Milestone in preservation of historical monuments
Chemical shield protects buildings from acid rain and bacteria

Ulm University

The Ulm Minster, the Colosseum in Rome and the pyramids of Giza share mutual "enemies": acid rain and biofilms destroy their facades. Local Professor of Chemistry Carsten Streb and his team of researchers have now developed a "protective shield" that makes stones impervious to adverse environmental influences. The water-repellent and acid-resistant liquid (POM-IL) can be applied to natural stones as a transparent protective film. The research team presented the new surface coating, which could very well mark a milestone in the fields of construction and historical monument preservation, in the scientific journal Angewandte Chemie.

Corrosion of natural stone poses a threat to our cultural heritage, including protected buildings and antique statues. The deterioration results from two main sources: first of all, acid rain, triggered by industrial pollution, destroys the facades. The second source is a thin, microbial biofilm, which causes stones to become shabby and porous. Combating these harmful environmental influences necessitates a material that is both water repellent and antibacterial.

In collaboration with their colleagues from Zaragoza (Spain) and Reims (France), researchers from the Ulm Institute of Inorganic Chemistry I have now developed a multifunctional surface coating for natural stones commonly used in construction that protects them from the adverse effects of acid rain and microbes.  The researchers use an ionic (saline) liquid that has already proved successful in protecting metals against corrosion. The major advantage, explains Professor Carsten Streb, is that "With this polyoxometalate-ionic liquid, POM-IL for short, cations and anions can be altered independently of one another. This enables us to adapt the properties of the coating depending on the respective environmental conditions".

During the course of the now published study, the researchers produced two variations of the surface shield, POM-IL1 and POM-IL2, and tried them out on three different types of natural carbonate rock with differing chemical compositions as well as porosity (Belgian blue stone, Dom stone and Romery stone).

Acid tests for new corrosion shield

The new liquids were subjected to genuine "acid tests" in the lab. Firstly, a coating of POM-IL1 or POM-IL2 was applied to samples of the types of limestone commonly found in buildings in Belgium and northern France. Both the treated samples and the reference samples were then kept in a glass vessel for 72 hours, where they were subjected to acetic acid vapour. The scientists then cleaned, dried and weighed the samples. The effects of the protective shield were already visible to the naked eye: "The stones that were treated with the POM-IL liquid retained their shape, while the surface of the untreated stones showed signs of weathering. The scales confirmed this loss of material", lead author Archismita Misra reports. POM-IL1 demonstrated a superior protective effect overall, proving that the properties of the liquids can be adapted.

In a second experiment, the scientists investigated the intactness of the POM-IL layer on the natural stone. For this experiment, the samples were sprayed with simulated acid precipitation for a period of three hours. The protective effect of the coating was confirmed when the samples were subsequently weighed. Despite the aggravated conditions, the coating remained mechanically and chemically intact.

Protective film also effective against bacteria

Researchers also investigated whether POM-IL coatings can provide protection against microbes and thus against harmful biofilms. Using various methods (including cell growth studies, electron-, fluorescence- and confocal microscopy), they tracked the growth and activity of bacteria, including E. coli, on treated and untreated stone samples. "By counting the bacteria colonies, we were able to prove the effectiveness of the new coating against biofilms: POM-IL reduced the number of microbes considerably, and the remaining bacteria displayed 'symptoms of stress'", sums up Professor Carsten Streb.

As a result, the research team developed a multi-functional, transparent protective film for commonly used natural stones. The liquid is easy to apply and is effective against both acids and bacteria. The film also withstands extreme weather influences and can be adapted to various conditions by altering the cation. In order to evaluate the long-term effects of POM-IL, however, it will be necessary to carry out further studies under real conditions. Close attention should be paid to the effectiveness of the liquid against fungi.

The research group from Ulm, Zaragoza and Reims continues to receive funding from Ulm University as well as Spanish sponsors.

Misra, A. , Franco Castillo, I. , Müller, D. P., González, C. , Eyssautier‐Chuine, S. , Ziegler, A. , de la Fuente, J. M., Mitchell, S. G. and Streb, C. (2018), Polyoxometalate‐Ionic Liquids (POM‐ILs) as Anticorrosion and Antibacterial Coatings for Natural Stones. Angew. Chem. Int. Ed. doi:10.1002/anie.201809893

Text and media contact: Annika Bingmann

Ulm Minster (Symbolic image: Rosa Grass)
Researchers coated various stone samples (BB, RO, DO) with POM-IL1 or POM-IL2 and then subjected them to acetic acid vapour. The image shows that the samples coated with the corrosion shield exhibit very little damage (column 1, column 2). In comparison, the untreated reference samples (column 3) were heavily damaged following the vapour treatment. (Image: Institute of Inorganic Chemistry I)
Prof Carsten Streb performs research at the Institute of Inorganic Chemistry I at Ulm University. (Photo: Eberhardt/Uni Ulm)