Complex Materials Discovery
Portfolio Partnership
PRACTICAL OUTREACH
ACTIVITIES FOR SCHOOLS
Preparation of a ferrofluid
A ferrofluid is a suspension of
ferromagnetic nanoparticles in
a liquid which may be an
organic solvent or water.
Ferrofluids
have
many
technological applications e.g.
to form liquid seals around
spinning drive shafts in hard
drives, as a radar absorbent
paint to make stealth bombers
invisible to radar, as contrast
agents in magnetic resonance
imaging (MRI). They also show some fascinating behaviour in the presence
of a strong magnetic field. In this practical exercise, students will use
aqueous solutions of FeCl2, FeCl3, dilute aqueous ammonia, and a surfactant
to prepare a ferrofluid. This is an optimized procedure based on published
methods.
Suitability
Years 11-13
[Year 11 students will not be familiar with
the details of transition metal chemistry]
Time
Facilities
Specialized equipment
Ferrofluids
Approx 1 hour
Standard lab bench
Rare earth disc magnets (20 mm x 5
mm)
Safety
SAFETY
These procedures are intended for use only by persons with prior training in
the field of chemistry. In the checking and editing of these procedures, every
effort has been made to identify potentially hazardous steps and to eliminate
as much as possible the handling of potentially dangerous materials; safety
precautions have been inserted where appropriate. If performed with the
materials and equipment specified, in careful accordance with the instructions
and methods in this text, we believe the procedures to be very useful tools.
However, these procedures must be conducted at one's own risk.
• Wear your safety glasses and lab coats (fastened) at all times in the
lab, even when you are just writing or reading
• No eating or drinking in the lab
• Always follow instructions from your demonstrator
• Report any spillages or breakages to your demonstrator
• If the fire alarm sounds, stop what you are doing and follow your
demonstrator out of the lab
• Wash your hands on leaving the lab
• Be extremely careful with rare earth magnets. These magnets are very
powerful and can accelerate a great speeds toward each other and toward
ferrous material. When these magnets come together quickly, they can
shatter and break sending particles at high speed. These magnets can also
pinch strongly if allowed to come together against the skin. You should
always wear eye protection when handling strong rare-earth magnets.
COSHH Assessment
Chemical
FeCl2 solution
FeCl3 solution
Tetramethylammonium
hydroxide solution
Dilute NH3 solution
Ferrofluids
Hazard
Irritant
Irritant
Toxic – absorbed
through skin
Corrosive
Irritant
Assessment
Standard
standard
Wear gloves
standard
Equipment
Equipment and chemicals
Solutions (for 50 students; allowing for some repeat experiments)
FeCl2 2M
FeCl3 1M
NH3 0.5M
[Me4N][OH], 25% in water
100 cm3
500 cm3
5 litres
100 cm3
Equipment (per student)
Item
Clamp and clamp stand
50 cm3 burette
250 cm3 conical flask
250 cm3 beaker
Rare earth disc magnet (20 mm x 5 mm)
Pasteur pipette
Wash bottle with distilled H2O
Glass stirring rod
Sample vial
Cotton wool or filter paper
Disposable vinyl gloves
Quantity
1
1
1
1
1
1
1
1
1
1 pair
FeCl2 solution, 2M (must be made fresh)
FeCl2.4H2O (39.8 g)
Dissolve in distilled H2O and make up to 100 cm3
Provide one container of solution and 1 x graduated pipette & filler per 4 students
FeCl3 solution, 1M (must be made fresh)
FeCl3.6H2O (135 g)
Dissolve in distilled H2O and make up to 500 cm3
Provide one container of solution and 1 x 1 graduated pipette & filler per 4 students
Ammonia, 0.5M
Conc NH3 (167.5 cm3)
Dilute to 5 litres with distilled H2O
[Me4N][OH], 25% in water (best used fresh)
Solution purchased ready made up
Provide one container of solution and 1 x graduated plastic Pasteur pipette per 4 students
Ferrofluids
Background
Background to the synthesis of ferrofuids
If a relatively small number of atoms are stuck together, to form a particle less than 100
nm (1 nm =10-9 m) across, we get a nanoparticle.
Ferrofluids are interesting materials which consist of ferromagnetic nanoparticles
suspended in a liquid e.g. water. They were originally developed by scientists at NASA
(the American space exploration agency), who were looking for a way to control the flow of
liquids in space. They discovered that nanoparticles of magnetic metal oxide could be
dispersed in oil or water and that the liquid could then be moved by a magnet. The
technology was never used but it led to the development of what we now know as
ferrofluids.
The fluid is not magnetic itself but when it is subjected to a magnetic field the particles
align along the field lines of force and produce an attractive ‘hedgehog’ pattern. Since the
liquid can be controlled by a magnetic field this property has led to important uses in a
wide variety of applications.
Ferrofluids can be used for lots of different things such as damping in loud speakers and
radar absorbent material used to paint stealth planes to make them nearly invisible to
radar. They are also used as magnetic inks in printing US paper currency.
If we try to make a ferrofluid using iron filings in water it doesn’t work. The iron filings settle
out of the solution as a precipitate and the water is left clear. The iron filings are too large
and dense to be suspended in water.
Ferrofluids are made by the suspension of nanoparticles of an iron containing compound,
Fe3O4 or magnetite. The magnetite is synthesised from the reaction of aqueous iron (II)
and iron (III) salts with an alkaline solution. These iron salt solutions are acidic and are
therefore neutralised upon addition of the alkali solution.
In the synthesis of ferrofluids, ammonium hydroxide is added to neutralise the iron (II) and
iron (III) salts, in the process forming the desired iron oxide (Fe3O4, magnetite) with a byproduct of ammonium salt.
To make the ferrofluids behave as a ‘fluid’, an extra compound is added to the water so
that the magnetic particles (Fe3O4 in this case) do not settle or clump together (since they
are attracted to each other via strong ionic inter-molecular forces) and stay in the water which makes it black. The name of the group of ‘extra compounds’ used to do this is
‘surfactants’.
The word surfactant is derived from surface acting agent. The surfactant molecule has
one part which is very soluble in water (hydrophilic) and one part that is hydrophobic and
very soluble in non-polar solvents such as hydrocarbons. The surfactant that you will use
in today's experiment is tetramethylammonium hydroxide ([N(CH3)4]+[OH]-). In water, it
dissociates to form OH- anions and [N(CH3)4]+ cations. The OH- ions are attracted to the
surface of the magnetite nanoparticles, giving them an overall negative charge. The
tetramethylammonium cations are then attracted to these negatively charged
nanoparticles and form a protective positively charged shell around them as shown in the
diagram below.
Ferrofluids
Background
OH-
OH-
OH-
Fe3O4
-
OH
OH
=
CH3
H3C N
CH3
H3C
OH-
-
OH
-
OH-
This enables the particles to stay apart from each other, so that they don’t clump together
and don’t fall out of suspension as a precipitate.
These structures in solution resemble the micelles formed by soaps and detergents, which
are also surfactants.
In the case of soaps, the non-polar tails (four CH3 groups for the ferrofluid surfactant) are
directed towards the interior, where they surround water-insoluble oils or fats, while the
polar ends (N+ and -O for the ferrofluid surfactant) are attracted to the water molecules by
ion-dipole forces.
Ferrofluids
Background
Ferrofluid Images
Here are a few images of the ferrofluid of the form you produced, synthesised by two
Chemistry undergraduates and imaged with the University of Liverpool's Transmission
Electron Microscope (TEM), operated by Dr Calum Dickinson.
An individual nanoparticle of
magnetite (approx 5 nm across
A collection of nanoparticles
A sample of ferrofluid at lower
magnification. The length scale
is now 500 nm (0.5 mm)
Ferrofluids
Method
Be extremely careful with rare earth magnets. These magnets are very powerful and can
accelerate a great speeds toward each other and toward ferrous material. When these
magnets come together quickly, they can shatter and break sending particles at high
speed. These magnets can also pinch strongly if allowed to come together against the
skin. You should always wear eye protection when handling strong rare-earth magnets.
1.
To a 250 cm3 conical flask add 4 cm3 of the 1 M FeCl3 solution and 1cm3 of the 2 M
FeCl2 solution. Use the two graduated pipettes to measure out these solutions.
Do not confuse the pipettes or cross-contamination can occur.
2.
Fill the burette with 50cm3 of 0.5 M ammonium hydroxide.
3.
Add to the conical flask 50 cm3 of the 0.5 M ammonia solution dropwise over 5
minutes with constant swirling of the conical flask to ensure that the two solutions
mix. You should try to add at a rate of 10 cm3 a minute or 1 cm3 every six seconds.
Upon addition of ammonia solution initially a brown precipitate will form, followed by
the formation of a black precipitate (Magnetite, Fe3O4).
4.
Hold a magnet underneath the conical flask. The magnetite will separate out of the
solution and settle on the bottom of the flask. The solution should become clear; this
may take a minute. The remaining clear liquid can now be poured off, using the
magnet to keep the black precipitate in place ensuring no loss of magnetite.
5.
Now pour approx. 20 cm3 of distilled water into the flask to wash the black precipitate.
Again let the magnetite particles attract to the magnet on the bottom of the flask and
pour off the clear liquid. Repeat this process twice.
6.
Now pour approx. 7 cm3 of distilled water into the flask to make a black suspension.
Transfer the magnetite suspension to a sample vial.
7.
Remove the water as in steps 5 and 6, using the magnet to prevent loss of magnetite.
8.
Any waste up to this point can be disposed of down the sink.
9.
Add 1 cm3 of tetra-methyl ammonium hydroxide (25% solution) to the sample vial
(you can use a plastic pipette for this) and stir well by moving the magnet underneath
the vial for up to 10 minutes. Use the Pasteur pipette to mix as well to ensure good
mixing.
10. Remove any excess liquid from the sample vial using the Pasteur pipette (this time it
will be black), using the magnet to prevent loss of the ferrofluid. Holding the magnet
firmly in place on the sample vial to attract the black liquid/solid; drain off any thin
brown/black liquid.
11. Dab the black liquid with some cotton wool or filter paper to get rid of the excess
black liquid
12. Move the magnet around underneath the sample vial, removing any further excess
liquid until spikes appear in the ferrofluid.
Ferrofluids
Demonstrator notes
Hints & tips on the practical aspects
! Many students are unfamiliar with the equipment, contained within their
experimental booklet is an equipment guide which should help, but be wary of
exotic burette and pipette filling techniques.
! Many of the students fail to realise the importance of the rate of addition of the
ammonia solution. It is worthwhile to tell them they need to add at a rate of 10 cm3 a
minute or 1 cm3 every six seconds.
! Some students produce a brown solution after addition of the ammonia solution to
the mixed iron chloride solutions. This is a good indication the experiment has
failed, it is often due to incorrect measuring of the aliquots of the iron solutions, or
cross contamination of the stock solutions with the pipettes. Unfortunately they
must start again.
! Students must be exceptionally cautious when dabbing off the excess tetramethyl
ammonium hydroxide solution from the ferrofluid. The best way is to draw the
ferrofluid onto the side of the vial with the magnet and tilt the vial away from the
magnet, this should leave a pool of tetramethyl ammonium that can be carefully
dabbed off. If they are not careful they will draw up all the ferrofluid into the cotton
wool.
Ferrofluids
Helen Aspinall 2012
http://creativecommons.org/licenses/by-nc-sa/3.0/