Ferrofluid Nanotechnology

Introduction

A magnetic liquid, also known as a ferrofluid, may seem like a space-age concept. That’s because it is—the idea was conceived by NASA in the 1960s to control the flow of liquid fuels in space! This activity provides a simple procedure for preparing a ferrofluid and demonstrating its properties. Magnetite (Fe₃O₄) is prepared by combining Fe²⁺ and Fe³⁺ ions with a weak base (ammonia) in dilute aqueous solution. The magnetite produced in this manner consists of extremely small, solid-phase particles that are only about 10 nm in diameter. Shrinking the size of particles to the nanometer scale (one-billionth of a meter) changes their physical and chemical properties. Rather than settle out of solution as a solid, the so-called nanoparticles form a stable colloid, giving rise to a magnetic liquid!

Concepts

• Nanotechnology
• Colloids vs. Solutions
• Magnetic properties
• Ferrimagnetism

Materials

Safety Precautions

Tetramethylammonium hydroxide solution is a corrosive liquid—it may cause skin burns and is especially dangerous to the eyes. The solution is toxic by ingestion and skin absorption and may cause respiratory tract irritation. Hydrochloric acid solution is a corrosive liquid and is toxic by ingestion and inhalation. Ammonia vapors are irritating to the lungs and eyes. Perform this demonstration in a fume hood or in a well-ventilated lab only. Iron(II) and iron(III) chlorides are slightly toxic by ingestion and are body tissue irritants. The potential health effects of nanoparticles have not been fully identified. Avoid contact of all chemicals with eyes and skin. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The waste ammonia solution in step 9 may be neutralized with hydrochloric acid, if needed, before being discarded down the drain with plenty of excess water according to Flinn Suggested Disposal Method #10. The colloidal ferrofluid may be stored in an open container (such as the weighing dish in which it is prepared) in the hood until all of the liquid has evaporated and only a solid remains. The solid may then be packaged for landfill disposal according to Flinn Suggested Disposal Method #26a.

Prelab Preparation

1. Prepare a 2 M solution of iron(II) chloride in hydrochloric acid by adding 4.0 g of FeCl₂•4H₂O to 10 mL of 2 M hydrochloric acid (provided with the kit) in a 50-mL Erlenmeyer flask or beaker. Stir to dissolve and mix well. Note: This solution does not store well and should be prepared fresh the day of use. Only 1 mL of the solution is needed per demonstration—prepare the solution in smaller-volume batches as needed if the demonstration will be repeated on different days. (Eight grams of iron(II) chloride are provided with the kit.)
2. Prepare a 1 M solution of iron(III) chloride in hydrochloric acid by adding 5.4 g of FeCl₃•6H₂O to 20 mL of 2 M hydrochloric acid (provided with the kit) in a 50-mL Erlenmeyer flask or beaker. Stir to dissolve and mix well. This solution may be stored for up to one week.

Procedure

1. Using a graduated cylinder, measure and add 4.0 mL of 1 M FeCl₃ solution to a 100-mL beaker.
2. Rinse the graduated cylinder with distilled water, then measure and add 1.0 mL of 2 M FeCl₂ solution to the beaker.
3. Place a stir bar in the combined Fe(II)/Fe(III) solution and place the beaker on a magnetic stirrer. (If a magnetic stirrer is not available, use a stirring rod to continuously mix the solution as the ammonia water is added in step 5).
4. Fill a clean, 50-mL buret with 1 M ammonia solution. Clamp the buret to a ring stand and position the tip of the buret directly over the 100-mL beaker containing the combined Fe(II)/Fe(III) solution.
5. Partially open the buret stopcock and allow the ammonia solution to drip slowly, with continuous stirring, into the combined Fe(II)/Fe(III) solution. Try to add the ammonia at the rate of 1 mL every 10 seconds. (Magnetite will precipitate out of the solution in the form of a brownish black solid.)
6. When 50 mL of ammonia has been added, turn off the magnetic stirrer.
7. Remove the magnetic stir bar using a stirring bar retriever. Rinse the stir bar over the beaker with a small amount of distilled water.
8. Place the neodymium magnet under the beaker to “pull” the black solid to the bottom of the beaker.
9. Decant the clear liquid from the beaker into a waste flask. (Continue holding the magnet under the beaker to avoid losing the solid when pouring off the liquid.)
10. Transfer the solid to a weighing dish by rinsing with a small amount of distilled water from a wash bottle. Rinse the beaker with 2–3 mL of water to get as much of the solid as possible into the weighing dish.
11. Decant (pour off) as much water as possible from the solid in the weighing dish. (Hold a magnet under the weighing dish to keep the solid in the weighing dish.)
12. Wash the solid twice more with distilled water: Add 1–2 mL of water, pour off the water by holding the magnet under the weighing dish, and repeat.
13. Using a glass (Pasteur) pipet, add 2–3 mL of tetramethylammonium hydroxide solution to the solid (magnetite) in the bottom of the weighing dish.
14. Using a glass stirring rod, stir the product and the tetramethylammonium hydroxide solution for at least one minute to get thorough mixing and to suspend the magnetite particles in the liquid.
15. Move the magnet around under the weighing dish to attract the “ferrofluid” or magnetic liquid to the center of the dish. Discard any extra liquid that is not attracted to the magnet. The ferrofluid should be neither too “thick” (viscous) nor too “thin” (watery).
16. With the weighing dish in one hand, hold the magnet in the other hand, about 1 cm below the weighing dish (do not touch the dish directly with the magnet). Slowly move the magnet around under the dish until small spikes are observed in the ferrofluid. (This step may take some practice, but the spikes get larger and more noticeable when the magnet is positioned just right with respect to the “magnetic liquid.”)
17. (Optional) The ferrofluid may be stored in a sealed glass vial. For best results, transfer the solid directly from the beaker to a vial in step 10, before washing the solid and suspending the solid in the tetramethylammonium hydroxide solution.

Teacher Tips

• This kit contains enough materials to perform the demonstration as written seven times: 350 mL of 1 M ammonia water, 8 grams of iron(II) chloride, 15 grams of iron(III) chloride, 100 mL of 2 M hydrochloric solution, 20 mL of 25% tetramethylammonium hydroxide solution, 7 disposable glass (Pasteur) pipets, 7 weighing dishes and one neodymium magnet. Note: These amounts include “extra” iron(II) chloride and iron(III) chloride, as well as more hydrochloric acid, in case the Fe(II) and Fe(III) solutions will not be used up within the recommended storage time.
• It is difficult to transfer the ferrofluid after it has been prepared. If the ferrofluid will be stored, carry out the washing and rinsing steps directly in a vial, as described in the optional step 17.
• Solutions and colloids differ in the size of the particles that are dispersed in the liquid phase. The following table summarizes the properties of solutions, colloids and suspensions. Notice that the particle size range for each type of mixture is just that, a range and not an absolute or fixed value.
  
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Discussion

Nanoscience or nanotechnology involves the preparation, characterization, and uses of nano-sized particles having dimensions in the 1–100 nm range (1 nm = 1 x 10^–9 m). Nanoparticles have unique physical and chemical properties that are very different from the macroscopic properties of traditional or “bulk” solids. Many of these properties have taken on special importance in recent years as the applications of nanotechnology have been intensively studied. In particular, the electronic, magnetic and optical properties of nanoparticles have proven to be very useful in the creation of new products using nanotechnology. Magnetic liquids, also known as ferrofluids, are stable colloids containing nanocrystalline magnetite particles.

Magnetite, Fe₃O₄, also known as ferrosoferric oxide, is a naturally occurring, strongly magnetic, mixed iron(II)/iron(III) oxide. (There are two Fe³⁺ ions for every Fe²⁺ ion in the crystal structure.) Magnetite is prepared in this demonstration by reacting FeCl₃ and FeCl₂ in a 2:1 mole ratio with dilute ammonia, NH₃. The basic ammonia solution provides hydroxide ions, OH^–, which combine with iron cations to produce the Fe₃O₄ oxide after loss of water molecules (Equation 1). The concentration of reactants is the main factor influencing the size of the magnetite particles produced in this reaction. Dilute solutions favor the formation of magnetite nanoparticles that are less than 10 nm in diameter.

In order to produce a stable colloid, the Fe₃O₄ nanoparticles in the ferrofluid must be coated with a substance that will prevent them from “clumping together” and settling out of solution. This is accomplished by washing the initial brownish black precipitate with water and then adding a solution of tetramethylammonium hydroxide, which acts as a surfactant. Hydroxide anions bind to the surface of the oxide nanoparticles, coating the particles and giving them a net negative charge. Tetramethylammonium cations form a positively charged outer shell around the anions and stabilize the nanoparticles. The surface-coated, charged magnetite particles in the ferrofluid repel each other and are therefore prevented from forming clusters and precipitating out of solution.

Magnetite is an example of a ferrimagnetic or “superparamagnetic” substance—it is polarized by and strongly attracted to an external magnetic field. Fe(II) and Fe(III) ions are both paramagnetic due to the unpaired d electrons in their electron structures, but they have different numbers of unpaired electrons and thus different magnetic moments. In the crystal structure the magnetic domains formed by alignment of the unpaired electrons in Fe²⁺ and Fe³⁺ ions are antiparallel. Because the magnetic moments are not equal, however, Fe₃O₄ has a net magnetic moment or magnetization. In the absence of an external magnetic field, the ferrofluid flows and behaves like a “normal” albeit viscous liquid. When a magnet is brought near a dish or vial containing the ferrofluid, the “solid” nanoparticles are attracted to and will “follow” the magnet around the dish or vial. The ferrofluid forms interesting three-dimensional shapes or structures as the magnetic moments of the nanoparticles align themselves with the external magnetic field. Noticeable peaks or spikes in the ferrofluid correspond to the magnetic field lines.

Ferrofluids are more than just an intellectual curiosity. They have innovative commercial or practical applications, including as dampeners or heat sinks in loudspeakers, as seals in high speed computer disk drives, as magnetic inks for laser printers and even, apparently, as radar-absorbing paints that allow military aircraft to escape radar detection.

References

Berger, P. et al. “Preparation and Properties of an Aqueous Ferrofluid,” J. Chem. Educ. 1999, 76, 943.

Chun, D. et al. “Synthesis of an Aqueous Ferrofluid,” California NanoSystems Institute and Materials Creation Training Program, http://voh.chem.ucla.edu/outreach.php3 (accessed December 2006).