Synthesis of an Aqueous Ferrofluid
The California NanoSystems Institute & Materials Creation Training Program
University of California, Los Angeles
Science Outreach Program
Doris Chun, Steven Karlen, Chris Kolodziej, Bob Jost,
Shabnam Virji, Michelle Weinberger
Students prepare a ferrofluid – a liquid that contains small particles, approximately 10 nanometers in
diameter, that spontaneously magnetizes in the presence of a magnetic field – through solution chemistry materials.
Teacher Pre-Lab – 20 minutes
Prepare solutions of 2M FeCl, 1M FeCl, and 0.5M NHOH 234
Distribute Supplies to Work Areas
Student Procedure – 45 minutes
Synthesize magnetite nanoparticles from iron chloride and ammonia
Isolation of the magnetite nanoparticles
Stabilize the magnetite with tetramethylammonium hydroxide
Teacher Post-Lab – 10 minutes
Collect, neutralize, and dispose waste
Collect Supplies and Clean Up
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Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 1
California State Science Standards Grades 9-12
Addressed by the Solar Cell Experiment
1m. *Students know how to solve problems involving the forces between two electric charges at a
distance (Coulomb's law) or the forces between two masses at a distance (universal gravitation). 5j. *Students know electric and magnetic fields contain energy and act as vector force fields. 5f. Students know magnetic materials and electric currents (moving electric charges) are sources of
magnetic fields and are subject to forces arising from the magnetic fields of other sources.
Ferromagnetism is the permanent magnetic dipole that results from the alignment of unpaired electron spins in elements such as iron, cobalt, nickel, etc. In this experiment, students will experiment with a fluid they create that exhibits ferromagnetism. They will synthesize magnetic nanoparticles from iron chlorides and disperse them into a tetramethylammonium hydroxide surfactant to form a colloidal suspension. They will then study the behavior of this ferrofluid in the presence of an external magnetic field.
Discussion questions 3, 4, and 5 relate to the concept of
Coulomb’s Law, which describes the magnitude of
electrostatic force, repulsion or attraction, between two
charged particles at a finite distance. The tetramethyl-
ammonium hydroxide surfactant used in this experiment is + -composed of two charged species, (CH)NandOH. The 34
hydroxide anions adhere to the surface of magnetite
particles, and these negative charges attract their counter
ions, tetramethylammonium cations, forming a positively
charged outer shell. Since like charges repel, the
electrostatic repulsion between positively charged outer
shells prevent magnetite particles from agglomerating. This
results in a colloidal suspension of magnetite nanoparticles,
which is what we called a ferrofluid. Discussion question 6
deals with magnetic fields and vector force fields.
Ferromagnetic materials respond to external magnetic
fields by aligning their unpaired electron spins with the
vector fields. When a magnet is far away from the solution,
no external vector fields interact with the ferrofluid, thus
there is nothing interesting to see except a black solution.
When a magnet is brought closer to the solution, the
magnetic force is large enough to dominate the forces of
surface tension and gravity, the ferrofluid forms spikes in the
direction of the magnetic field lines. The stronger the
vector field lines, the larger the spikes.
Nano particle picture from: Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. J. of Chemical Education 1999, 76, 943-8.
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 2
2f. *Students know how to predict the shape of simple molecules and their polarity from Lewis dot
2h. *Students know how to identify solids and liquids held together by van der Waals forces or hydrogen
bonding and relate these forces to volatility and boiling/ melting point temperatures. 5a. Students know the observable properties of acids, bases, and salt solutions.
6a. Students know the definitions of solute and solvent.
6b. Students know how to describe the dissolving process at the molecular level by using the concept
of random molecular motion.
6d. Students know how to calculate the concentration of a solute in terms of grams per liter, molarity,
parts per million, and percent composition.
The formation of ferrofluid involves various types of forces that hold the different components together. On the molecular level, magnetite (FeO) is held together by ionic interactions in the crystal lattice, while 34
tetramethylammonium and hydroxide are covalent molecules held together by ionic interactions. Ionic attractions between hydroxide anions and tetramethylammonium cations enable the coating of magnetite nanoparticles, while electrostatic interparticle repulsions among tetramethylammonium cations allow colloidal suspension of the magnitite in solution. Without tetramethylammonium hydroxide as a surfactant, magnetite nanoparticles tend to aggregate due to van der Waals forces. Therefore, it is critical to have the appropriate surfactant to stabilize an aqueous ferrofluid. In this experiment, students will learn that these forces are responsible for the formation of ferrofluid.
Discussion questions 7, 8, and 9 deal with basic quantitative chemistry in which students practice balancing equations, determining oxidation state in metals, and calculate solution concentrations. Students can also practice writing out the Lewis dot structures of chemicals used in this experiment to + –identify their charges, for example (CH)NandOH. Discussion question 10 deals with the packing and 34
layer sequence of magnetite in the crystal lattice.
Investigation and Experimentation
1b. Identify and communicate sources of unavoidable experimental error.
1c. Identify possible reasons for inconsistent results, such as sources of error or uncontrolled conditions. 1d. Formulate explanations by using logic and evidence.
Discussion questions 1 and 2 address the importance of adding ammonium hydroxide at a slow rate in the early stage of the experiment. In order for magnetite particles to remain in suspension their diameters must be on the order of 10nm (100Å) or less. By adding ammonium hydroxide slowly, one can ensure nanoscale particle formation. If ammonium hydroxide was added too quickly, large chunks of magnetite will form instead of the desired nanoparticles, consequently the experiment will fail. Toward the end of the synthesis, it is important to decant excess liquid out of the ferrofluid such that it has the right viscosity to form spikes in response to a nearby magnetic field. If the ferrofluid has too much excess liquid in it, it will not form spikes. Experiment with the ferrofluid by placing the magnet under the solution. If no spikes form, continue decanting until the right viscosity is achieved.
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 3
*****Tip for Teachers*****
Read the entire teachers manual before you begin this experiment with your students! There are a
number of ways in which students may be assessed on this experiment. You may choose to assign some
of the discussion questions from the student manual for credit, you may ask the students to keep a lab
notebook, or you may ask the students to prepare a lab report.
Ferrofluid Supplies List
Reusable Supplies Included in Kit:
; 40 Safety Glasses
; 250 mL Plastic Bottle
; 125 mL Plastic Bottle
; 2 L Plastic Jug
; 100 mL Graduated Cylinder
; 9 (10 mL) Graduated Cylinders
; ~70 (150 mL) Plastic Beakers
; ~300 Large Weigh Boats
; ~100 Pipettes
; 20 Magnets
; 1 Stainless Steel Scoopula Spatula
; 3 pk Gloves
Consumable Supplies Included in Kit: (Reorder requests: http://voh.chem.ucla.edu/outreach.php3)
; 500g FeCl•6HO (Ferric Chloride) 32
; 200g FeCl•4HO (Ferrous Chloride) 22
; 500 mL Ammonium Hydroxide (29%)
; 220 mL Tetramethylammonium Hydroxide
; 500 g Citric Acid
; pH paper
Supplies to be Obtained by Teacher:
; Distilled Water
Each Group of 2-5 Students Will Need:
; 3 Plastic Beakers (150 mL)
; 1 Pasteur Pipette
; 1 Large Weigh Boat
; 1 Magnet
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 4
Teacher Pre-Lab – 20 Minutes
Prepare the FeCl, FeCl, and NHOH Solutions 234
The empty 250 mL bottle will be used for the 1M FeCl solution. The empty 125 mL bottle will be used for 3
the 2M FeCl solution. To prevent oxidation of FeCl, minimize exposure of FeCl solid and solution to air by 222
keeping bottles capped when not in use. The 2L jug will be used for the 0.5M ammonium hydroxide solution. Two liters is enough for 40 experiments. If over 40 experiments are needed, use a larger container if available, or split the solution into 2 batches: preparing the second after the first is consumed. The experiment should be done with 2-5 students per experiment. The teacher will determine how many experiments are required given class sizes, student attention, and time. For solution prep calculations, adding excess experiments (~10%) to the minimum # of experiments required would be a good idea, allowing for spills/ accidental overuse of certain reagents.
1M FeCl: 1.0813 g of FeCl•6HO is required for each experiment. Therefore: 332
# of Experiments X 1.0813 = Grams FeCl•6HO Required for Teacher to Measure 32
FeCl•6HO solution requires 4 mL of water per experiment. Therefore: 32
# of Experiments X 4 = mL Water Required for the FeCl Solution 3
2M FeCl: 0.39762 g of FeCl•4HO is required for each experiment. Therefore: 222
# of Experiments X 0.39762 = Grams FeCl•4HO Required for Teacher to Measure 22
FeCl•4HO solution requires 1 mL of water per experiment. Therefore: 22
# of Experiments X 1 = mL Water Required for FeCl Solution 2
0.5M Ammonium Hydroxide: 1.6667 mL of concentrated (29%) Ammonium Hydroxide is required for each experiment. Therefore:
# of Experiments X 1.6667 = mL Concentrated Ammonium Hydroxide Required
for Teacher to Measure
0.5M Ammonium Hydroxide requires dilution to 50 mL per experiment. Therefore:
# of Experiments X 50 = Total Volume to Dilute Concentrated Ammonium
Hydroxide (Affords 0.5M Solution)
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 5
Distribute the Supplies
Each Group of 2-5 Students Should Have:
; 2 Graduated Cylinders (10 mL)
; 3 Plastic Beakers (150 mL)
; 1 Pasteur Pipette
; 1 Large Weigh Boat
; 1 Magnet
Set Up FeCl Station With: 3
; 250 mL Bottle of 1 M FeCl 3
; Several Plastic Pipettes
; 4 Graduated Cylinders (10 mL)
Set Up FeCl Station With: 2
; 125 mL Bottle of 2 M FeCl 2
; Several Plastic Pipettes
; 4 Graduated Cylinders (10 mL)
Set Up NHOH Station With: 4
; 2 L Jug of 0.5 M NHOH 4
(If time is an issue, this station can be pre-poured 150 mL beakers of 50 mL 0.5 NHOH) 4
Set Up Rinse Water Station With:
; Distilled Water (NOT Tap Water)
(If time is an issue, this station can be pre-poured 150 mL beakers of 50 mL HO) 2
Set Up (CH)NOH Station With: 44
; 250 mL Bottle of tetramethylammonium hydroxide
; Several Plastic Pipettes [only use these pipettes with (CH)NOH] 44
*****Tip for Teachers*****
When preparing the FeCl, FeCl, and NHOH solutions we recommend preparing an excess 10%, this will 234
allow for any spills or accidental overuse of the reagents that might occur. The four solution stations
should be in different parts of the classroom, this will help to keep a flow to and from the reagents to a
minimum. Also, if time is an issue, pre-pour the NHOH and distilled water into beakers for your students. 4
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 6
Student Procedure – 45 Minutes
Note: Procedure contains more detail and advanced terminology than the student manual
Preparation of the Ferrofluid A
1. Add 4 mL of the FeCl solution (0.004 mol) and 1 mL of the FeCl 32
solution (0.002 mol) to a 150 mL beaker.
2. While swirling the iron chloride solution, slowly add 50 mL of 0.5
M ammonium hydroxide dropwise over 5 minutes. Picture A It is
important that the ammonium hydroxide is added dropwise,
especially at the beginning. The students can add the ammonium
hydroxide more quickly at the end (the last 10-20 mL) if they are
short on time.
3. A black precipitate should form during the slow addition. This is
magnetite. Picture B The students should see this precipitate
form with the first drops of ammonium hydroxide, however as
long as the students add the ammonia slowly at first these will be small particle that will dissolve back into solution.
4. After all the ammonium hydroxide has been added, stop swirling. 5. Place one of the bar magnets under the beaker. It should pull all B of the magnetite out of the solution, and the water should
become clear. Picture C
6. Keeping the magnet on the bottom of the beaker, pour off the
excess water. This technique is called decanting. If the magnet
is removed then the particles will pour off into the waste
7. Add a minimal amount of water and transfer the magnetite to a weigh boat. Students may want to use a second portion of water to help transfer all the particles to the weighboat. 8. Place the magnet under the weigh boat to settle the magnetite.
C 9. Pour off the excess water.
10. Rinse the magnetite two more times by adding a small amount of
water, using the magnet to settle the magnetite, and discarding
the clear water. These rinsings remove the excess ammonium
hydroxide from the particles.
11. Remove as much water as necessary to form a viscous fluid. Be
careful NOT to remove all of the water, or you will form a solid.
The sample has the correct consistancy when there is no flow of
nanoparticles when the weigh boat is turned sidways and the
magnet is removed. It is important to achieve this consistancy
before stabilizing the ferrofluid. Once stabilized the magnetite
nanoparticles become water soluble and will be lost when
removing off excess water.
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 7
12. Add 1 mL of the 25% tetramethylammonium hydroxide solution, D and mix the ferrofluid for 2 minutes by moving the weigh boat over the magnet.
13. Once thoroughly mixed, place the magnet under the weigh boat
and remove the excess black liquid into an empty beaker, as you
did before during the rinsing. We recommend having the
students use the empty rinse water beaker just incase they pour
off too much of the ferrofluid.
14. Place the magnet under the ferrofluid and move it until you see
spiking. Picture D You may want to ask your students to bring
magnets from home for this experiment. Different magnets will
have different field lines.
NOTE: The ferrofluid is extremely difficult/impossible to remove from clothing. It is also difficult to remove from
magnets. Students should take care to avoid direct contact of the ferrofluid with clothing and magnets.
Teacher Post-Lab – 10 Minutes
Collect, Neutralize, and Dispose of Waste
Collect the waste from all the experiments in a large container and neutralize the base with citric acid. Once the pH is between 6 and 10 you can pour waste down the drain followed by plenty of water (to ensure that all suspended solids are flushed down the drain). The pH can be measured using the pH paper provided in the kit, there is a color scale on the side of the pH paper container.
Suggested Topics for Discussion
1. What is the molarity of the FeCl and FeCl solutions? 32
1M for FeCl and 2M for FeCl. 32
2. Why do you think slow (dropwise) addition of ammonium hydroxide is important? What might happen if you add ammonium hydroxide quickly?
The ammonium hydroxide solution is added slowly to ensure nanoscale particle formation, rather
than formation of large chunks of magnetite. Thus, if it is added quickly, large chunks of
magnetite will form instead of the desired nanoparticles.
3. Magnetite, FeO, consists of iron in what oxidation states? 34
Oxidation states of iron are +2 and +3.
4. . Why do you place a magnet underneath the beaker while removing water?
In the presence of a magnetic field (i.e. a magnet) the magnetite is magnetic. By placing a
magnet underneath the beaker, the magnetite is attracted to magnet and product loss is
minimized while the water is removed.
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 8
5. What is the purpose of the stabilizing agent tetramethylammonium hydroxide? What might happen if NO stabilizing agent is used?
Tetramethylammonium hydroxide is a stabilizing ligand that is used to keep the nanoparticles in
solution and from sticking to each other. That is, it adheres to the particles creating a net
repulsion between them so the particles do not agglomerate. In the absence of a stabilizing
agent the particles will agglomerate. These conglomerates will then precipitate from the solution
as a black solid.
6. Describe what happens when a magnet is brought near a ferrofluid. What happens when the magnet
is removed from the ferrofluid?
When the magnet is far away from the solution there is nothing interesting to see except a black
solution. When the magnet is brought closer to the solution then you see spikes corresponding to
the magnetic field lines. The stronger the field lines, the lager the ferrofluid spikes along the line.
9. ADVANCED: Balance the following equation.
2FeCl + FeCl + 8NH + 4HO FeO + 8NHCl 3232344
210. Chemistry classes may wish to discuss the crystal packing of magnetite, shown below.
This is one of the crystallographic planes of the magnetite crystal lattice. +3+2-2 In this diagram there is a ratio of 2 Fe : 1 Fe : 4 O
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 9
Topics for Advanced Students
What is a Ferrofluid?
A ferrofluid is a collection of superparamagnetic nanoparticles that are suspended in a liquid. These nanoparticles are approximately 10 nm in diameter. The majority of nanoparticles, like the ones you make in this lab, are iron-based, such as magnetite (FeO). If as-synthesized nanoparticles are put 34
into solution, they aggregate and form clusters due to van der Waal interactions. These clusters are too large to be kept in suspension by Brownian motion and settle to the bottom of the container. The use of a magnet can accelerate the settling of these clusters allowing for easy washing of the particles.
Nanoparticles will remain suspended in a solution as long as they do not aggregate. A technique to prevent aggregation is to ‘stabilize’ the particles by encapsulating them with an outer shell. The general
method to achieve this is to use a surfactant. Surfactants are molecules with two contrasting properties. They can be a linear molecule with a hydrophilic region and a hydrophobic region, or a cation anion pair. The former works by having the hydrophilic end of the molecules attach to the magnetite nanoparticles positioning the hydrophobic end to form a ‘greasy’ layer around the particle. This ‘greasy’ layer prevents nanoparticles from getting close enough to each other to aggregate. The later is the method used in this experiment. The ion pair keeps particles separated through Coulombic repulsion by encapsulating the particles with a cationic outter shell. The anions adhere to the surface of the magnetite nanoparticles, and they attract its counter cations to form the positively charged outer shell. Since like charges repel, the positively charged outer shell prevent magnetite nanoparticles from aggregating.
Applications of Ferrofluids
There are many applications of ferrofluids. Most applications are based on these properties:
1) The ferrorfluid will go to where the strongest magnetic field is and stay there (this is called
2) Ferrofluids absorb electromagnetic energy at convenient frequencies and heat up.
3) The physical properties of ferrofluids change when a magnetic field is applied.
Teacher Manual Last Updated: 5/23/12 UCLA—CNSI 10