WHEN it comes to repelling gunk, Teflon and car wax are among the best materials available. They could, however, be better. A good measure of their lack of stickiness is the angle of contact between them and a drop of water. For car wax this is 90º and for Teflon, 95°. If those angles were higher, the result would be cleaner surfaces. On top of that Teflon, car wax and other materials like them are built up from multiple microscopic layers. These tend to get disconnected over the course of time and the protection they grant thus fails. If a way could be found to make the layers bond more strongly with one another, that problem might go away. And, in a two-for-one Tong Lin, a chemist at Deakin University in Australia, thinks he has solved both using an idea familiar from high-school chemistry: the covalent bond.
At the moment, the layers of Teflon, car wax and substances like them are held together by ionic bonds. In these one or more electrons leap from one atom to another in order to fulfil a requirement deep in the heart of quantum theory that such particles to pair up if they can, so that their spins cancel out. The atom with the surplus electrons thus acquires a negative charge while the one with the deficit acquires a positive one. Since unlike charges attract, the charged atoms stick together.
The problem with ionic bonds is that they are easily disrupted by acids and bases—chemicals which, respectively, add electrons to, and subtract them from, other molecules. In a covalent bond, by contrast, the pairs are formed by atoms sharing electrons, rather than transferring them. Covalent bonds are less susceptible to disruption.
In view of this, Dr Lin and his colleagues tried to build up covalent bonds between the layers of a hydrophobic material called cellulose acetate butyrate, which is used to make water-resistant glosses for fabrics. In doing so they not only made it acid and base proof, they also improved its gunk-resistance.
They performed this trick, as they describe in Langmuir, using a substance called phenyl azido, which readily forms covalent bonds with its neighbours when exposed to ultraviolet light. They immersed films made of cellulose acetate butyrate in solutions of phenyl azido and silica (which is positively charged) and phenyl azido and poly-allylamine hydrochloride (which is negatively charged). They then spread these films in alternate layers on top of sheets of cotton fabric and exposed the result to ultraviolet light for 30 minutes, thus bonding the layers covalently. Then, once the material was cured, they tested it to see how readily it repelled water, acids, bases and detergents.
Hydrophobically, it performed extraordinarily. Its contact angle was 154° out of a possible maximum of 180°. Standard cellulose acetate butyrate has a contact angle of 76°. It was also impressively robust. Even after being immersed for 95 hours in hydrochloric acid or 44 hours in sodium hydroxide it maintained a contact angle greater than 150°. It also did so after enduring 50 visits to one of the most hostile environments known on the planet, the home washing machine.
You would not, it is true, want cellulose acetate butyrate on either your car or your frying pan. But besides being a way of making better raincoats, this is also a proof of principle. If waterproof coating for clothes can be made this way, then other materials might be, too.