CA2315704A1 - A magnetic fluid, a process and a device for its production - Google Patents
A magnetic fluid, a process and a device for its production Download PDFInfo
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- CA2315704A1 CA2315704A1 CA002315704A CA2315704A CA2315704A1 CA 2315704 A1 CA2315704 A1 CA 2315704A1 CA 002315704 A CA002315704 A CA 002315704A CA 2315704 A CA2315704 A CA 2315704A CA 2315704 A1 CA2315704 A1 CA 2315704A1
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- magnetic
- magnetic fluid
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- carrier liquid
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
- B03C1/00—Magnetic separation
- B03C1/32—Magnetic separation acting on the medium containing the substance being separated, e.g. magneto-gravimetric-, magnetohydrostatic-, or magnetohydrodynamic separation
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
- H01F1/00—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties
- H01F1/44—Magnets or magnetic bodies characterised by the magnetic materials therefor; Selection of materials for their magnetic properties of magnetic liquids, e.g. ferrofluids
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B82—NANOTECHNOLOGY
- B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
- B82Y25/00—Nanomagnetism, e.g. magnetoimpedance, anisotropic magnetoresistance, giant magnetoresistance or tunneling magnetoresistance
Abstract
The invention relates to a novel magnetic liquid and to a method and device for the production thereof. The aim of the invention is to produce a magnetic liquid which is based on a polar carrier liquid having a high saturation magnetization, i.e. having a high concentration of nanometer particles and a low viscosity, and to provide a method and device for the production thereof. To this end, a magnetic liquid is provided which comprises a saturation magnetization of at least 30 mT and a viscosity under 100 mPas at 40 ~C, whereby the carrier liquid contains essentially no dissolved components of the outer adsorption layer. In addition, a method and device are provided for the production thereof.
Description
A Magnetic Fluid, a Process and a Device for Its Production Specification The invention relates to a new magnetic fluid and to a process and a device for its production in accordance with the preambles of claims 1, 5 and 18.
Magnetic fluids are stable dispersions having superparamagnetic properties. The solid partiGes contained as dispersed phase in such a dispersion undergo sedimentation neither in a gravitational nor a magnetic field.
Magnetic fluids essentially consist of three components. The dispersed magnetic component comprises solid particles of ferro- or ferrimagnetic materials having a size of 3-50 nm. The dispersed phase present in the form of nanometer particles is stabilized by surfactants. The nanometer particles are homogeneously and stably dispersed in a dispersant herein refer-ed to as carrier liquid.
Surfactant molecules are amphiphilic molecules having both hydrophilic and lipophilic properties. The hydrophilic groups of the surfactants are chemically fixed at the surface of the particles to form monomolecular adsorption layers. For example, carboxylic acid groups, sulfonate groups, sulfate groups, phosphate groups, or phosphonate or amino groups are suitable as hydrophilic, chemically sorbable molecules. Both polar and non-polar solvents are suitable as carrier liquids.
To stabilize the nanometer particles in polar carrier liquids such as water, mainly two adsorption layers - one inner and one outer layer - are formed, where the amphiphilic molecules of the outer layer can be anionic, cationic or non-ionic surfactants, and those of the inner layer consist of anionic surfactants such as fatty acids. Particularly in case of magnetic fluids having an aqueous carrier liquid, a second, outer adsorption layer is necessary to stabilize the nanometer particles.
_2_ While the inner adsorption layer is attached chemically to the particle surface via the hydrophilic group of the surtactant, the outer layer is adsorbed to the hydrophobic portions of the molecules in the inner layer by weaker physical interactions. To enable formation of the outer layer, an excess of surfactant must be present in the aqueous phase. Frequently, the concentration of the outer layer surfactants in the aqueous phase is exceedingly high, giving rise to high volume viscosity and their massive accumulation during concentrating the aqueous phase, thus crucially limiting the values of magnetic saturation. The saturation magnetization is a measure for the concentration of magnetic particles in the magnetic fluid.
Water-based magnetic fluids are well-known. According to DE 195 16 323 A1, they exhibit magnetic saturations of up to 25 mT, and the nanometer particles as magnetic components are comprised of maghemite (Y-FezOa), magnetite (Fe30,) or mixed oxides such as cobalt ferrite or manganese-zinc ferrite. Also, such water-based magnetic fluids involve the drawback of having a relatively high concentration of surfactants in the aqueous phase. As a result, they also have a relatively high viscosity. Moreover, the high level of surfactants prevents achieving substantial magnetic saturations. Ultimately, high levels of surfactants may be critical in ecological terms and also, economically disadvantageous when producing substantial quantities.
Furthermore, it is well-known that magnetic fluids on an organic basis generally are treated by reprecipitation according to US 3,917,538 in order to reduce the surfactant concentration in the dispersion medium. The particles precipitate as a sediment, the dispersant containing surfactants is decanted and replaced by one free of surfactants. Following heating, the particles are redispersed to form a magnetic fluid. This process involves the proviso that a suitable precipitant is at hand and the surfactant will not be removed from the particles by the precipitant.
Hence, it is an energy-consuming, relatively rough procedure. Such a process cannot be used for magnetic fluids wherein the particles are stabilized by inner and outer adsorption layers. In such a case, the outer layer merely adsorbed in a physical fashion will normally be removed from the particles stabilized in the polar dispersion medium.
Methods of removing the superfluous surfactants from the outer adsorption layer after completed physical adsorption are not known as yet.
According to DE 41 30 268 A1, the particles are modified using a carboxy-functional polymer, where the dispersant includes both the carboxy-functional polymer and non-ionic wetting agents at a high concentration. For production, the magnetite particles are precipitated in the presence of the carboxy-functional polymers, a sediment then is formed from the modified particles which is redispersed in the dispersant having the above-mentioned composition. The saturation magnetization of the magnetic fluid having formed is below 10 mT. In addition, the electrical conductivity - being 900 w'cm~' - is very low, thus giving rise to flocculation of the particles when adding electrolyte.
US 4,208,294 describes aqueous magnetic fluids stabilized by two adsorption layers comprised of lauric acid and laurates. Similarly, this magnetic fluid includes an excess of lauric acid salt. The aqueous dispersion medium must be slightly alkaline. The attainable magnetic saturation is about 25 mT. One negative effect is that the particles undergo precipitation when shifting the pH from the neutral to the acidic range.
DE 43 27 826 A1 describes water-based magnetic fluids wherein the magnetite particles are stabilized by an internal fatty acid and the outer layer is comprised of ethoxylated fatty alcohols. Due to the conditions of production, the dispersant contains a large excess of ethoxylated fatty acids, resulting in a relatively high viscosity of the magnetic fluid and an attainable saturation magnetization of merely 25 mT at maximum.
It is the basic object of the invention to provide a magnetic fluid having high saturation magnetization, i.e., high concentration of nanometer particles, and low viscosity, and to provide a process and a device for the production thereof.
Said object is accomplished by the characterizing features of claims 1, 5 and 15.
The magnetic fluids according to the invention having an aqueous carrier liquid exhibit hitherto unknown magnetic saturations of between 30 and 100 mT, and the viscosity is below 100 mPaAS at 40EC. Owing to the high content of manometer particles, resulting in high magnetic saturations of the magnetic fluids according to the invention, in combination with a relatively low viscosity, these fluids are particularly suited as actuating fluids in medical pumps, in sensor technology, as well as in magnetohydrostatic separation of substances.
The fact that the carrier liquid does not contain surfactants anymore, results in ecological as well as economical advantages.
The magnetic fluids of the invention having high saturation magnetization are produced using the process and device according to the invention.
Surprisingly, it has been found that the surfactants can be removed by exposing the per se known magnetic fluids having relatively low saturation magnetization and high concentration of surfactants to an external magnetic field, in combination with using measures so as to reduce the solubility of the surfactants in the carrier liquid. For example, such removal is possible in such a way that the per se known aqueous magnetic fluids stabilized by one inner and one outer adsorption layer are heated to about 30-95EC, resulting in a reduced solubility of the surfactants in the carrier liquid. The heated magnetic fluid then is exposed to an external magnetic field so as to generate a strong inhomogeneous magnetic field - a magnetic field gradient -in the aqueous magnetic fluid. For example, this can be accomplished in such a way that a permanent magnet made of rare earths, which have surface magnetic saturation values of up to 0.5 T, acts to fix a heated aqueous magnetic fluid to the wall of a vessel. Following a magnetic field exposure period of about 15-120 minutes, mainly those surfactants making up the second outer adsorption layer and being present dissolved in the aqueous carrier liquid at a high concentration are removed from the magnetic manometer particles, entraining part of the aqueous carrier liquid, and forced to the surface, from where they can flow off. What remains is a concentrated magne~c fluid. By repeating this procedure, it is possible to increase the concentration of manometer particles step by step, so that magnetic saturations of 70 mT can be achieved. Because the surfactants are largely removed from the carrier liquid, remarkably low viscosities of the concentrated magnetic fluids are achieved, ranging between 5 and 30 mPaAS at 27EC.
Such low viscosities of the magnetic fluid represent one precondition for further concentrating by removing aqueous carrier liquid, e.g. by evaporation in a rotary evaporator. In this way, magnetic saturation values of 80 mT at a viscosity of only 70 mPaAS at 27EC have been achieved. Magnetic saturation values of up to 100 mT have been achieved by further withdrawal of water. Of course, the viscosity re-increases massively at such exceedingly high magnetic saturation values.
This process can be used with polar as well as nonpolar carrier liquids.
This concentrating procedure also is advantageous in that the dissolved surfactant from the carrier liquid, which has been separated from the nanometer particles in accordance with the process of the invention, can be recovered by evaporation and in this way, the surfactant can be used once.more in the production of an aqueous magnetic fluid.
According to the invention, other measures for reducing the solubility of surfactants in the carrier liquid are the following:
addition of agents that change the pH value;
changing the concentration of surfactants by withdrawing carrier liquid;
adding solvents reducing the solubility and/or solids such as salts and water-soluble polymers;
addition of substances that form aggregates with the surfactant molecules.
With reference to the drawings, the device of the invention will be illustrated in more detail.
Fig. 1 shows a device for quasi-continuous removal, and Fig. 2 shows a device for batchwise removal.
According to Fig. 1, a container 1 accommodates a magnetic fluid 10 to be concentrated. A heating element 5 is arranged beneath container 1. A feed line extends from the bottom of container 1 to the separation surfaces 2. The feed line 8 can be opened and closed by means of a stop valve 9. Two strong magnets 3 and are arranged above separation surface 2 in close vicinity thereof. The separation effect can be optimized via the slope angle of separation surface 2. A magnet tray 7 and a surfactant tray 6 are arranged beneath the separation surface 2. By switching on the heating element 5, the temperature of the magnetic fluid 10 is increased to about 60EC, thereby dramatically decreasing the solubility of the surfactants in the carrier liquid of the magnetic fluid 10. When opening the stop valve 9, the magnetic fluid 10 will flow via feed line 8 to the underside of separation surface 2.
Owing to the magnetic field gradient which is present and generated by magnet 3, a bulge-like accumulation of magnetic fluid 10 is formed at separation surface 10.
Following an exposure period of about 10 minutes, the first droplets of carrier liquid including accumulated surfactant come off, dripping into the surfactant tray 6. When switching off magnet 3 and simultaneously switching on magnet 4, the magnetic fluid 10 will be drawn to the separation surface beneath magnet 4 where additional surfactant is removed. After switching off magnet 4, the remaining, highly concentrated magnetic fluid 10 then is collected in magnet tray 7.
Fig. 2 illustrates a device for batchwise removal of surfactants from a carrier liquid at various stages of the process.
According to a_, the magnetic fluid 10 is heated in the first step, using heating element 5. According to b, the magnetic fluid - that is, the manometer particles present - accumulates at the separation surface 2 after switching on magnet 3. According to c_, the concentration process at separation surface 2 is completed, and the concentrated particles can be collected at the bottom. The magnetic fluid collected at the bottom may subsequently be refed in container 1, and an additional separation process may follow.
The process according to the invention will be illustrated in more detail with reference to the Examples below.
Example 1 A water-based 15 mT magnetic fluid containing magnetite particles having a layer of lauric acid affixed to the partiGes and a second non-ionic layer of ethoxylated alcohols with ethoxy groups is concentrated as follows:
100 ml of magnetic fluid is heated to 80EC in a refractory vessel. A rare earth permanent magnet having a magnetic saturation of 0.3 T at its surface is subsequently attached to the exterior wall of the vessel so as to hold the magnetic fluid at the opposite side of the magnet. After some minutes, a non-magnetic viscous solution begins to separate from the magnetic fluid. The magnetic fluid, becoming more and more concentrated over time, forms the typical peaks after some time, becoming fixed to the magnet more and more tightly. The separation process can be promoted by keeping the magnetic fluid in motion either by moving the magnet or by mechanical agitation of the magnetic fluid, or by reheating the magnetic fluid to 80-90EC. The final product has a saturation magnetization of 50 mT and a kinematic viscosity of 5 mPaAS at 27EC.
It was possible to increase the m.s. value to 80 mT by evaporating the aqueous phase, where the viscosity of the magnetic fluid increased to only 70 mPaAS. Further evaporation resulted in a highly viscous magnetic dispersion having an m.s. value of 100 mT.
Example 2 A water-based 10 mT magnetic fluid containing magnetite particles having a layer of oleic acid affixed thereto and a second non-ionic layer of sorbitan monooleate is treated as follows:
The magnetic fluid is heated to 90EC in a vessel. Subsequently, a foil-covered rare earth permanent magnet is introduced directly into the magnetic fluid.
The magnetic fluid sticking to the magnet is transferred into a new vessel where the separation is carried out. The final product attains an m.s. value of 50 mT at a viscosity of 10 mPaps at 27EC.
Example 3 A water-based 20 mT magnetic fluid containing cobalt ferrite particles as magnetic component, but othervvise consisting of the surfactant layers mentioned in the above Examples, is subjected to the following semi-continuous process:
The magnetic fluid first is heated to 80EC. A strong electromagnet is mounted on a glass pane or a plastic board and set up in a slightly slanted fashion.
Thereafter, the heated magnetic fluid is conveyed to the underside of the pane or board through a tube-like feed line. The separation process begins, and the surfactant solution from the magnetic fluid drips to the ground. Magnetic fluid is continuously supplied to the magnet until the concentrated magnetic fluid accumulates in such an amount that part of it is about to flow off from the magnet as well.
Now, the magnetic field of the electromagnet is gradually decreased so that the concentrated magnetic fluid is allowed to flow off separately into a collecting means. The process then is restarted by switching on the magnet and supplying magnetic fluid. A final product having an m.s. value of 60 mT and a viscosity of 20 mPaAS at 27EC was produced.
Example 4 A water-based 20 mT magnetic fluid containing magnetite particles stabilized with a bilayer of lauric acid in an alkaline medium is brought to a pH value of about 7 by adding dilute hydrochloric acid, where the magnetic fluid becomes slightly unstable. This is heated to 80EC and subjected to further treatment as in Example 2. Using a concentrated ammonium hydroxide solution, the final product is brought to a pH value > 8 where the particles undergo redispersion. The final _g_ product has an m.s. value of 60 mT at a viscosity of 5 mPaAS.
Example 5 The starting magnetic fluid is an aqueous magnetite magnetic fluid adjusted to be alkaline, wherein the particles are stabilized by an inner adsorption layer of lauric acid and an outer adsorption layer of lauric acid ammonium salt according to US 4,208,294, and which has a saturation magnetization of 15 mT.
The surfactants dissolved in the aqueous carrier liquid are caused to form surfactant aggregates by slow addition of ethanol and a dilute solution of hydrochloric acid, without destroying the magnetic fluid. Thereafter, separation of a part of the dispersion medium and the surfactants contained therein is effected in a magnetic field gradient. Subsequently, an alkaline pH value is readjusted by adding ammonium hydroxide to the concentrated magnetic fluid. The saturation magnetization of the concentrated magnetic fluid is 80 mT at a viscosity of 100 mPaAS at room temperature.
Example 6 A magnetite magnetic fluid based on petroleum, which is stabilized with a monolayer of oleic acid and has a magnetic saturation of 30 mT, is used as starting magnetic fluid. The oleic acid containing petroleum is condensed by adding ethanol in a ratio of 1:2. Upon exposure to an external magnetic field, the saturation magnetization increases to 100 mT, and the viscosity is 20 mPaAS at 27EC.
Reference list 1 Container 2 Separation surface 3 Magnet 4 Magnet 5 Heating element 6 Surfactant tray 7 Magnet tray 8 Feed line 9 Stop valve 10Magnetic fluid
Magnetic fluids are stable dispersions having superparamagnetic properties. The solid partiGes contained as dispersed phase in such a dispersion undergo sedimentation neither in a gravitational nor a magnetic field.
Magnetic fluids essentially consist of three components. The dispersed magnetic component comprises solid particles of ferro- or ferrimagnetic materials having a size of 3-50 nm. The dispersed phase present in the form of nanometer particles is stabilized by surfactants. The nanometer particles are homogeneously and stably dispersed in a dispersant herein refer-ed to as carrier liquid.
Surfactant molecules are amphiphilic molecules having both hydrophilic and lipophilic properties. The hydrophilic groups of the surfactants are chemically fixed at the surface of the particles to form monomolecular adsorption layers. For example, carboxylic acid groups, sulfonate groups, sulfate groups, phosphate groups, or phosphonate or amino groups are suitable as hydrophilic, chemically sorbable molecules. Both polar and non-polar solvents are suitable as carrier liquids.
To stabilize the nanometer particles in polar carrier liquids such as water, mainly two adsorption layers - one inner and one outer layer - are formed, where the amphiphilic molecules of the outer layer can be anionic, cationic or non-ionic surfactants, and those of the inner layer consist of anionic surfactants such as fatty acids. Particularly in case of magnetic fluids having an aqueous carrier liquid, a second, outer adsorption layer is necessary to stabilize the nanometer particles.
_2_ While the inner adsorption layer is attached chemically to the particle surface via the hydrophilic group of the surtactant, the outer layer is adsorbed to the hydrophobic portions of the molecules in the inner layer by weaker physical interactions. To enable formation of the outer layer, an excess of surfactant must be present in the aqueous phase. Frequently, the concentration of the outer layer surfactants in the aqueous phase is exceedingly high, giving rise to high volume viscosity and their massive accumulation during concentrating the aqueous phase, thus crucially limiting the values of magnetic saturation. The saturation magnetization is a measure for the concentration of magnetic particles in the magnetic fluid.
Water-based magnetic fluids are well-known. According to DE 195 16 323 A1, they exhibit magnetic saturations of up to 25 mT, and the nanometer particles as magnetic components are comprised of maghemite (Y-FezOa), magnetite (Fe30,) or mixed oxides such as cobalt ferrite or manganese-zinc ferrite. Also, such water-based magnetic fluids involve the drawback of having a relatively high concentration of surfactants in the aqueous phase. As a result, they also have a relatively high viscosity. Moreover, the high level of surfactants prevents achieving substantial magnetic saturations. Ultimately, high levels of surfactants may be critical in ecological terms and also, economically disadvantageous when producing substantial quantities.
Furthermore, it is well-known that magnetic fluids on an organic basis generally are treated by reprecipitation according to US 3,917,538 in order to reduce the surfactant concentration in the dispersion medium. The particles precipitate as a sediment, the dispersant containing surfactants is decanted and replaced by one free of surfactants. Following heating, the particles are redispersed to form a magnetic fluid. This process involves the proviso that a suitable precipitant is at hand and the surfactant will not be removed from the particles by the precipitant.
Hence, it is an energy-consuming, relatively rough procedure. Such a process cannot be used for magnetic fluids wherein the particles are stabilized by inner and outer adsorption layers. In such a case, the outer layer merely adsorbed in a physical fashion will normally be removed from the particles stabilized in the polar dispersion medium.
Methods of removing the superfluous surfactants from the outer adsorption layer after completed physical adsorption are not known as yet.
According to DE 41 30 268 A1, the particles are modified using a carboxy-functional polymer, where the dispersant includes both the carboxy-functional polymer and non-ionic wetting agents at a high concentration. For production, the magnetite particles are precipitated in the presence of the carboxy-functional polymers, a sediment then is formed from the modified particles which is redispersed in the dispersant having the above-mentioned composition. The saturation magnetization of the magnetic fluid having formed is below 10 mT. In addition, the electrical conductivity - being 900 w'cm~' - is very low, thus giving rise to flocculation of the particles when adding electrolyte.
US 4,208,294 describes aqueous magnetic fluids stabilized by two adsorption layers comprised of lauric acid and laurates. Similarly, this magnetic fluid includes an excess of lauric acid salt. The aqueous dispersion medium must be slightly alkaline. The attainable magnetic saturation is about 25 mT. One negative effect is that the particles undergo precipitation when shifting the pH from the neutral to the acidic range.
DE 43 27 826 A1 describes water-based magnetic fluids wherein the magnetite particles are stabilized by an internal fatty acid and the outer layer is comprised of ethoxylated fatty alcohols. Due to the conditions of production, the dispersant contains a large excess of ethoxylated fatty acids, resulting in a relatively high viscosity of the magnetic fluid and an attainable saturation magnetization of merely 25 mT at maximum.
It is the basic object of the invention to provide a magnetic fluid having high saturation magnetization, i.e., high concentration of nanometer particles, and low viscosity, and to provide a process and a device for the production thereof.
Said object is accomplished by the characterizing features of claims 1, 5 and 15.
The magnetic fluids according to the invention having an aqueous carrier liquid exhibit hitherto unknown magnetic saturations of between 30 and 100 mT, and the viscosity is below 100 mPaAS at 40EC. Owing to the high content of manometer particles, resulting in high magnetic saturations of the magnetic fluids according to the invention, in combination with a relatively low viscosity, these fluids are particularly suited as actuating fluids in medical pumps, in sensor technology, as well as in magnetohydrostatic separation of substances.
The fact that the carrier liquid does not contain surfactants anymore, results in ecological as well as economical advantages.
The magnetic fluids of the invention having high saturation magnetization are produced using the process and device according to the invention.
Surprisingly, it has been found that the surfactants can be removed by exposing the per se known magnetic fluids having relatively low saturation magnetization and high concentration of surfactants to an external magnetic field, in combination with using measures so as to reduce the solubility of the surfactants in the carrier liquid. For example, such removal is possible in such a way that the per se known aqueous magnetic fluids stabilized by one inner and one outer adsorption layer are heated to about 30-95EC, resulting in a reduced solubility of the surfactants in the carrier liquid. The heated magnetic fluid then is exposed to an external magnetic field so as to generate a strong inhomogeneous magnetic field - a magnetic field gradient -in the aqueous magnetic fluid. For example, this can be accomplished in such a way that a permanent magnet made of rare earths, which have surface magnetic saturation values of up to 0.5 T, acts to fix a heated aqueous magnetic fluid to the wall of a vessel. Following a magnetic field exposure period of about 15-120 minutes, mainly those surfactants making up the second outer adsorption layer and being present dissolved in the aqueous carrier liquid at a high concentration are removed from the magnetic manometer particles, entraining part of the aqueous carrier liquid, and forced to the surface, from where they can flow off. What remains is a concentrated magne~c fluid. By repeating this procedure, it is possible to increase the concentration of manometer particles step by step, so that magnetic saturations of 70 mT can be achieved. Because the surfactants are largely removed from the carrier liquid, remarkably low viscosities of the concentrated magnetic fluids are achieved, ranging between 5 and 30 mPaAS at 27EC.
Such low viscosities of the magnetic fluid represent one precondition for further concentrating by removing aqueous carrier liquid, e.g. by evaporation in a rotary evaporator. In this way, magnetic saturation values of 80 mT at a viscosity of only 70 mPaAS at 27EC have been achieved. Magnetic saturation values of up to 100 mT have been achieved by further withdrawal of water. Of course, the viscosity re-increases massively at such exceedingly high magnetic saturation values.
This process can be used with polar as well as nonpolar carrier liquids.
This concentrating procedure also is advantageous in that the dissolved surfactant from the carrier liquid, which has been separated from the nanometer particles in accordance with the process of the invention, can be recovered by evaporation and in this way, the surfactant can be used once.more in the production of an aqueous magnetic fluid.
According to the invention, other measures for reducing the solubility of surfactants in the carrier liquid are the following:
addition of agents that change the pH value;
changing the concentration of surfactants by withdrawing carrier liquid;
adding solvents reducing the solubility and/or solids such as salts and water-soluble polymers;
addition of substances that form aggregates with the surfactant molecules.
With reference to the drawings, the device of the invention will be illustrated in more detail.
Fig. 1 shows a device for quasi-continuous removal, and Fig. 2 shows a device for batchwise removal.
According to Fig. 1, a container 1 accommodates a magnetic fluid 10 to be concentrated. A heating element 5 is arranged beneath container 1. A feed line extends from the bottom of container 1 to the separation surfaces 2. The feed line 8 can be opened and closed by means of a stop valve 9. Two strong magnets 3 and are arranged above separation surface 2 in close vicinity thereof. The separation effect can be optimized via the slope angle of separation surface 2. A magnet tray 7 and a surfactant tray 6 are arranged beneath the separation surface 2. By switching on the heating element 5, the temperature of the magnetic fluid 10 is increased to about 60EC, thereby dramatically decreasing the solubility of the surfactants in the carrier liquid of the magnetic fluid 10. When opening the stop valve 9, the magnetic fluid 10 will flow via feed line 8 to the underside of separation surface 2.
Owing to the magnetic field gradient which is present and generated by magnet 3, a bulge-like accumulation of magnetic fluid 10 is formed at separation surface 10.
Following an exposure period of about 10 minutes, the first droplets of carrier liquid including accumulated surfactant come off, dripping into the surfactant tray 6. When switching off magnet 3 and simultaneously switching on magnet 4, the magnetic fluid 10 will be drawn to the separation surface beneath magnet 4 where additional surfactant is removed. After switching off magnet 4, the remaining, highly concentrated magnetic fluid 10 then is collected in magnet tray 7.
Fig. 2 illustrates a device for batchwise removal of surfactants from a carrier liquid at various stages of the process.
According to a_, the magnetic fluid 10 is heated in the first step, using heating element 5. According to b, the magnetic fluid - that is, the manometer particles present - accumulates at the separation surface 2 after switching on magnet 3. According to c_, the concentration process at separation surface 2 is completed, and the concentrated particles can be collected at the bottom. The magnetic fluid collected at the bottom may subsequently be refed in container 1, and an additional separation process may follow.
The process according to the invention will be illustrated in more detail with reference to the Examples below.
Example 1 A water-based 15 mT magnetic fluid containing magnetite particles having a layer of lauric acid affixed to the partiGes and a second non-ionic layer of ethoxylated alcohols with ethoxy groups is concentrated as follows:
100 ml of magnetic fluid is heated to 80EC in a refractory vessel. A rare earth permanent magnet having a magnetic saturation of 0.3 T at its surface is subsequently attached to the exterior wall of the vessel so as to hold the magnetic fluid at the opposite side of the magnet. After some minutes, a non-magnetic viscous solution begins to separate from the magnetic fluid. The magnetic fluid, becoming more and more concentrated over time, forms the typical peaks after some time, becoming fixed to the magnet more and more tightly. The separation process can be promoted by keeping the magnetic fluid in motion either by moving the magnet or by mechanical agitation of the magnetic fluid, or by reheating the magnetic fluid to 80-90EC. The final product has a saturation magnetization of 50 mT and a kinematic viscosity of 5 mPaAS at 27EC.
It was possible to increase the m.s. value to 80 mT by evaporating the aqueous phase, where the viscosity of the magnetic fluid increased to only 70 mPaAS. Further evaporation resulted in a highly viscous magnetic dispersion having an m.s. value of 100 mT.
Example 2 A water-based 10 mT magnetic fluid containing magnetite particles having a layer of oleic acid affixed thereto and a second non-ionic layer of sorbitan monooleate is treated as follows:
The magnetic fluid is heated to 90EC in a vessel. Subsequently, a foil-covered rare earth permanent magnet is introduced directly into the magnetic fluid.
The magnetic fluid sticking to the magnet is transferred into a new vessel where the separation is carried out. The final product attains an m.s. value of 50 mT at a viscosity of 10 mPaps at 27EC.
Example 3 A water-based 20 mT magnetic fluid containing cobalt ferrite particles as magnetic component, but othervvise consisting of the surfactant layers mentioned in the above Examples, is subjected to the following semi-continuous process:
The magnetic fluid first is heated to 80EC. A strong electromagnet is mounted on a glass pane or a plastic board and set up in a slightly slanted fashion.
Thereafter, the heated magnetic fluid is conveyed to the underside of the pane or board through a tube-like feed line. The separation process begins, and the surfactant solution from the magnetic fluid drips to the ground. Magnetic fluid is continuously supplied to the magnet until the concentrated magnetic fluid accumulates in such an amount that part of it is about to flow off from the magnet as well.
Now, the magnetic field of the electromagnet is gradually decreased so that the concentrated magnetic fluid is allowed to flow off separately into a collecting means. The process then is restarted by switching on the magnet and supplying magnetic fluid. A final product having an m.s. value of 60 mT and a viscosity of 20 mPaAS at 27EC was produced.
Example 4 A water-based 20 mT magnetic fluid containing magnetite particles stabilized with a bilayer of lauric acid in an alkaline medium is brought to a pH value of about 7 by adding dilute hydrochloric acid, where the magnetic fluid becomes slightly unstable. This is heated to 80EC and subjected to further treatment as in Example 2. Using a concentrated ammonium hydroxide solution, the final product is brought to a pH value > 8 where the particles undergo redispersion. The final _g_ product has an m.s. value of 60 mT at a viscosity of 5 mPaAS.
Example 5 The starting magnetic fluid is an aqueous magnetite magnetic fluid adjusted to be alkaline, wherein the particles are stabilized by an inner adsorption layer of lauric acid and an outer adsorption layer of lauric acid ammonium salt according to US 4,208,294, and which has a saturation magnetization of 15 mT.
The surfactants dissolved in the aqueous carrier liquid are caused to form surfactant aggregates by slow addition of ethanol and a dilute solution of hydrochloric acid, without destroying the magnetic fluid. Thereafter, separation of a part of the dispersion medium and the surfactants contained therein is effected in a magnetic field gradient. Subsequently, an alkaline pH value is readjusted by adding ammonium hydroxide to the concentrated magnetic fluid. The saturation magnetization of the concentrated magnetic fluid is 80 mT at a viscosity of 100 mPaAS at room temperature.
Example 6 A magnetite magnetic fluid based on petroleum, which is stabilized with a monolayer of oleic acid and has a magnetic saturation of 30 mT, is used as starting magnetic fluid. The oleic acid containing petroleum is condensed by adding ethanol in a ratio of 1:2. Upon exposure to an external magnetic field, the saturation magnetization increases to 100 mT, and the viscosity is 20 mPaAS at 27EC.
Reference list 1 Container 2 Separation surface 3 Magnet 4 Magnet 5 Heating element 6 Surfactant tray 7 Magnet tray 8 Feed line 9 Stop valve 10Magnetic fluid
Claims (21)
1. A magnetic fluid, consisting of a polar carrier liquid and magnetic nanometer particles stabilized by two monomolecular adsorption layers, characterized in that the magnetic fluid has a saturation magnetization of at least 30 mT and a viscosity below 100 mPaAS at 40EC, the carrier liquid essentially containing no dissolved components of the outer adsorption layer.
2. The magnetic fluid according to claim 1, characterized in that the polar carrier liquid is water and/or a water-miscible liquid such as a glycol or a formamide.
3. The magnetic fluid according to claim 1 or 2, characterized in that the magnetic nanometer particles have a size of 3-50 nm.
4. The magnetic fluid according to any of claims 1-3, characterized in that the saturation magnetization is 30-100 mT.
5. A process for producing highly concentrated magnetic fluids based on nonpolar and polar carrier liquids and magnetic nanometer particles stabilized by one or two adsorption layers comprised of surfactants, characterized in that a magnetic fluid having a polar or nonpolar carrier liquid containing surfactants is exposed to an external magnetic field after supplying or adding agents reducing the solubility of the surfactants, and that following said exposure, the surfactants depositing in the carrier liquid are separated from the nanometer particles.
6. The process according to claim 1, characterized in that an external magnetic field having a strength of 0.2 T at minimum is used for exposure.
7. The process according to claim 5 or 6, characterized in that the magnetic fluid to be concentrated is heated to at least 30EC prior to magnetic field exposure.
8. The process according to any of claims 5-7, characterized in that the magnetic fluid is heated to 30-95EC, particularly 60-80EC.
9. The process according to any of claims 5-8, characterized in that agents altering the pH value, such as acids, bases or salts are added.
10. The process according to any of claims 5-8, characterized in that solvents reducing the solubility and/or solids such as other surfactants, salts and/or water-soluble polymers are added.
11. The process according to any of claims 5-8, characterized in that agents solely absorbing the carrier liquid are added.
12. The process according to any of claims 5-11, characterized in that the magnetic fluid is exposed to the magnetic field for at least five minutes.
13. The process according to any of claims 5-12, characterized in that the magnetic fluid is exposed to an external magnetic field of 0.1 - 1.5 T.
14. The process according to any of claims 5-13, characterized in that the magnetic fluid is exposed to multiple magnetic fields with increasing strength.
15. The process according to any of claims 5-14, characterized in that the separated carrier liquid is reused.
16. The process according to any of claims 5-15, characterized in that magnetic fluids having polar carrier liquids such as water and/or water-miscible fluids such as glycols or formamides are used.
17. The process according to any of claims 5-15, characterized in that magnetic fluids having nonpolar carrier liquids are used.
18. A device for producing highly concentrated magnetic fluids based on polar carrier liquids and magnetic nanometer particles having two monomolecular adsorption layers, consisting of - a container (1) for magnetic fluids, - separation surfaces (2), and - magnetic field gradient generators (3), the magnetic field gradient generators (3) being arranged at the separation surfaces (2).
19. The device according to claim 18, characterized in that the container (1) has a heating element (9) for the magnetic fluids.
20. The device according to claim 18 or 19, characterized in that the magnetic field gradient generators (3) are oriented in the direction of gravity.
21. The device according to claim 18-20, characterized in that multiple magnetic field gradient generators (3) with increasing magnetic strength are arranged in line.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE19758350.4 | 1997-12-22 | ||
| DE19758350A DE19758350C1 (en) | 1997-12-22 | 1997-12-22 | Magnetic fluid comprising magnetic nano-particles dispersed in a polar carrier fluid |
| PCT/EP1998/008232 WO1999033072A1 (en) | 1997-12-22 | 1998-12-17 | Magnetic liquid and method and device for the production thereof |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2315704A1 true CA2315704A1 (en) | 1999-07-01 |
Family
ID=7853655
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002315704A Abandoned CA2315704A1 (en) | 1997-12-22 | 1998-12-17 | A magnetic fluid, a process and a device for its production |
Country Status (9)
| Country | Link |
|---|---|
| EP (1) | EP1051715A1 (en) |
| JP (1) | JP2001527283A (en) |
| KR (1) | KR20010033478A (en) |
| CN (1) | CN1285950A (en) |
| AU (1) | AU2162299A (en) |
| CA (1) | CA2315704A1 (en) |
| DE (1) | DE19758350C1 (en) |
| RU (1) | RU2203516C2 (en) |
| WO (1) | WO1999033072A1 (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE102006041495A1 (en) * | 2006-08-29 | 2008-03-20 | Friedrich-Schiller-Universität Jena | Substance mixture, useful for locally limited of magnetic nanoparticle within or directly at the edge of tumor affected physical tissue, where the magnetic particle is mixed with a fluid thixotropic carrier substance |
| UA87177C2 (en) | 2007-07-04 | 2009-06-25 | Государственное Предприятие "Международный Центр Электронно-Лучевых Технологий Института Электросварки Им. Е.О.Патона Национальной Академии Наук Украины" | method of producing nanoparticles for magnetic fluids by electron-beam evaporation and condensation in vacuum, method of producing magnetic liquid and magnetic liquid produced by said method |
| CN102441488B (en) * | 2011-09-06 | 2013-07-31 | 北京交通大学 | Slide-type gas-liquid interface jigging magnetic separation controllable device |
| US9409148B2 (en) | 2013-08-08 | 2016-08-09 | Uchicago Argonne, Llc | Compositions and methods for direct capture of organic materials from process streams |
| EP3610564B1 (en) * | 2017-04-14 | 2022-09-28 | Carrier Corporation | Electrical machine winding inductance enhancement |
| CN107195419A (en) * | 2017-08-02 | 2017-09-22 | 新疆大学 | Ethylene glycol magnetic fluid and preparation method thereof |
| CN108037700A (en) * | 2017-12-13 | 2018-05-15 | 杭州电子科技大学 | A kind of magnetic current body controlling means |
| JP7059358B2 (en) | 2018-03-30 | 2022-04-25 | 富士フイルム株式会社 | Separation device and separation method |
Family Cites Families (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3917538A (en) * | 1973-01-17 | 1975-11-04 | Ferrofluidics Corp | Ferrofluid compositions and process of making same |
| US4208294A (en) * | 1979-02-12 | 1980-06-17 | The United States Of America, As Represented By The Secretary Of The Interior | Dilution stable water based magnetic fluids |
| JPH04107802A (en) * | 1990-08-28 | 1992-04-09 | Nippon Seiko Kk | Production of magnetic fluid |
| US5240626A (en) * | 1990-09-21 | 1993-08-31 | Minnesota Mining And Manufacturing Company | Aqueous ferrofluid |
| DE4327826C2 (en) * | 1993-08-16 | 1995-06-08 | Ikosta Gmbh Inst Fuer Korrosio | Magnetic liquid |
| DE19514515C2 (en) * | 1995-04-12 | 1997-03-20 | Dirk Dipl Chem Guenther | Magnetizable dispersions |
| DE19516323C2 (en) * | 1995-04-27 | 1997-02-27 | Dirk Dipl Chem Guenther | Process for the preparation of magnetizable dispersions and their use |
| US5667716A (en) * | 1996-07-01 | 1997-09-16 | Xerox Corporation | High magnetization aqueous ferrofluids and processes for preparation and use thereof |
-
1997
- 1997-12-22 DE DE19758350A patent/DE19758350C1/en not_active Expired - Fee Related
-
1998
- 1998-12-17 RU RU2000119040/02A patent/RU2203516C2/en not_active IP Right Cessation
- 1998-12-17 JP JP2000525894A patent/JP2001527283A/en active Pending
- 1998-12-17 KR KR1020007006979A patent/KR20010033478A/en not_active Application Discontinuation
- 1998-12-17 CN CN98813070A patent/CN1285950A/en active Pending
- 1998-12-17 WO PCT/EP1998/008232 patent/WO1999033072A1/en not_active Application Discontinuation
- 1998-12-17 EP EP98965839A patent/EP1051715A1/en not_active Withdrawn
- 1998-12-17 AU AU21622/99A patent/AU2162299A/en not_active Abandoned
- 1998-12-17 CA CA002315704A patent/CA2315704A1/en not_active Abandoned
Also Published As
| Publication number | Publication date |
|---|---|
| KR20010033478A (en) | 2001-04-25 |
| AU2162299A (en) | 1999-07-12 |
| EP1051715A1 (en) | 2000-11-15 |
| WO1999033072A1 (en) | 1999-07-01 |
| JP2001527283A (en) | 2001-12-25 |
| CN1285950A (en) | 2001-02-28 |
| RU2203516C2 (en) | 2003-04-27 |
| DE19758350C1 (en) | 1999-03-11 |
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