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Synthesis of an Aqueous Ferrofluid

By Jonathan Gonzalez,2014-05-23 17:57
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Synthesis of an Aqueous Ferrofluid

    Synthesis of an Aqueous Ferrofluid

    Version 3.0

    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

    November 2005

Overview

    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.

Outline

     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

     Observe spiking

     Teacher Post-Lab 10 minutes

     Collect, neutralize, and dispose waste

     Collect Supplies and Clean Up

     For the latest update to the manuals, visit

    http://voh.chem.ucla.edu/outreach.php3

    Discussion board password: nano

    or email: tolbert@chem.ucla.edu

Teacher Manual Last Updated: 5/23/12 UCLACNSI 1

    California State Science Standards Grades 9-12

    Addressed by the Solar Cell Experiment

Physics

    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 UCLACNSI 2

Chemistry

    2f. *Students know how to predict the shape of simple molecules and their polarity from Lewis dot

    structures.

    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 UCLACNSI 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 UCLACNSI 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 UCLACNSI 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 UCLACNSI 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

     container.

     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 UCLACNSI 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 UCLACNSI 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 UCLACNSI 9

    Further Disscussion

    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

    localizability).

    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 UCLACNSI 10

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