Zinc oxidation

This week, a protective compound that brings strong childhood memories to Brian Clegg

Brian Clegg

When I was young I seemed to spend most of the summer with my extremities – particularly my ears – coated in a pink lotion. These were the days before sun block was common. If you had fair skin and you went out in the sun, there was only one option - you burned. And when you’d burned, on went the calamine. Although the familiar pink colouration of this soothing balm comes from a small proportion of iron oxide, the main active ingredient is an inorganic compound that has had a place in medical chests for at least 2, 000 years. It is zinc oxide.

This simple compound, with one oxygen atom for each of zinc, is a plain white powder and one of the most versatile substances around. Apart from the use to treat skin conditions, which goes back to the Ancient Greeks, and may have been used even earlier in India, it has found its way into paints, ceramics, rubber and even food.

In that calamine lotion, the zinc oxide has an antiseptic action, and the lotion was supposed to reduce itching, though there is some doubt how effective it was, beyond the basic cooling effect of the evaporating liquid. It would also have a secondary effect of reducing any further exposure to sunlight that would cause extra burning, as zinc oxide is a good reflector of ultraviolet light. It plays this role in modern sun blocks, which lack the traditional thick, white appearance because the zinc oxide is present in the form of nanoparticles, alongside those of another strong white pigment, titanium dioxide and a range of organic compounds that absorb UV.

Pigments provide the earliest known examples of nanotechnology, although zinc oxide does not provide coloration in a sun block, where its tiny particles are practically transparent to visible light. This is only the beginning of the application of the oxide on the extremely small scale. A whole range of nanostructures can be grown with zinc oxide, including nanowires and films. These are of interest because zinc oxide is an n-type semiconductor, meaning it has a few extra electrons which allow it to conduct. This means it is potentially of use in nanoscale electronics, and it already provides a conductive film for LCDs and solar panels.

The more traditional use of zinc oxide as a pigment has been in the paint box, where it has cropped up since the mid-1800s. Although it isn’t as starkly opaque as titanium dioxide, it provides a good, strong white pigment that forms the basis of the colour Chinese white and is used to give paper a bright white coat. It even finds its way into that most subtle of paint palettes, the cosmetic box, where it forms the bulk basis of the popular mineral makeups. Zinc whites compare well with earlier white pigments like lead oxide as it isn’t poisonous nor blackened by airborne sulfides. It works best mixed with other pigments – as a pure white background it tends not to be used as is quite brittle and can easily crack.

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