Repelling rain to preserve the past

 

A team of scientists from Cardiff University and the University of Iowa have synthesised a ‘super-hydrophobic’ compound to protect limestone buildings from the damaging effects of acid rain.

Many historical buildings throughout Europe are made from limestone, and so fears over the damage that could be done to such cultural treasures has spurred scientists to devise methods of protection. One of the worst culprits for damaging old limestone buildings is acid rain – specifically the type that forms when sulphates and other sulphur-containing oxides dissolve in water.

Limestone is particularly susceptible to sulphur decay because the calcite it contains reacts with the sulphuric acid in acid rain to form calcium salts such as gypsum. The blackened edges that can be seen on old limestone buildings and their ornamentation – such as gargoyles – are visible evidence of this damaging process.

The compound created by the Cardiff-Iowa team protects the limestone from acid rain using a phenomenon called the “lotus effect”, so named for the interesting way in which lotus leaves repel water to avoid getting wet. The lotus effect has been exploited in many other areas already – it’s used in medical science to protect microscope slides from water-borne contaminants, and it’s possible to buy water-repellent sprays for car windscreens which work on the same principles.

The substance itself is made up of mostly fatty acids – these allow it to coat the limestone surface in a thin, conformable layer – and small quantities of fluorinated alkylsilanes which contain fluorine and silicon, and confer a “super-hydrophobicity” on the limestone.

The team used the particle accelerator at Diamond Light Source to better understand how the hydrophobic compound works. Because the coating is intended for use on important cultural landmarks, it’s critical to understand what the coating does to the limestone on a molecular level in order to assess if the coating is damaging it. For this reason, rather than just applying the coating to limestone and testing for an observable hydrophobic effect, the team used high intensity x-ray absorption spectroscopy to examine the effects of the coating. The results showed that the super-hydrophobic coating greatly slowed the reaction whereby acid rain converts calcite in limestone to gypsum, by greatly reducing the size of the calcium-sulphur accretions.

The strength of these results has already taken the scientists out of the lab. York Minster has already been coated with this substance by the Cardiff team. There are many more cultural landmarks across Northern Europe and the USA which could benefit from such coatings, and whilst limestone might be a small first step, these projects could have a dramatic effect on preserving our cultural and historical landscape.

REFERENCE: Walker, R et al. Scientific reports, 2012 DOI: 10.1038/srep00880

IMAGE: vpickering, flickr.

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