Low viscosity, electrically conductive ferrofluid composition and method of making and using same

A low viscosity, electrically conductive ferrofluid composition and method of making and using same, which composition comprises: a first polar liquid carrier; a second polar liquid carrier miscible in the first polar carrier and comprising a polyol, such as a tetraethylene glycol dimethyl ether; magnetic particles to impart magnetic characteristics to the ferrofluid composition; and a surfactant, particularly a cationic surfactant in an amount sufficient to stabilize and disperse the magnetic particles, the surfactant dissociated or ionized predominately by the second polar liquid carrier.

U.S. patent application Ser. No. 713,757 describes an electrically 
conductive ferrofluid composition, which ferrofluid composition comprises 
a liquid carrier having a colloidal dispersion of ferromagnetic particles 
in an amount sufficient to provide magnetic properties to the ferrofluid 
composition and carbon particles in an amount sufficient to provide 
electrically conductive properties to the ferrofluid composition. The 
ferromagnetic and carbon particles are stabilized in the ferrofluid 
composition by a surface active dispersing agent. The electrically 
conductive ferrofluid composition is usefully employed in a ferrofluid 
exclusion seal apparatus to provide an electrically conductive seal 
apparatus particularly useful for computer disk drives. 
U.S. patent application Ser. No. 736,388 discloses a stable ferrofluid 
composition and the method of preparing and using the ferrofluid 
composition, such as in a ferrofluid seal apparatus. The ferrofluid 
composition comprises: a liquid carrier; ferromagnetic particles 
sufficient to provide magnetic properties to the liquid carrier; and a 
dispersing amount of a cationic surfactant, such as a quaternary ammonium 
compound soluble in the carrier, to provide a stable ferrofluid 
composition. The stable ferrofluid compositions have improved electrical 
conductivity and are useful in sealing computer disk drives. 
U.S. patent application Ser. No. 773,627 describes a low viscosity, 
electrically conductive ferrofluid composition, which composition 
comprises a liquid carrier, and contains in combination an electrically 
conductive amount of a cationic surfactant and dispersed carbon particles, 
to obtain a low-electrical-resistivity ferrofluid having a low viscosity 
and suitable for use in a ferrofluid bearing apparatus. 
BACKGROUND OF THE INVENTION 
Ferrofluids or magnetic colloids are liquids with magnetic properties in 
which ferromagnetic materials are colloidally suspended. Such ferrofluids 
or magnetic liquids must show a high degree of stability (gravitational 
and magnetic field) in order to perform well in various commercial devices 
and be responsive to external magnetic fields. Generally a stable magnetic 
colloid or ferrofluid in a high magnetic field gradient require small 
ferromagnetic particles of generally less than 100 angstroms in diameter. 
The ferromagnetic particles are typically coated with one or several 
separate layers of surfactants to prevent agglomeration in any particular 
liquid carrier. 
Ferrofluids are widely known and used, and typical ferrofluid compositions 
are described, for example, in U.S. Pat. No. 3,700,595, issued Oct. 24, 
1972, wherein anionic surfactants, such as fatty acids, alcohols, amines 
or amids and other organic acids are employed as dispersing surface active 
agents; U.S. Pat. No. 3,764,504, issued Oct. 9, 1973, wherein aliphatic 
monocarboxylic acids are employed as dispersing agents; U.S. Pat. No. 
4,208,294, issued June 17, 1980, wherein a water based magnetic liquid is 
produced by the employment of C.sub.10 to C.sub.15 aliphatic 
monocarboxylic acids as acid dispersing agents; and U.S. Pat. No. 
4,430,239, issued Feb. 7, 1984, wherein a stable ferrofluid composition is 
provided employing a phosphoric acid ester of a long-chain alcohol as a 
surfactant. 
Various processes have been described for preparing magnetic colloids and 
ferrofluids, such as described more particularly in U.S. Pat. No. 
3,917,538, issued Nov. 4, 1975, which provides a process for preparing an 
irreversibly flocked magnetic particle through the use of different 
dispersing agents which includes a variety of nonionic and anionic 
surfactants, such as various petroleum sulfonates as the anionic 
surfactants and wherein the ferrofluids are prepared employing a grinding 
or ball mill technique; U.S. Pat. No. 4,019,994, issued Apr. 26, 1977, 
which employs a petroleum sulfonate with an aqueous carrier; U.S. Pat. No. 
4,356,098, issued Oct. 26, 1982, which describes ferrofluid compositions 
composed of a silicone-oil carrier and a dispersing amount of an anionic 
surfactant which forms a chemical bond with the surface of the magnetic 
particles as a tail group compatible or soluble in the silicone-oil 
carrier; and U.S. Pat. No. 4,485,024, issued Nov. 27, 1984, wherein a 
ferrofluid is produced through controlling the pH of the aqueous 
suspension of the ferromagnetic particles of an organic solvent together 
with surface active agents, such as fatty carboxylic acids. 
A properly stabilized ferrofluid composition typically undergoes 
practically no aging or separation, remains liquid in a magnetic field and 
after removing of the magnetic field shows no hysteresis. Such a 
stabilized ferrofluid exhibits stability by overcoming generally three 
principal attractive forces: van der Waals, interparticles-magnetic and 
gravitational forces. The average particle needed in a ferrofluid depends 
on the selection of the ferromagnetic materials and typically may range 
from 20 to 300 angstroms, for example 20 to 200 angstroms, and for use in 
a very high magnetic field gradient may range up to 100 angstroms in 
diameter. Typically, the ferromagnetic particles must be covered by one or 
more layers of the selected surfactant in order to provide stability in an 
external magnetic field gradient. While there are many known ways to 
obtain small particles of the ferrites, cobalts, irons and other 
ferromagnetic materials, the type of surfactant and dispersing agent 
needed to stabilize these particular particles is an important aspect of 
the formation of stable ferrofluid compositions and the method of 
preparing such compositions. 
The ferrofluid compositions have been used in a wide variety of commercial 
applications, such as for ferromagnetic seals, as dampening liquids in 
inertia dampers, as heat transfer liquids in the voice coil of 
loudspeakers, as bearing liquids, as ferrolubricants, for domain 
detection, for oil prospecting, and other applications. 
Electrically conductive ferrofluid compositions are usually employed in 
computer disk drive applications, for example to provide a conventional 
sealing ring, and further for the conduction of electrical charges from 
the shaft so as to prevent charge build up on the disk. In the computer 
industry, the static charge build up at the disk in a rotating spindle 
needs to be grounded in addition to sealing hermetically the disk cavity 
for contamination-free operation. Electrically conductive ferrofluids 
which contain finely divided dispersed carbon paticles are quite useful; 
however, there is a need to restrict the amount of carbon black employed 
in the ferrofluid compositions to avoid gradual increases in the viscosity 
of the composition and absorption of the fluid into the carbon particles 
with time. The addition of carbon black to a typical ferrofluid 
composition provides for a composition which tends to be pseudoplastic in 
amounts greater than about 5 percent of carbon black, while low 
concentrations of carbon black provide for a Newtonian composition. 
Therefore, it is desirable to provide for stable, low viscosity, highly 
electrically conductive ferrofluid compositions both with or without the 
use of carbon black, and particularly for use in the computer seal, as 
well as other devices wherein a stable, low viscosity, highly electrically 
conductive ferrofluid composition is useful. 
SUMMARY OF THE INVENTION 
The present invention concerns low viscosity, stable, electrically 
conductive ferrofluid compositions and the method of preparing and using 
such compositions, such as for example as a ferrolubricant and bearing 
apparatus and as a ferrofluid composition in seals. In particular, the 
invention relates to a ferrofluid composition which includes a 
dissociation or ionization liquid agent for the dissociation and 
ionization of the surfactant employed in the ferrofluid composition to 
disperse the magnetic particles. 
The electrically conductive, low viscosity, stable ferrofluid composition 
comprises: a first polar liquid carrier, typically a low viscosity, 
nonvolatile-type liquid carrier; an agent which is miscible or soluble in 
the polar liquid carrier; magnetic particles dispersed in the liquid 
carriers in a sufficient amount to provide for the desired magnetic 
properties of the ferrofluid composition; and a surfactant, such as, for 
example, but not limited to, a cationic surfactant which acts as a 
dispersing agent for the ferromagnetic particles or other particles in the 
ferrofluid composition and which also helps to provide for electrical 
conductivity properties in the composition. 
Importantly, the polar agent employed in the ferrofluid composition is 
selected to provide for dissociation or ionization of the surfactant. 
Typically the agent or liquid carrier comprises an oxygen-containing 
compound, such as for example, a polyol compound, such as a polyol ether 
and more particularly a glycol ether, so as to provide for dissociation of 
the surfactant. Optionally but preferably, the ferrofluid composition may 
include dispersed carbon particles in the ferrofluid composition, 
typically in an amount so as not to provide for a substantial increase in 
ferrofluid viscosity, since the addition of the carbon particles further 
increases the electrical conductivity of the composition. Thus, the 
ferrofluid composition may also include controlled amounts of dispersed 
carbon particles, such as finely divided carbon black, which in 
combination particularly with a cationic surfactant and the polyol ether 
provides for a very stable, but low electrical conductivity ferrofluid. 
When carbon particles are employed in the ferrofluid composition, it is 
essential to precoat the carbon particles with any of the constituents of 
the ferrofluid to avoid absorption and with a surfactant in order to 
provide for the proper dispersion of the carbon particles in the 
ferrofluid. 
The liquid carrier employed in the preparation of the ferrofluid 
composition may be any type of a liquid polar carrier and typically is a 
stable, nonvolatile liquid, for example but not limited to: fluorocarbons; 
amino-alcohol; polyphenyl ethers; polyglycols; amido-amines; esters; 
polyolesters; glycol esters; and various other esters, such as for 
example, and more particularly, a polyol ester oil liquid or a 
polyalphaolefin base. The polar liquid carrier, as well as the second 
polar liquid agent, employed may contain or have various additives, such 
as stabilizers, antioxidants, corrosion inhibitors, viscosity index 
additives, or various minor amounts of other additives to improve the 
selected quality or property of the ferrofluid composition. 
The magnetic particles employed in the ferrofluid compositions may be those 
magnetic-type of particles commonly employed in preparing ferrofluid 
compositions and typically comprise finely divided single domain particles 
of ferromagnetic materials, such as, but not to be limited to: magnetite; 
ferrites; iron; nickel; cobalt; metal carbides or metal oxides; metal 
alloys; and other finely divided materials which have or can be made to 
have magnetic properties when dispersed in a liquid carrier. One typical 
and preferred ferromagnetic particle comprises magnetite particles. The 
ferromagnetic particles employed are finely divided and are generally less 
than 1000 angstroms, but more typically less than 300 angstroms, such as 
for example about 20 to 200 angstroms in single domain particles. 
The ferromagnetic particles are dispersed and stabilized and used in 
combination with the employment of a dispersing agent or surfactant, 
typically any surfactant may be employed, such as a nonionic, cationic or 
anionic surfactant. However, it is desirable to employ a cationic 
surfactant in order to provide for increased or improved electrical 
conductivity. The amount and nature of the surfactant employed may vary 
depending on the particular liquid carriers used, the ferromagnetic 
particles and the size thereof, and the type and stability of the 
dispersion desired. 
Typically, the ratio of the surfactant as a dispersing agent to the 
ferromagnetic particles may vary, but generally in the ferrofluid 
composition ranges from about 0.5:1 to 20:1 by weight, with the 
ferromagnetic particles generally present in the composition in an amount 
ranging from 5 to about 80 percent by weight of the liquid carrier. The 
ferrofluid compositions prepared have varying saturization and 
magnetization values, and typically range from 20 to 800 gauss, for 
example 100 to 500 gauss, and range in viscosity from about 25 cp to 5000 
cp. 
Particularly preferred cationic surfactants comprise: the quaternary 
ammonium surfactant molecules, generally with two structural parts; a 
polar head group comprising the quaternary ammonium which attaches to the 
ferromagnetic particle surface or to a layer of another surfactant; and a 
tail group with properties similar to the surrounding fluid matrix or the 
polar carrier liquids, and typically being compatible with and soluble in 
the organic liquid carrier. Therefore, the selection of the particular 
tail group of the quaternized ammonium surfactant should be matched with 
the particular polar liquid carrier employed, for example a hydrocarbon 
tail group of a quaternized ammonium compound would typically be soluble 
in hydrocarbon oil or polyalphaolefin nonpolar-type organic liquid 
carriers, while tail groups having a hydroxyl or an oxygen-containing 
tail, e.g. OH groups, would be soluble and compatible in glycols, 
polyglycols, esters, esterglycols and the like. For example, a 
polyalkylene oxide, such as a polypropylene oxide tail group of a 
quaternized ammonium cationic surfactant, is typically soluble in a polar 
organic liquid carrier, such as esters, polyol esters, polyglycols or 
glycolesters. The employment of cationic surfactants as dispersing agents 
permits the dispersing of ferromagnetic particles in an entirely new 
family of organic liquid carriers or lubricants, such as polyol esters, 
glycols, silahydrocarbons and various other organic liquid carriers which 
may be used as oils, lubricants, bearing fluids and the like. Cationic 
surfactants which are insoluble in water also allow the possibility of 
making ferrofluid compositions of magnetic colloids compatible with water 
or alcohols. The ferrofluids employing cationic surfactants may be used in 
all known applications of ferrofluids, such as in sealing, dampening, 
lubrication, heat transfer, domain detection, bearing fluids, and in other 
applications. 
A wide variety of cationic surfactants may be employed as the dispersing 
agent of the invention where the tail portion of the surfactant molecule 
is soluble in or compatible with the carrier liquid, and in addition is of 
sufficient length in order to provide a stable ferrofluid composition, for 
example, typically the length of the molecular structure of the tail 
portion of the cationic surfactant should be more than about 20 angstroms 
to stabilize particles having a diameter of about 100 angstroms or more. 
The particularly preferred cationic surfactants of the invention comprise 
the quaternary ammonium cationic surfactant composed of a quaternary 
ammonium head group and a long tail portion, such as for example, ranging 
in each surfactant molecule from about 10 to 50 angstroms in overall 
length or having C.sub.6 -C.sub.30, e.g. C.sub.8 -C.sub.18, carbon atoms 
or length. 
Some quaternary ammonium cationic surfactants useful as dispersing agents 
in the ferrofluid composition would comprise, but not be limited to the 
following representative classes, such as: quaternary imidazoline salts 
which contain a heterocyclic ring which contains a quaternary ammonium 
group and includes a long chain, such as a fatty acid substituent group; 
aliphatic quaternary ammonium salts where at least one of the groups and 
often two of the groups through the quaternary ammonium and nitrogen 
comprises a long chain group, such as a fatty acid group; and quaternary 
acylated polyamine salts which contain, for example, a long chain alkoxy 
group such as an ethoxy or propoxy group, which compounds are particularly 
useful as dispersing agents where the liquid carrier comprises an 
oxygen-containing liquid, such as an ester, glycol or polyol ester. The 
cationic surfactant should be employed in an amount sufficient to provide 
for a stable dispersion, and may be used alone or in combination with 
other surfactants, such as nonionic surfactants (but not directly with 
anionic surfactants), and with other various additives or used in 
combination. 
The carbon particles employed in the ferrofluid composition, which is 
optional but preferred, can be derived from any carbon material; however, 
it has been found that the best electrical conductivity is obtained by 
dispersing electrically conductive carbon black particles having a 
particle size of about 50 to 300 angstroms and a surface area of about 100 
to 3000 meters per square gram, such carbon black particles are typically 
produced by various processes. The carbon black derived from the furnace 
process is particularly useful. 
The agent employed is typically a high molecular weight, high boiling 
point, for example of over 250.degree. C., low evaporation, 
oxygen-containing liquid which dissociates the surfactant or ionizes the 
surfactant. The surfactant selected which is dissociated may be one or 
more of the surfactants used in the ferrofluid composition and is present 
in excess quantity and should be dissociated by the polar agent, and more 
particularly dissociated or ionized by hydroxyl-type groups, or 
alkoxy-type groups, such as ethoxy and methoxy groups. The use of a polar 
agent which provides for such dissociation and ionization permits the 
preparation of low viscosity ferrofluid compositions having electrical 
resistivity of less than generally about 1.times.10.sup.7 and in 
combination with amounts of up to 5 percent of carbon particles with 
electrical conductivity of less than 1.times.10.sup.6 ohms-centimeter. The 
resistivity was found to depend on temperature more dramatically with 
carbon present than in the absence of carbon. 
The polar agent employed should have one, and preferably two or more, 
hydroxyls or ethoxy or methoxy groups, typically separated by hydrocarbon 
or other alkoxy groups and must be miscible with the first liquid polar 
carrier. The polar agent may comprise for instance a polyol, such as 
glycol, and more particularly polyethylene and polypropylene glycols. 
While ethylene glycol and propylene glycol may be suitable, particularly 
in small amounts, they are not generally preferred in that they evaporate 
at too low a temperature and therefore limit the life of the ferrofluid 
compositions. The low molecular weight glycols provide higher conductivity 
values for the ferrofluid compared with the higher molecular weight 
glycols. 
More particularly preferred as the polar agent are those alkyl ether 
compounds of polyalkylene polyols, and more particularly the lower alkyl 
ethers of such compounds wherein the polyalkylene comprises an ethylene or 
propylene. Particularly preferred compounds for use in the invention would 
include the di-, tri- and tetraethylene glycol dialkyl ethers and 
particularly those having a boiling point about 200.degree. C., and more 
particularly about 250.degree. C. Suitable specific compounds include, but 
not to be limited to: the diethylene glycol dibutyl ether; the 
tetraethylene glycol dimethyl ether, as well as triethylene glycol 
dimethyl ether and a lower boiling point diethylene glycol diethyl ether, 
as well as various low molecular weight liquid polyethylene and 
polypropylene glycol compounds, such as polyethylene glycol having 
molecular weights ranging from about 400-800. 
The ferrofluid composition may be prepared employing the usual and 
generally accepted techniques of ball milling and grinding, or 
precipitation as in the prior art, to prepare the ferrofluid compositions. 
The cationic surfactants may be used in conjunction with anionic and 
nonionic surfactants or may be used as one surfactant layer; while other 
surfactants, anionic, nonionic or other cationic surfactants, may be used 
in another layer or to complete the first layer. For example, the 
ferromagnetic particles may be dispersed first with an anionic surfactant, 
and then a separate surfactant to complete the layer or as a second layer 
of a cationic surfactant is employed, or vice versa, to provide stable 
ferrofluid compositions. The techniques for such multiple layer dispersing 
of magnetic particles is known in the art for anionic surfactants. Thus, 
the stable ferrofluid composition may comprise cationic surfactants 
together with other surfactants, typically a first dispersing layer of an 
anionic fatty acid surfactant followed by a final dispersing of the 
anionic surfactant magnetic particles with a cationic surfactant or other 
surfactant which can be dissociated. 
While not wishing to be better than any particular theory or method of 
operation, it is suspected that the employment of a second polar liquid 
which dissociates predominately the excess surfactant provides for the 
traveling of the electric charge by the dissociated surfactant between the 
colloidally dispersed carbon particles where the carbon particles are 
employed, and which carbon particles then act as small electrodes. The 
dispersion of the small electrode carbon particles in the ferrofluid make 
the connection between the rotating member, such as the shaft, and the 
stationary member such as the pole piece, to provide for electrical 
conductivity. If the carbon particles are individually colloidally 
suspended the resistivity will be high, while for example where the carbon 
particles are less than 100 angstroms; however, carbon particles may be 
employed in clusters or aggregates, for example over 1000 angstroms, in 
which case the resistivity will be low. Thus, the carbon particles may be 
employed in aggregate form and it is not needed to be broken up, or may be 
employed in individual particle broken up aggregates. 
Ferrofluid compositions of improved electrical conductivity may comprise, 
for example, general formulations as follows: 
(1) Ferromagnetic particles such as Fe.sub.3 O.sub.4, about 3-7 percent by 
volume, for example 5 percent; 
(2) Carbon black coated particles with any of the constituents of 
ferrofluid to minimize absorption from the furnace process, 100 angstroms 
or less, about 1-6 percent by volume, for example about 2.5 percent by 
volume; 
(3) First polar liquid carrier to make up the balance of the ferrofluid 
compositions; 
(4) polar liquid carrier, that is the dissociation or ionization agent, 
from about 3-20 percent by volume, more particularly 5-12 percent; 
(5) First surfactant, typically an anionic surfactant, such as oleic acid, 
for precoating the ferromagnetic particles in an amount of from 5-10 
percent by volume, for example 5 percent; 
(6) Second surfactant, preferably a cationic surfactant, such as a 
quaternary ammonium surfactant, and more particularly a quaternary 
ammonium alkoxy surfactant, from about 2-12 percent by volume, and 
typically 5 percent. 
The invention will be described for the purposes of illustration only in 
connection with certain embodiments; however, it is recognized that 
various changes, additions, modifications, and improvements in the 
invention may be made to these illustrative embodiments by those persons 
skilled in the art and all falling within the spirit and scope of the 
invention.

DESCRIPTION OF THE EMBODIMENTS 
Example 1 
The magnetite (Fe.sub.3 O.sub.4) particles were prepared using 240 grams of 
FeSO.sub.4, 425 ml of 45 percent FeCl.sub.3 and 600 ml water. The solid 
components were well dissolved in water at 5.degree. to 70.degree. C. 
under constant agitation. 
Fe.sub.3 O.sub.4 was precipitated from the above solution by the slow 
addition of ammonia solution 30 percent NH.sub.3 to reach the pH of 12. 
100 ml of soap is added under a strong agitation. The soap composition is 
oleic acid (or isosteric acid):ammonia (30 percent NH.sub.3) solution in 
volume ratio of 1:1. 
The suspension of magnetite particles covered with oleic acid is mixed for 
30 minutes at 50.degree.-90.degree. C., then 350 cc of an Isoparaffinic 
hydrocarbon fraction (ISO-G having a boiling point range of 160.degree. 
to 176.degree. C. of Exxon Chemical Co.) is added to the suspension under 
constant agitation. 
After 60 minutes of strong agitation, all the magnetite particles, well 
covered with oleic acid, are disperse in hydrocarbon fraction. The mixture 
was settled for 120 minutes. The upper layer containing the hydrocarbon 
base magnetic colloid was transferred into another beaker. The hydrocarbon 
base magnetic colloid 300 cc was mixed with 200 cc of acetone. The 
magnetite was allowed to settle and the supernatant liquid was siphoned 
off. The operation is repeated two more times in order to eliminate the 
excess of oleic acid. 
The acetone wet slurry was added in a beaker containing 500 cc of heptane. 
Under the strong agitation, the slurry was heated to 80.degree. C. in 
order to remove the acetone. 
The remaining fluid is 450 cc heptane based magnetic colloid having 
approximately 400 gauss saturation magnetization. 
A total of 120 grams of a polypropoxy quaternary ammonium acetate 
surfactant (EMCOL CC55 of WITCO Chemical Co.) was added to the heptane 
base magnetic colloid at 70.degree. C. under constant stirring. Prior to 
using EMCOL CC55, it was cleaned in a vacuum chamber by continuous pumping 
for 24 hours. By this process, residual solvents present in the surfactant 
were removed. The surfactant amount is sufficient to provide a stable 
colloid and to suspend subsequent addition of carbon particles and to 
provide the necessary ionization to electrical conductivity. After 30 
minutes of agitation at 70.degree. C., the adsorption of the second 
surfactant was considered complete. A total of 300 cc of liquid carrier of 
Polyolester (Mobil Ester P-42, having a pour point of -51.degree. C., 
flash point 243.degree. C. and viscosity index of 134) of Mobil Chemical 
Co. was added under the agitation to the heptane base magnetic colloid. 
The mixture was held at 100.degree. C. until all the heptane was removed. 
The ferrofluid was kept in high magnetic field gradient at 80.degree. C. 
for 24 hours in order to remove the large aggregates. The supernatant 
fluid was filtered and the final magnetic colloid was 350 gauss and 60 cp 
viscosity at 27.degree. C. 
Ten grams of Conductex 40-220 carbon black particles (Columbia Chemicals 
Company) are added to a 100 cc mixture of heptane and polyolester P-42 (30 
percent P-42 in heptane) and mixed thoroughly for 60 minutes. The heptane 
was removed at 100.degree. C. in 72 hours. The polyolester oil was 
absorbed on carbon black. The carbon black slurry was kept at 100.degree. 
C. in vacuum oven (30 inch vacuum) for another 24 hours at 100.degree. C. 
to thoroughly remove the heptane. The carbon particles may also be coated 
with a glycol the same way or with a cationic surfactant. 
The magnetic colloid having 350 gauss was diluted down to 250 gauss using a 
mixture of Tetraglyme, a tetraethylene glycol dimethyl ether, (boiling 
point 275.degree. C. of Ferro Corp.) and a polyol glycol (POLYOL P-425 of 
Dow Chemical Company). 
The mixture consisting of 50 percent glymes (organic compounds in the 
family of symmetrical glycol diethers) and 45 percent Polyglycol P-425 and 
5 percent of triethanolamine. This mixture acts as a strong polar solvent 
for the cationic surfactant CC55 in final electrically conductive 
ferrofluid. 
Forty grams of carbon black slurry (10 grams carbon black, 30 grams P-42, 
obtained as above) are added to 100 cc of magnetic colloid having 250 
gauss and mixed in a motor mill for about 30 minutes to assure a proper 
dispersion of carbon black in the magnetic colloid. The electrically 
conductive ferrofluid has finally about 200 gauss saturation magnetization 
and an electrical resistivity of about 10.sup.6 ohm-cm. The colloid shows 
a thixotropic behavior. 
The electrically conducting magnetic ferrofluid thus prepared was tested 
and had a 250 gauss saturation magnetization and an electrical resistivity 
of about 10.sup.7 ohm-cm. 
In the foregoing example, the carbon black particles optionally employed in 
the ferrofluid composition were precoated with a polyol ester oil. Where 
carbon black particles are not coated, the electrical conductivity is 
generally lower than that obtained in Example 1. The precoating of the 
carbon black particles has been found to be important in order to prevent 
gradual increases in viscosity in the ferrofluid composition. Thus, where 
carbon particles are employed, the carbon particles should be precoated 
with any of the constituents of ferrofluid and then introduced into the 
ferrofluid composition.