Materials Science
Synthesis of an Aqueous Ferrofluid
Ferrofluids are materials with the fluid properties of liquids and the magnetic properties of a solid.
They contain tiny particles (~10 nm in diameter) of a magnetic solid suspended in a liquid
medium. These particles must be small in order to remain suspended in the liquid medium. The
most effective ferrofluids are made from particles of magnetite, Fe 3O4. Magnetite particles are
made by combining Fe3+ and Fe2+ salts in a basic solution:
2 FeCl3 + FeCl2 + 8 NH3 + 4 H2O → Fe3O4 + 8 NH4Cl
The slow addition of NH3 leads to the formation of nano-sized particles. However, these
magnetite particles are attracted to each other by London dispersion forces. If left untreated, they
will agglomerate into larger particles and precipitate from the liquid medium. In order to keep the
magnetite particles separated, they are coated in a surfactant such as tetramethylammonium
hydroxide (Fig. 1).
Fig. 1
The hydroxide ions in this surfactant are attracted to the surface of each magnetite particle,
forming a negatively charged layer at the magnetite surface. The tetramethylammonium cations
are attracted to this negatively charged layer, forming a positive layer. When magnetite particles
approach each other the repulsions between their positively-charged layers keeps them from
getting too close (Fig. 2).
N(CH3)4+
Fe3O4
Fe3O4
OHrepulsion between (+) charges
keeps Fe3O4 particles separated
Materials
1.0 M FeCl3 in 2 M HCl
2.0 M FeCl2 in 2 M HCl
1.0 M NH3
25% tetramethylammonium hydroxide in water
deionized water in a wash bottle
100 or 150 mL beaker
Fig. 2
stir plate and magnetic stirbar
buret, buret clamp, and ring stand
plastic weighing boat
strong craft magnet
glass stirring rod
10 mL graduated cylinder
Reference: Adapted from: http://mrsec.wisc.edu/Edetc/nanolab/ffexp/index.html
Procedure
1. Add 4.0 mL of 1.0 M FeCl3 and 1.0 mL of 2.0 M FeCl2 solution to a 100 or 150 mL beaker.
Add a magnetic stirring bar and begin stirring on a stir plate.
2. While stirring, SLOWLY add 50 mL of 1.0 M aqueous NH3 solution by dripping it from a
buret. This addition should last about 5 minutes. After an initial brown precipitate, a black
precipitate will form (magnetite).
3. Remove the beaker from the stir plate. Immediately use a strong magnet on the OUTSIDE of
the beaker to work the stir bar up the walls of the beaker. Remove the stir bar with tongs or a
gloved hand BEFORE it touches the magnet. Rinse off the stir bar with water into a waste
container, and clean it with a paper towel.
4. Let the magnetite in the beaker settle. You can speed the settling process by putting a magnet
under the beaker.
5. Decant (pour off) and discard the clear liquid from the beaker without losing a substantial
amount of solid. This works best if you keep a magnet under the beaker while you pour.
6. Transfer the solid to a plastic weighing boat with the aid of a few squirts of water from a wash
bottle.
7. Put a strong magnet under the weighing boat to attract the ferrofluid to the bottom of the
weighing boat.
8. Pour off and discard as much clear liquid as possible, again keeping the magnet under the
weighing boat. Rinse with water from a wash bottle and decant the rinse as before.
9. Use a 10-mL graduated cylinder to add 1-2 mL of 25% tetramethylammonium hydroxide.
Gently stir with a glass rod for at least a minute to suspend the solid in the liquid. Use a strong
magnet to attract the ferrofluid to the bottom of the weighing boat. Pour off and discard the dark
liquid. Move the strong magnet around and again pour off any liquid. If the ferrofluid does not
spike, continue to move the strong magnet around, pouring off any liquid.
10. What happens when you move a magnet under the ferrofluid? If you are using a very strong
magnet you might get more interesting results by varying the distance of the magnet below
the boat.
Cleanup: Rinse the weighing tray off with water (into a waste container) and dispose of it in the
trash can. Pour all aqueous waste into the “metal cation waste” container.
Conclusion Questions (answer in your lab notebook)
1. Were you able to prepare ferrofluid? Describe what you observed.
2. How many spikes were immediately around the central spike and how were they arranged?
3. Examine the starting FeCl2 and FeCl3 solids used to prepare magnetite (leave the solids in the
sealed zip-loc bags.) How do they respond to a magnet?
4. What is the purpose of the surfactant? Explain chemically how the surfactant works to keep the
nanoparticles suspended.
5. Why is it important that the magnetite particles be nano-sized?