Room-temperature ionic liquids are finding applications including environmentally benign reaction solvents and separation agents. Li Niu and colleagues1 now exploit the fact their counter ions can be facilely exchanged in solution to expand potential applications. Their approach enables rapid, controlled release using ionic liquids, which has wide potential for use in applications such as oil transportation.

The researchers used ion exchange and the high hydrophobic nature of fluorine atoms for controlling release. They started with an amphiphilic ionic molecule, which consists of a long hydrophobic tail and a hydrophilic head group with a positive charge that is counterbalanced by a negative bromine ion. In aqueous solution, this amphiphile formed aggregates or micelles, with the head groups all facing out towards the water and the hydrophobic tails contained in the centre of the micelles.

Fig. 1: The addition of hexafluorophosphate ions to the micelle solution means the bromine ions are displaced and the head groups become hydrophilic. This makes the micelles collapse.

When Niu and colleagues added a hexafluorophosphate ion to the solution, which displaces the bromine ion, the head groups of the amphiphiles became hydrophobic because of the fluorine atoms on the replacement ions. Thus the micelle structure was no longer favorable and the aggregates broke up, releasing any contents (Fig.1). The hydrophobic molecules then precipitated out of solution.

This simple process was rapid, taking only one or two seconds from addition of the hexafluorophosphate ions to precipitation being formed. Furthermore, this procedure was much quicker than dissolution of micelles in response to other external stimuli such as light, heat or pH.

The authors highlight a potential application for transportation of oil, which is easier to pump through pipelines in an emulsified state that is less viscous than oil alone. However, the oil must be easily recoverable from this emulsified state after transportation. Niu and co-workers showed that their ionic liquid approach satisfies these criteria.

Niu explains that the use of the modulation of surface wettability by ion-exchange enables the researchers “to create a simple, efficient, inexpensive, rapid, incremental and highly controllable smart system.”

In further work they hope to find other uses for the materials. “Future plans include applications of these smart surfactants in the fields of nanoscience and polymerization,” says Niu.