A simple way to charge low-conductivity organic liquids

Electrostatically charged liquids underpin numerous applications from coatings and high-resolution printing to mass spectrometry. The charge provides a handle by which to control the droplets. Invariably the liquids are water-based because the high electrical conductivity of aqueous solutions makes them easy to charge.

Organic liquids have been sidelined in these applications because they resist charging, especially common solvents that are insulating and nonpolar such as hexane and toluene. That may soon change thanks to new work that shows how to charge organic liquids in a remarkably simple way. The finding broadens the range of chemical compounds suitable to today’s charged-droplet applications and may open the door to new uses for these materials.

Although researchers have developed a few ways to apply an electrical charge to low-conductivity organic liquids, the methods have shortcomings. In electrospray ionization mass spectrometry, for example, scientists sometimes blend an organic liquid with water and conductive additives, such as salts, to boost the electrical conductivity of the liquid, then zap it with high voltage to form charged droplets. But that method can alter the solution’s composition and chemical properties and it requires expensive and potentially dangerous equipment.

Researchers at National University of Singapore have now come up with a workaround that’s almost as simple as running a balloon across your hair to make the balloon stick to a wall. The team, which includes Kang Hui Lim, Yajuan Sun, Wei Chun Lim, and Siowling Soh, reports that rubbing two strips of plastic insulators against one another to accumulate an electrostatic charge on their surfaces, then drizzling organic droplets across the surfaces, imparts long lasting charge to the droplets (J. Am. Chem. Soc. 2020, DOI: 10.1021/jacs.0c06000).

The do-it-yourself method works well with nylon, poly(tetrafluoroethylene), poly(vinylchloride), and other common insulating polymers. And the electrostatic charge that results from rubbing those insulators together quickly charges hexane, cyclohexane, xylenes, toluene, ethylbenzene, diethyl ether, chloroform, and other tough-to-charge liquids to a level comparable to the charge that builds in aqueous liquids charged via high-voltage methods.

The researchers showed that the method is tunable. The amount of static charge can be controlled by the rubbing pressure. And the polarity of the charged droplets simply depends on whether they were in contact with a positively or negatively charged polymer surface.

To demonstrate that the charged droplets can be controlled with an electric field, which is the basis of most applications involving charged liquids, the group used charged strips of polymer to steer various types of charged droplets, causing them to collide and react, in one case forming a charged polymer product.

“This is one of the most inspired pieces of work I have seen in a very long time,” says James H. Davis, Jr., a chemist at the University of South Alabama who has long worked with ionic liquids, another type of charged liquid. The notion that molecular liquids can have a charge imparted to them that they retain for a long time, even while being manipulated, is “extraordinary,” he says.

Davis adds that this work shows that with a good idea and hard, careful work, fundamental discoveries can still be made even with simple tools.

“I have a hunch that these researchers may have just created a brand new subfield in liquids research that could have an impact rivaling that which ionic liquids has had over the past twenty years.”