What Is the Chemical Composition of Ferrofluids?

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The chemical composition of ferrofluids can be very important for their usefulness. A magnetic fluid that is not properly stabilized will quickly become solid, exhibiting jagged spikes when placed in a magnet. This is a well-known phenomenon called Rosensweig instability, named after the late Ronald Rosensweig, an American chemist who developed some of the first magnetic fluids.

To prevent this, the iron oxide (magnetite) particles in ferrofluids need to be coated with surfactants. This keeps the magnetic particles in suspension. Previous attempts to prepare stable silicone oil-based ferrofluids using surfactants such as oleic acid have been limited by the volatility of the silicon oils themselves. These volatile hydrocarbon liquids require a large volume of solvent to produce a ferrofluid, and their evaporation rapidly degrades the magnetic properties of the ferrofluid.

A team led by physical chemist Ben Erne at Utrecht University in the Netherlands has now developed a new kind of surfactant, which makes it possible to create magnetic fluids that stay liquid even when heated above 100°C. Their new formula also allows them to make magnetic fluids with different densities, allowing materials of different weights to float at different heights. This is very important in applications such as magnetic density separation (MDS), where flaked plastics from different sources flow through a ferrofluid over an electromagnet, attracting the magnet and repelling non-magnetic flakes.

The key to the new surfactant is a special type of molecule called b-cyclodextrin, which can attach itself to the iron oxide particles and form a cage around them. This cage makes it impossible for the magnetic particles to stick together, and also blocks their ability to interact with other iron oxides. As a result, the spins of the magnetite atoms are oriented strictly opposite each other, producing a checkerboard pattern that is strongly attracted to magnetism.