Surfactants with Magnetic Heads

The self-organization of surfactants is well known and results in the formation of architectures such as micelles or mesophases.

Sebastian Polarz and his team at the University of Konstanz, Germany, set out to investigate the behavior of magnetic surfactants, that is, a molecule with a magnetic metal complex attached to a chelating organic ligand furnished with a hydrophobic chain. They chose Dy³⁺ as the “head” group because of its high magnetic moment and a modified DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) ligand bearing a decyl chain. The solubility of these metal complexes is quite low, and depends on temperature and pH value. The molecules form small aggregates, and heating to 80 °C induces a growth process. After cooling, the formation of dumbbell-shaped aggregates with frayed moustache-like ends was observed. These materials possess both optical and structural anisotropy.

Transmission electron microscopy (TEM) studies showed that the aggregates comprise parallel fibers that are aligned parallel to the long axis of the molecule. The fibers are present in bundles, and appear to be hollow with 18 nm thick walls. Further studies showed that the decyl chains are partially interdigitated.

The researchers conclude that this special type of self-organization occurs because of the magnetic interactions between the paramagnetic Dy³⁺ ions in the heads of the molecules. These interactions result in the formation of hollow fibers rather than spherical vesicles.

The hypothesis was confirmed by a control experiment where the paramagnetic Dy³⁺ ions were replaced by diamagnetic Lu³⁺ ions and the formation of the moustache-shaped aggregates was not observed. Further magnetic measurements show that the structures are paramagnetic and actually show behavior similar to superparamagnetic materials as all the structures align when a weak magnetic field is applied. In fact, chainlike structures could be grown in the presence of an external magnetic field.

This use of an external trigger to control the macroscopic properties is very useful for potential applications of these systems.