Observation of the Unusual Properties of Ferrofluids
MS 90, Spring '01
When nanoparticles of magnetic materials are suspended in a fluid, they can form ferrofluid. The interest in ferrofluids arises from the fact that they act like fluids dynamically, but can be controlled directly by the application of a magnetic field. NASA was the first to develop and characterize ferrofluids in the 1960s. Since then, ferrofluids have been incorporated into a number of commercial and industrial devices, from actuators to information storage media. The behavior of a ferrofluid depends on the mean size and size distribution of the nanoparticles and rheological properties of the fluid, and the nature of the surfactant used to suspend the particles. A surfactant adheres to the surface of particles and mediates the interactions between the particles and the solution. In the case of ferrofluids, the surfactant causes a net repulsion between particles that keeps them from agglomerating (sticking together) and precipitating out of the solution. The key to making a ferrofluid is finding the right combination of magnetic particle, liquid medium, and surfactant.
Synthesize nanoparticle magnetite, Fe3O4, which has the inverse spinel crystal structure; disperse the particles in a colloidal solution; and observe ferrofluidic behavior. Estimate the particle size by X-ray powder diffraction.
Preparation and Properties of an Aqueous Ferrofluid, P. Berger et al., J. Chem. Ed. 76 (1999) 943-948. [Accessible on-line. The present lab is taken direcly from this paper.]
Physical/Chemical Aspects of Ferrofluids
Ferrofluids possess the fascinating property of spiking when exposed to a suitable magnet: when a magnetic field is applied to the fluid, the fluid forms sharp peaks pointing along the direction of the magnetic field lines. This spiking, which will be investigated in this lab, is a good indication of a high-quality ferrofluid. There are two basic steps in creating a ferrofluid. The first is to synthesize the magnetic nanoparticles and the second is to incorporate them into a colloidal suspension. In order to prepare a ferrofluid, the magnetic particles must be very small - on the order of 10 nm (100 Å) in diameter. The thermal energy of the particles must be large enough to overcome the magnetic interactions between particles, which occurs when the particles are sufficiently small. If the particles are too large, magnetic interactions will dominate and the particles will agglomerate and precipitate. Once properly sized particles are created, they can be suspended in the liquid medium with the aid of a surfactant. The surfactant should be introduced either while the particles are being made or shortly thereafter. As mentioned before, choosing the right surfactant is crucial.
In this synthesis of a ferrofluid, a number of safety rules must be followed. Hydrochloric acid (HCl) and ammonium hydroxide (aqueous NH3) will be used in this lab. Both are corrosive and should be handled with care. Gloves and goggles must be worn at all times. The Fe+2 chloride salt (also denoted iron (II) chloride, ferrous chloride, or FeCl2) is toxic, corrosive, and a mutagen. The Fe+3 chloride salt (iron (III) chloride, ferric chloride, or FeCl3) is corrosive. Tetramethylammonium hydroxide [N(CH3)4OH] is a strong base that is corrosive and flammable. Extreme caution must be exercised when handling all of these materials. Ferrofluids can be messy. The particular ferrofluid you will prepare will permanently stain almost any fabric. If it gets on your skin, wash it off immediately. Do not let the ferrofluid come directly into contact with any magnet; any time you are holding a magnet near a ferrofluid, keep the two well separated.
In this lab you and a partner will be making an aqueous-based ferrofluid. The process involves synthesizing the magnetic solid, magnetite (Fe3O4), and then suspending it in water with the aid of a surfactant. The magnetite will be synthesized by a precipitation reaction that occurs upon mixing FeCl2 and FeCl3 with a base (ammonium hydroxide, which is an aqueous solution of ammonia, NH3). The unbalanced equation for this reaction is as follows:
__ FeCl3 + __ FeCl2 + __ NH3 + __ H2O -> __ Fe3O4 + __ NH4Cl
Balance the equation. Determine the proper stoichiometric amounts of each reagent to use based on the following starting materials:
1 mL 2M FeCl2
__ mL 1M FeCl3
The magnetite formed in this particular synthesis generally has a particle diameter of between 5 and 20 nm, which is well within the range required for a suspension. The surfactant used in this synthesis is the salt tetramethylammonium hydroxide (TMAOH or N(CH3)4OH). The hydroxide (OH-) ions formed in solution tend to coordinate with the iron sites on the magnetite particles, creating a net negative charge on each particle. The positively-charged tetramethylammonium ions will then associate with the negatively-charged magnetite particles, forming a kind of shell around each magnetite particle. This charged shell raises the energy required for the particles to agglomerate, stabilizing the suspension.
This procedure should be performed IN A HOOD.
Work in pairs.
Make sure all glassware is clean before using.
If they are not already available, prepare the following starting solutions:
- 10 mL of 2M FeCl2
- 10 mL of 1M FeCl3
- 10 mL of 2M HCl
- 100 mL of 0.7M NH4OH (same as aqueous NH3)
Using burets, add to a 125 mL Erlenmeyer flask 1 mL of stock 2M FeCl2 in 2M HCl and the corresponding amount of stock 1M FeCl3 in 2M HCl that you calculated above. Place a magnetic stirring bar into the flask and begin stirring. Using a buret or a 50 mL volumetric pipet and rubber bulb (be sure to rinse out and shake dry the bulb before and after using), add dropwise while stirring vigorously 50 mL of stock 0.7M aqueous NH3 solution into the flask. Magnetite, a black precipitate, will form. Continue stirring throughout the addition of this ammonia solution and for an additional 5 min afterwards. Rinse out the buret or pipet as soon as you are done using it.
While the solution is stirring, transfer equal portions of it into several small centrifuge test tubes. A disposable plastic pipet should be used to transfer the solution from the flask into the centrifuge tubes. Centrifuge the solution for 1 min (Keck 136). After the centrifuge has stopped, remove the tubes and decant the liquid. The solid at the bottom is magnetite. Before adding the surfactant, set a small portion aside for X-ray powder diffraction.
Add 1 mL of the surfactant solution, 25% tetramethylammonium hydroxide (N(CH3)4OH), from the buret directly into each of the small test tubes and stir with a thin glass rod until the solid is completely suspended in the liquid. To remove excess ammonia, pour the contents of the tubes into a 50 mL vacuum flask, along with a medium-sized magnetic stir bar. Seal the top with a #4 rubber stopper, evacuate the flask and stir the solution under vacuum for 30 min. To do this, first turn on the magnetic stirrer, then turn on the aspirator, and finally connect the rubber hose to the vacuum flask; when finished, disassemble the setup by reversing the order of these steps.
The magnetic stirring bar in the filtration flask should be covered with a black sludge, which may or may not have spikes on it. The challenge now is to separate the stir bar from the nanoparticles (sludge). Move the stir bar and attached sludge into a plastic weighing boat, leaving behind the rest of the liquid. Remember that ferrofluids are messy and can easily and permanently stain any fabric. Carefully and slowly move a strong, block-shaped magnet (preferably a neodymium-iron-boron, Nd2Fe12B, magnet) up to the bottom of the plastic weighing boat containing only the stir bar and the magnetite sludge that is stuck to it, keeping the magnet away from the sides of the weigh boat, see the figure at the end of the lab. If the magnet hits the weigh boat very hard, the ferrofluid may splash and get on your clothes. The magnetite will be attracted to the strong magnet more than to the magnetic stir bar. Rotate the stir bar about its axis to remove the magnetite (see illustration below). Make sure to remove the magnetite from both ends of the stir bar. Using gloved fingers, remove the magnetic stir bar from the weighing boat - the magnetite should remain attracted to the strong magnet. This will be a little difficult due to the stronger magnet underneath the weigh boat. Again, be very careful; if the stir bar slips out of your grip, it will fall back against the stronger magnet, splashing ferrofluid. Take the stir bar back to your bench. With the strong magnet still underneath the weigh boat, pour any excess water into the nearby WASTE beaker. Finally, carefully remove the strong magnet from the bottom of the weigh boat. Again, the sludge can splash easily, so be cautious.
Hold a standard magnet up to the bottom of the weigh boat to check if the sludge exhibits spikes. If it does not, or the spikes are small, add ONE drop of distilled water, and move the end of the magnet under the plastic weighing boat to mix the ferrofluid, and see if spiking develops or improves. If spikes are still not seen, try adding another drop of distilled water. It should not take more than a few drops of water to obtain a good spiking effect. If too much water is added, the ferrofluid will become too dilute and not exhibit spikes. If your ferrofluid is too dilute, hold the strong block-shaped magnet against the bottom of the weigh boat in order to hold the magnetite particles in place and again pour any excess water into the nearby WASTE beaker.
X-ray powder diffraction
Your TA will show you how to collect X-ray powder diffraction patterns with the Siemens D-500 diffractometer. Spread your magnetite powder over a glass slide. You will compare the diffraction pattern of nanoparticle magnetite with standard magnetite. Grind a small portion of commercial magnetite in a mortar and pestle. Place double-sided tape on a glass slide, and them spread the ground powder over it.
Examine your diffraction patterns, and record the full-width half-max, b, for the most intense diffraction peak in radians. In the case of the commercial powder, the peak width is due, for the most part, to instrumental parameters (as opposed to the characteristics of the sample). Therefore, take bcommercial to be equal to binstr. Estimate the size of your nanoparticles according to the following relations.
bnano ~ bmeas - binstr
bnano ~ l/Dcosq
where l is the wavelength of the X-radiation used, q is the angle of the diffraction peak analyzed, and D is the particle size.
Provide some background on nanoparticles, magnetic oxides, colloidal dispersions and/or ferrofluids. Describe the synthetic procedure, and the crystal structure of the resultant Fe3O4. Can you identify where the hydroxides from TMAOH will associate with the structure? Describe the geometry of the spikes in the ferrofluid. How close does the magnet need to be in order for the spikes to appear? Examine the starting FeCl2 and FeCl3 solids used to prepare magnetite. How do they respond to a magnet? Index the peaks in your diffraction pattern of Fe3O4 (ao = 8.320Å). Give the estimated particle size.
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