Ferrofluids surfactant

Ferrofluids synthesized by chemical coprecipitation may be surfactant-stabilized ferrofluids or ionic ferrofluids. Surfactant ferrofluids are iron oxide nanoparticles coated by stabilizers or surfactant layer(s) for colloidal stability. The coating agents are polymers and surfactants, be they polar, non-polar or non-ionic. Fatty acid derivatives are most often used to stabilize these iron oxide nanoparticles either in organic or aqueous medium. If the particles are dispersed in an aqueous medium, a double layer of surfactant is needed to form a hydrophilic layer. On the other hand, if the particles are dispersed in an organic non-polar medium, one layer of surfactant forms a hydrophobic layer on the surface of the particles. 

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. 

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].)...

The Hamiltonian of a single isolated nanoparticle consists of the magnetic anisotropy (which creates preferential directions of the magnetic moment orientation) and the Zeeman energy (which is the interaction energy between the magnetic moment and an external field). In the ensembles, the nanoparticles are supposed to be well separated by a nonconductive medium [i.e., a ferrofluid in which the particles are coated with a surfactant (surface-active agent)]. The... 

We now focus on a ferrofluid of single-domain particles of the amorphous alloy Fei cC c (x 0.2-0.3). The particles were coated with a surfactant (oleic acid) and dispersed in a carrier hquid (xylene). The particle shape is nearly spherical (see Fig 3.14) and the average particle diameter d = 5.3 0.3nm. The... 

The encapsulation of magnetite particles into polystyrene particles was efficiently achieved by a miniemulsion process using oleoyl sarcosine acid [ 109] or the more efficient oleic acid as first surfactant system to handle the interface magnetite/styrene, and SDS to stabilize the interface styrene/water, thus creating a polymer-coated ferrofluid (Fig. 15b). Since the magnetite particles were very small (ca. 10 nm), each polymer particle was able to incorporate many inorganic nanoparticles. A content of 20 wt% could be incorporated in this way. 

The properties of ferrofluids seem now to be well understood, and numerous applications have been found. Unlike ER and MR fluids, particles in ferrofluids have permanent (magnetic) dipoles thus the particles must be small, around 10 nm, to prevent permanent clumping. If single-domain ferromagnetic particles this small are made, and coated with surfactant to prevent clumping by van der Waals forces, stable ferrofluids can be made whose properties are readily predicted from theory. 

The binding of surfactants to the surface of the particles can be done through electrostatic interaction or complexation with metal ions the surfactants molecules should have a complexing (chelating) groups in the latter case. Ferrofluids can also be stabilized by various low molecular weight compounds such as fatty acids, saccharides and the polymers polyvinyl alcohol (PVA), PEG, dextran, polyacrylamide. 

Fig. 11 TEM image of latex prepared with St, DVB and KPS (40 wt% DVB)]obtained from ferrofluid droplets stabilized with a functional polymeric surfactant. Reprinted from [135] with permission...

A ferrofluid is a liquid consisting of ferromagnetic nanoparticles suspended in a nonmagnetic carrier fluid, typically water or oil. The ferromagnetic nanoparticles are coated with a surfactant such as oleic acid to prevent their agglomeration. The magnetic particles have typical diameters on the order of 5-15 nm and a volume fraction of about 5-10 %. 

The reaction product is subsequently coprecipitated with concentrated ammonium hydroxide. Next, a peptization process transfers the particles from the water-based phase to an organic phase such as kerosene with a surfactant such as oleic acid. The oil-based ferrofluid can then be separated by use of a magnetic field. 

After evaporation of the octane, a water-based ferrofluid consisting of oleic add-coated aggregated magnetite dispersed in a water phase is obtained (Fig. 2.15). In other words, the magnetite aggregates must have a surfactant double layer the first layer is oleic acid, which provides a hydrophobicity of the partides for later encapsulation the second layer, bdng SDS, promotes the stabihzation in water. 

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