Metals Materials repel water almost completely

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Researchers at the Karlsruhe Institute of Technology (KIT) and the Indian Institute of Technology Guwahati (IITG) have modified metal-organic frameworks (MOFs) with a new process so that they can almost completely repel water.

Metal-organic frameworks (MOFs) consist of metals connected by linkages of organic molecules into networks with empty pores, similar to a sponge. Their volumetric properties—where two grams of this material, if unfolded, would cover the area of a football field—make them interesting for applications such as gas storage, carbon dioxide capture, or new technologies in the medical field.

Contact angle significantly higher than smooth surfaces and coatings

However, the external surfaces of these crystalline materials also offer unique opportunities, which the research team from the Karlsruhe Institute of Technology (KIT) and the Indian Institute of Technology Guwahati (IITG) have now utilized with a new idea: they anchored hydrocarbon chains on thin MOF films. A water contact angle of more than 160 degrees was observed—the larger the angle formed by the surface of a water droplet with a substrate, the more water-repellent the material is. "Our method produces superhydrophobic surfaces with contact angles significantly higher than those of other smooth surfaces and coatings," says Professor Christof Wöll from the Institute of Functional Interfaces at KIT. "While the wetting properties of MOF powder particles have been studied, the use of homogeneous MOF thin films for this purpose is a groundbreaking concept." These water-repellent properties are of interest for applications such as self-cleaning surfaces, which need to be robust against environmental influences, for example, in automobiles or architecture. 

Next generation of "superhydrophobic" materials

The team attributes these results to the brush-like arrangement (polymer brushes) of the hydrocarbon chains on the MOFs. These can form particularly good "tangles" after being anchored to the MOF materials—a state of disorder that science refers to as a "high entropy state," which is essential for the water-repellent properties. The researchers noted that this state has not been observed for anchored hydrocarbon chains on other materials.

Remarkably, the water contact angle did not increase even with the perfluorination of the hydrocarbon chains, which involves replacing hydrogen atoms with fluorine. In materials like Teflon, perfluorination leads to particularly water-repellent properties. However, in the newly developed material, it significantly reduced the water contact angle, according to the team. Further analyses in computer simulations confirmed that, unlike the hydrocarbon chains, the perfluorinated molecules cannot assume the energetically favorable high entropy state.

In addition, the research team varied the surface roughness of their SAM@SURMOF systems in the nanometer range. This further reduced adhesion. Water droplets then start to roll off at extremely small inclination angles, significantly enhancing the water-repellent or self-cleaning properties.

"Our work also provides a comprehensive theoretical analysis that links unexpected experimental behaviors to the high entropy state of molecules attached to MOF films," says Professor Uttam Manna from the Department of Chemistry at IITG. "This study will change the design and production of the next generation of materials with optimal hydrophobic properties."