Preparation of a ferrofluid

Transcription

1 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. 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 [Year 11 students will not be familiar with the details of transition metal chemistry] Time Facilities Specialized equipment Approx 1 hour Standard lab bench Rare earth disc magnets (20 mm x 5 mm)

2 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 Hazard Assessment FeCl 2 solution Irritant Standard FeCl 3 solution Irritant standard Tetramethylammonium hydroxide solution Toxic absorbed through skin Corrosive Wear gloves Dilute NH 3 solution Irritant standard

3 Equipment Equipment and chemicals Solutions (for 50 students; allowing for some repeat experiments) FeCl 2 2M 100 cm 3 FeCl 3 1M 500 cm 3 NH 3 0.5M 5 litres [Me 4 N][OH], 25% in water 100 cm 3 Equipment (per student) Item Quantity Clamp and clamp stand 1 50 cm 3 burette cm 3 conical flask cm 3 beaker 1 Rare earth disc magnet (20 mm x 5 mm) 1 Pasteur pipette 1 Wash bottle with distilled H 2 O 1 Glass stirring rod 1 Sample vial 1 Cotton wool or filter paper Disposable vinyl gloves 1 pair FeCl 2 solution, 2M (must be made fresh) FeCl 2.4H 2 O (39.8 g) Dissolve in distilled H 2 O and make up to 100 cm 3 Provide one container of solution and 1 x graduated pipette & filler per 4 students FeCl 3 solution, 1M (must be made fresh) FeCl 3.6H 2 O (135 g) Dissolve in distilled H 2 O and make up to 500 cm 3 Provide one container of solution and 1 x 1 graduated pipette & filler per 4 students Ammonia, 0.5M Conc NH 3 (167.5 cm 3 ) Dilute to 5 litres with distilled H 2 O [Me 4 N][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

4 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. 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. 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. are made by the suspension of nanoparticles of an iron containing compound, Fe 3 O 4 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 (Fe 3 O 4, 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 (Fe 3 O 4 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(CH 3 ) 4 ] + [OH] - ). In water, it dissociates to form OH - anions and [N(CH 3 ) 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.

5 Background OH - OH - OH - OH - Fe 3 O 4 OH - = H 3 C H 3 C CH3 N CH3 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 CH 3 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.

6 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)

7 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 cm 3 conical flask add 4 cm 3 of the 1 M FeCl 3 solution and 1cm 3 of the 2 M FeCl 2 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 50cm 3 of 0.5 M ammonium hydroxide. 3. Add to the conical flask 50 cm 3 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 cm 3 a minute or 1 cm 3 every six seconds. Upon addition of ammonia solution initially a brown precipitate will form, followed by the formation of a black precipitate (Magnetite, Fe 3 O 4 ). 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 cm 3 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 cm 3 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 cm 3 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.

8 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 cm 3 a minute or 1 cm 3 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. Helen Aspinall 2012