Have you ever tried bleaching sunscreen stains on clothing, only to be left with bright red results?
Professor Clare Mahon, from our Department of Chemistry, did just this and the scarlet staining instantly intrigued her.
Determined to uncover what was happening, she joined forces with colleagues to dig into the chemistry behind the reaction.
What came next was a fascinating dive into the interaction between two common household products – one that could just save summer wardrobes the world over!
Durham’s Chemistry Department is home to the ANTENNA project – an innovative collaboration with P&G and Imperial College London developing advanced cleaning and detergent formulations. So, we’re the perfect place to investigate such a surprising chemical reaction.
Clare led a team of our chemists in a detailed study of how commercial sunscreens reacted to bleach.
Of the 11 sunscreens the team tested, seven of them produced the red colouration when exposed to bleach. All seven of these contained the ingredient diethylaminohydroxybenzoyl hexyl benzoate (or DHHB for short).
Previous studies had found that parts of the DHHB molecule could be changed by adding chlorine (such as bleach). However, this change did not result in a strong colouration, making the cause of the red stains a mystery. Clare and her team suspected there was more at play.
They turned to nuclear magnetic resonance experiments to fully understand the structure of the molecules at play. Working with Professor Andy Beeby, Dr Alan Kenwright and Dr Eric Hughes the team found a molecular structure at odds with what standard chemistry rules would have predicted.
An unusual chemical change in the DHHB molecule was observed, where two chlorine atoms ended up in the same spot on one of its ring structures – a process called ipso-dichlorination.
Computational simulations, performed by Professor Mark Wilson showed that this new molecule would absorb most of the shorter and medium-range wavelengths of visible light, allowing only the longer red wavelengths to pass through.
The result – bright red colourisation, just like on the clothes that sparked the study.
To confirm their findings, the team developed an ipso-dichlorination resistant version of DHHB. In this version, there was very little change in its UV absorption – suggesting it could offer a formula to avoid sunscreen staining.
Any new formulations would require further research and testing. However, there is hope on the horizon for banishing these stains for good.
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