Experimental results

Preparation and properties of the magnetite particles The magnetite (Fe304) particles were prepared in the water carrier by coprecipitation of concentrated iron(II) and iron(III) salt solutions with ammonium hydroxide.

The particles have the following physical properties: Medium particle core diamter: 10 +/—1 nm Specific area (BET-method): 130 m2/g Saturation polarization: about 400 mT, No hysteresis in the magnetization curve.

Preparation of magnetite sols

Magnetite sols (ferrofluids) have been well known for about 30 years. Our preparation methods are based on Khalafallas' [19, 20] and Massarts' [21] suggestions.

Using the above-mentioned nm magnetite particles the following water based magnetic fluids were prepared:

a) Magnetic fluids in surfactant free acidic aqueous carriers at electrolyte concentrations of 1:1-electrolytes below an electrical conductivity of 10"3 Q-1 cm-1, corresponding to electrostatically stabilized magnetite sols.

b) Magnetic fluids in alkaline carriers stabilized by a double layer of carbonic acids [22] or perfluoropoly-ether-mono carbonic acids, corresponding to sterically plus electrostatically stabilized magnetite sols.

c) Magnetic fluids in neutral, acidic or alkaline carriers stabilized by two different layers: the first one consists of a carbonic acid, the second one of nonionics [23], corresponding to sterically stabilized magnetite sols.

These fluids are environmentally friendly and therefore suitable for the separation of non-ferrous metals using the magneto-levitation effect [24],

Adsorption of surfactants on the magnetite particles

To estimate the adsorption capacity of surfactants onto electrostatically stabilized, 10 nm magnetite particles at 80 °C the adsorption isotherm of an ammonium salt of a perfluoropolyether mono carbonic acid with the molecular weight 929, prepared at Ausimont (Italy), was determined.

The amount of the surfactant was increased step-by-step to determine the formation as well as of the mono-and the bilayer by checking the wetting and dispersing properties of the modified particles towards water and to Galden HT 90, a perfluoropolyether of a average molecular weight of 617 and a viscosity of 0.59 cSt at 40°C.

Experimentally, a strong wetting and dispersing of the magnetite particles in the Galden carrier was found at a surfactant concentration of 0.5 mMol surfactant/g Fe304. This oleophilic behavior should correspond to the monolayer coverage of the particles.

On the other hand, at higher surfactant concentrations the modified particles became more and more hydrophilic, and could be very well dispersed in the aqueous carrier at a coverage of 1.0 mMol surfactant/g Fe304. This concentration should correspond to a bilayer coverage of the particles.

The adsorption curve in Fig. 7 shows that the surfactant can be nearly totally extracted from the carrier under the conditions of formation of the monolayer possibly caused by the high chemical affinity to the surface. In the opposite case the formation of the second, external layer is caused by hydrophobic interaction. There is an equilibrium with the adsorbed amount and the concentra-

Fig. 7 Adsorption isotherm of an ammonium salt of the perfluoropolyether-acid S2 (mw = 929) at 80 °C onto electrostatically stabilized 10 nm magnetite particles

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Adsorption isotherm of S2NH4 at 80*C

S2NH4 concentration/Mol/l tion of the surfactant in the carrier. The adsorption is not so effective, but the molecules of the second adsorption layer can be extracted by suitable solvents or can be evaporated.

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