovery (147). Copper and zinc sulfides were similarly recoverable as a dewatered product from waste-rock slurries (148). 

FIGURE 16.43 When a magnet is pulled up from this viscous ferrofluid, the particles of iron(lll) oxide align themselves with the magnetic field. Because strong attractions exist between the particles and the detergent molecules in the oil, the liquid is pulled into the field along with the particles. 

In recent years, the growing numbers of publications are concerned with ultra-fine metal oxide structures because of their useful applications as bactericides, adsorbents, energy storage media, magnetic data storage, and ferrofluids and specifically as catalysts [6, 7]. 

Rosensweig, R.E. Method of substituting one ferrofluid solvent for another. US Patent (1970) 3,917,538. 

Gomes, J.D., Sousa, M.H., Tourinho, F.A. and Aquino, R. da Silva, G.J., Depeyrot, J., Dubois, E., Perzynski, R. (2008) Synthesis of core-shell ferrite nanopartides for ferrofluids chemical and magnetic analysis. Journal of Physical Chemistry C, 112 (16), 6220-6227. 

Geldwerth, D., Helley, D., de Jong, K., Sabolovic, D., Sestier, C., Roger, J., Pons, J.N., Freyssinet, J.M., Devaux, P.F. and Kuypers, F.A., 1999, Detection of phosphatidylserine surface exposure on human erythrocytes using annexin V-ferrofluid. Biochem. Biophys. Res. Commun., 258 199-203. 

Ferrofluid NMR studies can also be used in order to determine geometrical and physical properties of the super-paramagnetic crystals, like their specific magnetization or radius. They also give valuable information on the aggregation level and on anisotropy. 

The preparation of a ferrofluid emulsions is quite similar to that described for double emulsions. The starting material is a ferrofluid oil made of small iron oxide grains (Fe203) of typical size equal to 10 nm, dispersed in oil in the presence of an oil-soluble surfactant. The preparation of ferrofluid oils was initially described in a US patent [169]. Once fabricated, the ferrofluid oil is emulsifled in a water phase containing a hydrophilic surfactant. The viscosity ratio between the dispersed and continuous phases is adjusted to lie in the range in which monodisperse fragmentation occurs (0.01-2). The emulsification leads to direct emulsions with a typical diameter around 200 nm and a very narrow size distribution, as can be observed in Fig. 1.33. 

Figure 1.33. Scanning electron microscopy image of a ferrofluid emulsion (Courtesy of Ademtech Company.)...

In TFB technique, the thin film radius is typically of the order of 100 pm, far larger than the contact film radius likely to be formed when two micron-sized droplets approach. The magnetic chaining technique overcomes this limitation, allowing the direct measurement of force-distance profiles between liquid colloidal droplets. This technique exploits the properties of monodisperse ferrofluid... 

Case I (see Fig. 2.17) corresponds to the situation such that the emulsion is initially stabilized with SDS at 8 10 mold (CMC). The repulsive force as a function of distance between the ferrofluid droplets, stabilized with SDS alone is referred as 0% PVA. Then, PVA-Vac is introduced at different concentrations varying from 0.002 to 0.5 wt%. After each addition, the emulsion is incubated for 48 h to reach equilibrium. It can be seen that the force profiles remain almost the same as in the case of 0% PVA. As the surfactant concentration is equal to CMC, the expected decay length is 3.4 nm. The experimental value of the decay length obtained from the force profile, 2.9 nm (solid line), is in good agreement with the predicted value. Thus, if the emulsion is preadsorbed with surfactant molecules, the introduction of polymer does not influence the force profile significantly. 

Figure 2.17. Forces between the ferrofluid droplets as a function of the interdroplet spacing. The best fits, using Eq. (2.30) are shown by solid lines. Case I Droplets preadsorbed with sodium dodecyl sulfate (SDS) at 8 10 mol/1 and at various PVA-Vac concentrations. The solid line represents the average value of the best fit. Case 11 Droplets preadsorbed with sodium dodecyl sulfate (SDS) at 0.27 10 mol/1. Premixed PVA-SDS was added to the emulsion. In all the cases, the polymer concentration was 0.6 wt%. The surfactant concentrations are indicated in the inset. Case III Droplets preadsorbed with PVA-Vac. In all the cases, the polymer concentration was fixed at 0.6 wt%. The surfactant concentrations are indicated in the inset. (Adapted from [75].)...

Rgure 2.21. Force vs. distance profiles for ferrofluid emulsions stabilized with /3-casein at different concentrations (points). The ionic strength is 3 10 mol/1, pH = 6.2. The lines are the best fits (see the text for details). (Adapted from [78].)... 

Figure 2.22. (a) Disjoining pressure vs. thickness isotherm for an emulsion film stabilized by 0.1% BSA, ionic strength of 10 mol/1 NaCl, oil phase = hexadecane. The dots are the experimental data, dashed line is the double-layer contribution to the total disjoining pressure, and the solid line is the best fit done supposing additivity of the double-layer and steric forces, (b) Force vs. distance profiles for ferrofluid emulsions stabilized with mixed BSA-Tween-20 adsorption layers. The total concentration of the Tween-20 is kept constant = 5CMC, pH = 5.8. (Adopted from [78].)... 

Figure 2.25. Optical micrographs of a ferrofluid oil-in-water emulsion stabilized by 0.04 wt% BLG. The bar corresponds to 10 am. (a) On field corresponding to force of 1 pN. (b) 30 min after switching off the magnetic field. (Reproduced from [87], with...

Figure 7.1. (a) Transmission electron microscopy image of a collection of 200-nm magnetic emulsion droplets obtained from emulsifying an octane-based ferrofluid. (b) One droplet is shown after polymerization. A polymer shell is visible that encapsulates the iron oxide nanoparticles. (With permission of Ademtech). 

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