JPH01315103A - Manufacture of magnetic fluid composition - Google Patents

Abstract

PURPOSE:To facilitate controlling the diameter of fine particles of a ferromagnetic substance and to improve their dispersion properties independently by a method wherein an amount to be added of a surface-active agent in a first adsorption process is set to be less than 100% of a covering rate on the surface of the fine particle of the ferromagnetic substance and a surface-active agent in a second adsorption process is added in an aqueous phase. CONSTITUTION:A first ionic surface-active agent whose dispersion property is lower than that of a second surface-active agent is used intentionally; alternatively, when the first ionic surface-active agent whose dispersion property is equal to that of the second surface-active agent is used, an amount to be added is reduced and a covering rate is lowered. In addition, when the second ionic surface-active agent is used in an aqueous phase, the ionic surface-active agent is not adsorbed simply physically to the surface of a fine particle of a ferromagnetic substance in a part which has not been covered with the first surface-active agent; polar radicals of both agents can be bonded firmly and irreversibly. By this setup, a control operation of the fine particle of the ferromagnetic substance can be executed easily and independently of an enhancement of the dispersion property.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a CD magnetic fluid composition in which fine ferromagnetic particles are stably dispersed in a dispersion medium via a dispersant.

[Conventional technology]

The magnetic fluid is magnetite, ferrite, and iron.

It is an extremely stable colloidal solution in which fine ferromagnetic particles such as cobalt are dispersed in a liquid, and has the characteristic that the higher the concentration of fine ferromagnetic particles, the stronger the apparent magnetism of the liquid itself. Therefore, even though it is a liquid, its behavior can be freely restrained by magnets, etc., so it can be used as a damping agent, a sealing agent in the sealing mechanism of magnetic disks, etc.
Application fields are wide-ranging.

Various liquids can be used as a dispersion medium for magnetic fluids, but when oil is generally used as a dispersion medium, the surface of the ferromagnetic particles themselves is hydrophilic, so a dispersant such as a surfactant is used to make the particles hydrophilic. It is necessary to modify the oily surface.

The dispersibility of ferromagnetic fine particles whose surface properties have been modified to be lipophilic in an oil-based dispersion medium is affected by the following factors.

■Particle size of ferromagnetic fine particles (especially maximum particle size).

■ Dispersion power possessed by the dispersant adsorbed on the ferromagnetic fine particles.

■ Interparticle distance of ferromagnetic fine particles in the dispersion medium, that is, particle concentration.

Therefore, by controlling the particle size of the ferromagnetic particles and improving the dispersion force of the dispersant, it is possible to obtain a magnetic fluid with more stable dispersibility, which naturally increases the concentration of the ferromagnetic particles. It should be possible.

By the way, the following various magnetic fluid compositions have been proposed in which ferromagnetic fine particles are stably dispersed in a desired non-aqueous solvent using a dispersing agent with high dispersion power.

A magnetic fluid composition using an aliphatic hydrocarbon oil or an alicyclic hydrocarbon oil as a dispersion medium (carrier) and a hydrocarbon-soluble surfactant as a dispersant (US Pat. No. 3,700,595). A magnetic fluid composition in which a perfluorinated oil is used as a carrier and a surfactant having a perfluoropolyether as a hydrophobic group is used as a dispersant (US Pat. No. 3,784.471). Magnetic fluid compositions using phosphate esters of long chain alcohols as dispersants (U.S. Pat. No. 4,430.23.9). Compositions with polyphenyl ether as carrier (U.S. Pat. No. 4,315,827)
). A magnetic fluid composition in which silicone oil is used as a carrier and a surfactant consisting of a part soluble in silicone oil and a part chemically bonded to the particle surface is used as a dispersant (US Pat. No. 4,356).
098).

In each of the above-mentioned magnetic fluid compositions, a sufficient amount of surfactant to completely coat the surface of the ferromagnetic fine particles is added at one time during the manufacturing process.

Generally, there is a strong correlation between the particle size distribution of ferromagnetic fine particles in a magnetic fluid composition and the dispersing power of a dispersant, and the larger the dispersing power of a surfactant used, the larger the maximum dispersible particle size. However, it is difficult for large particles to continue to be dispersed as stably over a long period of time as fine particles. That is,
Simply using a surfactant with a large dispersion power does not necessarily guarantee long-term stable dispersibility of the resulting magnetic fluid composition.

Therefore, the applicant first added an ionic surfactant twice, and after the first addition formed a monomolecular layer of the surfactant on the surface of the ferromagnetic fine particles, a large Separate the particles and then.

Furthermore, we proposed a method for producing a magnetic fluid composition in which a second surfactant is added (Japanese Patent Application Laid-open No. 105093/1983).
.

[Problem to be solved by the invention]

However, in the conventional method of adding surfactant twice, the amount of ionic surfactant added in the first time is
The surface of all the ferromagnetic fine particles was set to 1i (100%) so that the surface of all the ferromagnetic particles could be completely covered with the polymeric particle layer, and the second addition of the ionic surfactant was carried out in a non-polar organic solvent.

As a result, the following problems occurred.

■ In non-polar organic solvents, ionic surface activity ・
When adsorbing the agent to the ferromagnetic fine particles, electrostatic force 1
In other words, ferromagnetic fine particles 1
The surface of the surfactant is modified to be lipophilic by being coated with a first surfactant with the lipophilic groups facing outward. Therefore, the second surfactant is oriented with its lipophilic groups toward the lipophilic group side of the first surfactant layer and its hydrophilic polar groups toward the outside, thereby adsorbing two layers.

This adsorption is a reversible physical adsorption based on van der Waals force, and the adsorption force is weak. Especially in a system where the particle concentration is high, a long-term effect of improving dispersibility cannot be expected.

(2) Since the second surfactant orients its hydrophilic polar groups outward in this way, the dispersibility in a non-polar organic solvent tends to deteriorate as the amount added increases. Therefore, the amount added must be limited to an extremely small amount, and a large dispersion effect cannot be expected.

(2) In order to increase the amount of the second surfactant added, it is necessary to select a surfactant whose hydrophilic group has a small effect on the dispersion system. For this reason, the surfactant itself to be used is limited.

(2) In order to obtain a long-term stable dispersion system, it is necessary to control the size of the dispersed particles by removing relatively large diameter particles from among the ferromagnetic fine particles in the system. In the conventional case described above, particle size is controlled only by physical means such as a centrifuge, or by adjusting conditions in the ferromagnetic particle synthesis process before adding a surfactant. Ta.

However, in order to keep the average particle size and particle size distribution within an extremely small range using only centrifugal force, ultra-high speed rotation is required, which requires enormous equipment costs.

On the other hand, it is extremely difficult to set conditions for adjusting the average particle size and particle size distribution with good reproducibility in the ferromagnetic fine particle synthesis process.

In summary, in the conventional two-time addition method of surfactant,
It has been difficult to independently control the particle size and improve the dispersibility of ferromagnetic fine particles.

The present invention has been made by focusing on such conventional problems, and its purpose is to suppress the dispersibility of ferromagnetic fine particles by the first surfactant, thereby improving the magnetic fluid obtained. In addition to controlling the maximum particle size of the dispersed particles in the composition as small as possible, a second surfactant is added in the aqueous phase to firmly bind the ionic surfactant to the surface of the ferromagnetic fine particles. It is an object of the present invention to provide a method for producing a magnetic fluid composition, in which a highly stable magnetic fluid composition can be obtained by adsorption to a magnetic fluid composition.

[Means to solve the problem]

In the present invention, after adding an unsaturated amount of a first ionic surfactant and a low-boiling point organic solvent to ferromagnetic fine particles to obtain a dispersion system in which the particle surface is incompletely covered with the surfactant, a particle size control step of removing relatively large ferromagnetic particles from the system; a step of removing a low boiling point organic solvent from the dispersion system to obtain highly lipophilic 41i particles; Water and a second ionic surfactant are added to the magnetic particles to form a second ionic surfactant by hydration on the surface of the strong C'zI particles, at least in areas not covered with the first ionic surfactant. ferromagnetic fine particles coated with the first and second surfactants are dispersed in a dispersion medium.

[Effect]

The first ionic surfactant is intentionally used with a lower dispersibility than the second surfactant, or if the first ionic surfactant is used with the same dispersibility as the second surfactant, the amount added Reduce the coverage rate. The surfaces of the ferromagnetic particles are only incompletely covered. Thereby, the dispersed particle diameter is adjusted while the dispersibility of the ferromagnetic fine particles is suppressed. Therefore, ferromagnetic fine particles with a relatively large particle size are not completely covered with surfactant and strongly protected by chemical dispersion force as in the past, and are unstable. . In order to remove these unstable large-diameter ferromagnetic particles from a low-viscosity, low-boiling point organic solvent system, very strong means are not required; for example, a typical centrifugal force of about 8,000 mm is sufficient. It is. In this way, only fine particles with a small particle size that can maintain good dispersibility over a long period of time are easily selected and remain. For example, by further centrifuging particles that have been precipitated with a centrifugal force of 8000 G to obtain a supernatant liquid, it is also possible to obtain a system in which the particle size distribution is closer to monodisperse.

In the present invention, further, by adding the second ionic surfactant in the aqueous phase, the second ionic surfactant is added to the surface of the ferromagnetic fine particles in the portion not covered with the first surfactant. The ionic surfactant No. 2 is not simply physically adsorbed, but its polar groups bond very strongly and irreversibly (chemical adsorption or chemical bonding).

Hereinafter, the method for manufacturing the magnetic fluid composition of the present invention will be explained in detail.

The first surfactant used in the first adsorption step of the present invention is:
Basically, it is soluble or compatible with the aqueous solvent used for the intermediate medium described later, and has a polar group that chemically adsorbs or chemically bonds with the surface of the ferromagnetic fine particles. in particular,
A hydrophobic group having 8 or more carbon atoms and, for example, sulfonic acids (synthetic sulfonic acids).

petroleum sulfonic acid, alkylnaphthalene sulfonic acid, etc.), carboxylic acids (alkylnaphthalene carboxylic acid, polyoxyethylene alkyl ether acetic acid, N-acylamino acids, unsaturated fatty acids, saturated fatty acids, etc.), phosphonic acids,
Or sulfuric acid ester.

Examples include anionic surfactants having a hydrophilic group moiety such as phosphoric acid esters, amines, alcohols, salts thereof, and quaternary ammonium salts, and silane coupling agents having alkoxysilanes. Several of these may be used in combination.

The number of carbon atoms in the hydrophobic group of the first surfactant is considered to be 10 or more in view of particle dispersibility in conventional magnetic fluid compositions, but this is not suitable for controlling the particle size of the present invention. In order to utilize a state in which particle dispersibility is intentionally lowered,
A material having a carbon number of about 8, which has poor dispersion power, is also applicable.

The first adsorption step in which these first surfactants are added is
This is to control the maximum particle size of the ferromagnetic fine particles in the finally obtained magnetic fluid composition.

That is, the amount of the first surfactant added is determined in consideration of the yield of the obtained lipophilic ferromagnetic fine particles and the target maximum particle diameter. The range is such that the coverage (described later) on the particle surface is 20% or more and less than 100%, preferably 50 to 80%.

If the particle surface is covered with 100% or more, the desired particle control effect cannot be obtained. In other words, the particle size control of the present invention is achieved by intentionally lowering the dispersion ability of the first surfactant that disperses the ferromagnetic fine particles into the intermediate medium, thereby efficiently removing only relatively large-sized particles from the system. removal, resulting in a long-term stable ferrofluid composition. Therefore, when a first surfactant with high dispersibility is selected, the coverage is lower than when a first surfactant with low dispersibility is selected.

Specifically, when selecting a combination of the dispersibility and IW coverage of the first surfactant, it is desirable to use the following numerical values as a guide. That is, the maximum particle diameter of the ferromagnetic fine particles in the obtained magnetic fluid composition increases the coverage of the dispersant by 100%.
The maximum particle size is at least 10% smaller than the maximum particle size in a conventional magnetic fluid composition obtained as a ferrofluid composition.

On the other hand, if the coverage is less than 20%, the above yield is, for example, 10%.
This will be disadvantageous in terms of profitability.

The second surfactant used in the second adsorption step of the present invention is
Except that the number of carbon atoms in the hydrophobic group part is 18 or more,
The same type of surfactant as the first surfactant may be used.

The low-boiling organic solvent of the present invention includes benzene, toluene, 8-xylene, hexane, cyclohexane, chloroform, diethyl ether, and the like.

Further, the dispersion medium (carrier) for the ferromagnetic fine particles used in the present invention may be a low-volatility, low-viscosity mineral oil, ester oil, ether oil such as alkyl polyphenyl ether, alkyl naphcrene oil, or polyα-olefin. A solvent such as a hydrocarbon-based synthetic oil such as oil is suitable, and is used as appropriate depending on the use of the magnetic fluid.

These preferably have a high affinity with the first and second surfactants described above. Therefore, the combination of a dispersion medium and a surfactant must be determined by considering compatibility and also by referring to, for example, the following experimental results.

"When the surfactant was N-acylamino acid, oleic acid, or isostearic acid and the dispersion medium was hexane, a dense dispersion of ferromagnetic particles was obtained.On the other hand, with the same surfactant, When the dispersion medium was poly-α-olefin, a dense dispersion of ferromagnetic fine particles could not be obtained.

As the ferromagnetic fine particles of the present invention, those obtained as a colloidal water suspension (slurry) by a well-known wet method may be used. The wet method here refers to the method of obtaining magnetite colloid by adding alkali to an acidic solution containing ferrous ions and ferric ions in a ratio of 1:2 to make the pH approximately 9 or higher, and aging it at an appropriate temperature. be. Alternatively, it may be obtained by a so-called wet pulverization method in which magnetite powder is pulverized in a ball mill in water or an organic solvent.

Furthermore, other materials obtained by a dry method may also be used.

In addition to magnetite, there is also manganese ferrite.

It is also possible to use ferromagnetic fine particles such as nickel ferrite, cobalt ferrite, composite ferrite of these and zinc, barium ferrite, and ferromagnetic metal fine particles such as iron and cobalt.

The content of ferromagnetic fine particles is not only within the range of 1 to 20% by volume, which is conventionally used for boat fishing, but also by manufacturing through an intermediate medium using a low boiling point organic solvent.
It also becomes possible to adjust the concentration even higher.

In the first adsorption step of the present invention, a low boiling point organic solvent is added to a predetermined amount of ferromagnetic fine particles having a particle size of 20 to 500 to form a suspension, and then a first surfactant is added. An intermediate medium may be obtained, or a mixture of the first surfactant and a low-boiling organic solvent may be added to obtain an intermediate medium. In addition, if ferromagnetic fine particles obtained by a wet method are used, a coating layer is formed by adding the required amount of the first surfactant to a suspension of ferromagnetic fine particles in an aqueous phase, and then the coating layer is washed. After drying to obtain hydrophobic ferromagnetic fine particles, a low boiling point organic solvent may be added to obtain an intermediate medium.

In the second adsorption step of the present invention, the ferromagnetic fine particles having lipophilic and hydrophilic amphoteric surface properties obtained in the first adsorption step are suspended in water, and a second surfactant is added thereto. do. As a result, the hydrophilic group of the second surfactant directly forms a strong chemical bond with the hydrophilic surface of the particle, and the entire surface of the ferromagnetic fine particle becomes lipophilic.

Thereafter, the lipophilic ferromagnetic fine particles are separated from water and dried. Finally, directly onto the dried lipophilic ferromagnetic particles.
A dispersion medium is added to obtain a magnetic fluid composition.

If you want to create a magnetic fluid composition with a particularly high particle concentration, add a low-boiling organic solvent and a dispersion medium as an intermediate medium, then heat and concentrate the low-boiling organic solvent, and add a ferromagnetic material to the concentrated product. What is necessary is to repeat adding a low boiling point organic solvent in which the fine particles are dispersed, separating the low boiling point organic solvent by heating, and concentrating it.

The present invention will be explained in detail below.

[Example 1] Relationship between the concentration of the dispersant (first surfactant) added and the coverage of the ferromagnetic fine particles in the first adsorption step: Dispersant: N-acyl amino acid Dispersion medium: hexane First, by wet method A magnetite slurry was produced. That is, 6N NaOHaq was added to an aqueous solution 11 containing 0.3 mol each of ferrous sulfate and ferric sulfate at pH 1.
1 or higher, the solution was heated at 60"C for 30
A slurry of magnetite colloid was obtained for 7 minutes.

To the above slurry containing 100 parts by weight of magnetite particles,
3N HClaq was added to adjust the pH to 5.5, and while maintaining the pH, 60 parts by weight of N-acyl amino acid [Nikko Chemicals Co., Ltd., Sarcosine] was added to 100 parts by weight of magnetite particles in the slurry. 1-LH(
Add an aqueous solution containing acyl chain length: CI□ is the main component).
After stirring thoroughly, add 3N HCl aq to adjust the pH.
was adjusted to 4, and the liquid temperature was finally raised to 70°C. As a result, the first surfactant was adsorbed onto the surface of the magnetite particles. Thereafter, the solution is allowed to stand still to coagulate and precipitate the magnetite particles in the solution, and the supernatant is discarded.

Add more water, stir, let stand again, and discard the supernatant. After repeating this water washing several times to remove the electrolyte in the aqueous solution, it was filtered and dehydrated, and dried to obtain powdered magnetite particles.

Next, hexane is added to this magnetite powder and shaken thoroughly to disperse the magnetite particles in hexane. This is centrifuged for 30 minutes under a centrifugal force of 8000 G to sediment and remove particles with poor dispersibility among the magnetite particles. It was observed that the magnetite particles in the obtained supernatant liquid were extremely stably dispersed.

Here, in order to remove unadsorbed or incompletely adsorbed first surfactant, MeOH is added as necessary. As a result, the magnetite particles lose their dispersibility and aggregate. On the other hand, since the unadsorbed or incompletely adsorbed surfactant remains dissolved in the solvent, the aggregated particles are filtered and dried. Then add hexane and redisperse.

In this way, after almost completely removing unstable dispersed particles in advance, the low-boiling organic solvent is separated by heating, and a ferromagnetic structure is created in which a surface modified to be lipophilic and a surface that remains hydrophilic coexist. body microparticles were obtained.

The results of examining the relationship between the concentration of N-acyl amino acid added as the first surfactant to the magnetite particles and the coverage in the above first adsorption step will be explained.

In the above process, the concentration of N-acylamino acid added to the weight of magnetite particles in the slurry was varied to obtain hexane-dispersed magnetite colloidal solutions having different concentrations (parts by weight).

These magnetite colloidal liquids were transferred to a rotary evaporator and kept at 90°C to evaporate hexane, and the resulting lipophilic magnetide particles were weighed.

The resulting yields of magnetite particles (!weight of IJI oil-based magnetite particles/weight of magnetite particles in slurry) at each addition concentration of N-acyl amino acid are shown in Table 1. However, the yield is 100% when the concentration of N-acyl amino acid added is 60 parts by weight.

From Table 1, when N-acyl amino acid is used as a dispersant, monomolecular adsorption is completed when the added concentration is 60 parts by weight. That is, the coverage of magnetite particles at this concentration is 100%. FIG. 1 shows the relationship between the above-mentioned additive concentration, yield, and coverage.

[Example 2] Effect of adding the dispersant (second surfactant) in the aqueous phase in the second adsorption step: In the same process as in Example 1, using hexane as the dispersion medium, the first N- as a surfactant
A magnetite colloid was obtained in which acyl amino acids were adsorbed to a coverage of 50%.

Transfer this colloid melt to a rotary evaporator,
Hexane was removed by evaporation at 90°C to obtain lipophilic magnetite fine particles. (The above is the first adsorption step).

Next, 2000 parts by weight of pure water was added to 100 parts by weight of the above lipophilic magnetite fine particles, and the mixture was heated while being irradiated with ultrasonic waves.
．． Add IN Na0Haq to adjust pH to 0,
Magnetite fine particles are combined with molecules fI1. I let it happen. Then, while stirring the dispersion, add a second surfactant aqueous solution containing 20 parts by weight of eicosylnaphthalene sulfonic acid, adjust the pH to 4 with 3N HCfaq, and heat the liquid temperature to 60°C. did. This completes the adsorption of the second surfactant, eicosylnaphthalene sulfonic acid, onto the surface of the magnetite particles. Then wash with water °ζ
After removing the electrolyte from the aqueous solution, the solution was filtered and dehydrated, followed by vacuum drying.

After drying, hexane was added and thoroughly shaken to disperse the magnetite particles in hexane. If necessary, it may be centrifuged here to remove particles that are not completely adsorbed.

120 in benitosan in which magnetite particles are dispersed.
Part by weight of eicosylnaphthalene was added as a dispersion medium, and after thorough mixing, the mixture was transferred to a rotary evaporator and kept at 90° C. to evaporate hexane. The magnetic fluid composition thus obtained was dark brown and transparent.

For comparison, a magnetic fluid composition in which a dispersant (second surfactant) was added in the oil phase as before was prepared in the following steps.

[Comparative example]

A first adsorption step similar to that in Example 2 was performed to obtain lipophilic magnetite fine particles. Next, 300 parts by weight of hexane was added to 100 parts by weight of the lipophilic magnetite fine particles. Furthermore, after adding 20 parts by weight of eicosylnaphthalene sulfonic acid, which is a second surfactant, the lipophilic magnetite fine particles are redispersed in hexane by treatment in a ball mill for 2 hours, and the fine particles are dispersed in oil. An operation was performed in which the surfactant No. 2 was adsorbed onto the surface of the ferromagnetic fine particles.

Thereafter, 120 parts by weight of eicosylnaphthalene was added as a dispersion medium to this magnetite colloidal liquid, and the mixture was thoroughly mixed. The mixture was transferred to a rotary evaporator and kept at 90°C to evaporate hexane.

The ferrofluid composition thus obtained was reddish-brown in color and cloudy.

Table 2 shows the comparative results of the dispersibility of the two types of magnetic fluid compositions obtained as described above.

Table 2 Example 3 Preparation of various magnetic fluid compositions according to the process of the present invention: Example 1. Various ferrofluid compositions prepared similarly to Example 2 are shown in Table 3.

In addition, as the ferromagnetic fine particles, magnetite I and other M
n-Zn ferrite could also be adjusted in the same way.

The carriers in the table are indicated by abbreviations.

OL niolefin, AP: alkyl polyphenyl ether, AN: alkyl naphthalene, FE: barfluoropolyether.

[Example 4] Particle diameter of ferromagnetic fine particles in the magnetic fluid composition obtained by the production method of the present invention and conventional method using only the first surfactant (without using the second surfactant) Comparison with the particle size in the magnetic fluid composition obtained by the conventional method: ■ Magnetic fluid composition by conventional method Amount of 1111 agent bisynthetic sulfonic acid added: Coverage rate 100% Dispersion medium: Added amount of poly α olefin: 80 weight First, in the same manner as in Example 1, a magnetite colloid was obtained using synthetic sulfonic acid as a dispersant and hexane as a dispersion medium. Polyα-olefin was added as a dispersion medium, and hexane was removed by evaporation to produce a magnetic fluid composition.

A transmission electron microscopy (TEM) image of magnetite particles in the magnetic fluid composition is shown in FIG.

■ Magnetic fluid composition dispersant according to the production method of the present invention: 1st
Addition of N-acyl amino acid as a surfactant ■: Coverage rate 0% Addition amount of synthetic sulfonic acid as second surfactant: 30 parts by weight Dispersion: Addition amount of poly α-olefin 2 80 parts by weight Same as Example 2 By the above procedure, a magnetic fluid composition was produced in which the first surfactant was an N-acylamino acid, the second surfactant was a synthetic sulfonic acid, and the dispersion medium was a polyα-olefin.

A transmission electron microscopy (TEM) image of magnetite particles in the magnetic fluid composition is shown in FIG.

From FIG. 2 and FIG. 3, it can be seen that the magnetite particles in the magnetic fluid composition prepared by the manufacturing method of the present invention are more concentrated than the magnetite particles in the magnetic fluid composition prepared by the conventional manufacturing method. approaching a monodisperse system. It is also obvious that the particle size is small, and it can be said that the effect of particle size control is clearly demonstrated.

〔Effect of the invention〕

As explained above, according to the present invention, the amount of surfactant added in the first adsorption step is less than 100% coverage of the surface of the ferromagnetic particles, and the amount of surfactant added in the second adsorption step is reduced to less than 100%. It was decided that this would be done in phase. Therefore, it is possible to control the particle size of ferromagnetic particles. An extremely stable magnetic fluid composition that can easily improve 1 Jl property independently, does not contain relatively large particles that impair long-term stability, and has a surfactant firmly adsorbed. However, it has the advantage that it can be obtained using conventional manufacturing equipment.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the concentration of the first surfactant added to the ferromagnetic fine particles, the coverage rate, and the yield of lipophilic ferromagnetic fine particles, and Figure 2 is a graph showing the relationship between the concentration of the first surfactant added to the ferromagnetic fine particles, and the yield of lipophilic ferromagnetic fine particles. FIG. 3 is a transmission electron microscope view showing the particle structure of the ferromagnetic fine particles in the magnetic fluid composition. FIG. It is a field diagram of a type electron microscope.

Claims (1)

[Claims] (1) A first adsorption step in which an unsaturated amount of a first ionic surfactant and a low-boiling organic solvent are added to ferromagnetic fine particles to obtain a dispersion system in which the particle surface is incompletely covered with the surfactant. a step of separating ferromagnetic fine particles having a relatively large particle size from the dispersion system, and then removing a low boiling point organic solvent from the dispersion system to obtain lipophilic ferromagnetic fine particles; Water and a second ionic surfactant are added to the fine particles, and at least the surface of the ferromagnetic fine particles not covered with the first ionic surfactant is coated with the second ionic interface in the aqueous phase. A magnetic fluid composition comprising a second adsorption step of adsorbing an active agent, and a step of drying the ferromagnetic fine particles coated with the first and second surfactants and then dispersing them in a dispersion medium. Production method.