Harmful and persistent “forever chemicals” build up in the environment and in the bodies of animals — including humans. But a new review article lays out a blueprint for replacing those chemicals in certain situations.
A research team has compiled more than a decade’s worth of work from multiple labs to detail chemical principles of per- and polyfluoroalkyl substances, otherwise known as PFAS. PFAS show up in products as varied as firefighting foams, nonstick cookware and stain-resistant fabrics. While none of the proposed substitutes outperform existing PFAS yet, the best alternatives are approaching the same performance in certain water-repelling applications, scientists report in the July 15 Journal of Colloid and Interface Science.
PFAS usually contain long chains of carbon atoms. Depending on the chemical, most or all of the carbon atoms have strong bonds to one or more fluorine atoms. Mixed with water, some PFAS act as surfactants, which cause water droplets to spread out rather than beading up, even in the presence of oily chemicals where water normally wouldn’t mix. This behavior relies on properties known as surface energy and surface tension. Molecules in a material with low surface energy or surface tension don’t mind being at the surface of a solid or a droplet of liquid, where come in contact with something dissimilar. PFAS surfactants lower the surface tension of water, so they excel in applications like foams that fight gasoline or grease fires.
Alternatively, when used as solid coatings, PFAS force liquids on a surface to bead up into droplets rather than spreading out, which gives PFAS-coated materials like nonstick pans their water- and oil-repelling properties.
But the strong carbon-fluorine bonds in PFAS don’t break down easily, says Julian Eastoe, an interface scientist at the University of Bristol in England. The chemicals steadily accumulate in the environment and in our bodies, a buildup that “can be considered as one of the great ticking time bombs in our civilization,” Eastoe says. PFAS have been linked to a range of health issues, from high cholesterol to cancer. Some researchers are investigating how to break down PFAS in the environment, while others — like Eastoe — are developing fluorine-free alternatives.
To replace PFAS, scientists need to find a way to keep a material’s surface energy low without invoking fluorine. Eastoe and colleagues report that for PFAS acting as surfactants, chains of mostly carbon and silicon atoms with a bulky, tree branch–like structure can take the place of fluorine-rich fragments.
The researchers determined the surface tension of solutions containing water and the fluorine-free surfactants at different concentrations, usually by measuring the force required to pull a metal plate out of each solution. These tests suggest that the surfactants’ “branches” pack tightly at the surface of a water droplet to reduce the surface tension. Some of the best-performing alternatives reduced the water’s surface tension about as well as PFAS surfactants in use today.
It’s much harder, however, to compete with PFAS in oil-repelling applications. Oils typically spread out easily, so designing a surface coating that rebuffs oils would require a material with a very low surface energy — a difficult feat without invoking fluorine, says Kevin Golovin, a mechanical engineer at the University of Toronto who was not involved in the work. To effectively repel oils with fluorine-free surfaces, “we really do need a breakthrough.”
Still, the research could facilitate a transition away from PFAS in certain applications and could help counter perceptions that PFAS cannot be replaced, says Martin Scheringer, a chemist at ETH Zurich who was not involved in the work. “We need scientists, chemists and materials scientists, who break out of that PFAS track.”