Researchers at the Karlsruhe Institute of Technology (KIT) and the Indian Institute of Technology Guwahati (IITG) have used a completely new method to modify metal-organic frameworks (MOFs). These, also known as MOFs for short, consist of metals that are connected by connecting struts of organic molecules to form networks with empty pores, similar to a sponge. Their volume properties are enormous: if you were to unfold two grams of this material, you would get the surface area of a soccer pitch. This makes them interesting for applications such as gas storage, carbon dioxide capture or new technologies in medicine.
Hydrocarbon chains anchored on MOF films
The research team made use of the outer surfaces of these crystalline materials. They anchored hydrocarbon chains on thin MOF films. A water contact angle of more than 160 degrees was observed - the greater the angle that the surface of a drop of water forms with a substrate, the more water-repellent the material is. "Our method produces superhydrophobic surfaces with contact angles that are significantly higher than those of other smooth surfaces and coatings," says Professor Christof Wöll from the Institute of Functional Interfaces at KIT.
Next generation of superhydrophobic materials
The team attributes these results to the brush-like arrangement of the hydrocarbon chains on the MOFs. After anchoring on the MOF materials, these can form "tangles" particularly well - a state of high entropy that is essential for the water-repellent properties. According to the researchers, this state has not been observed for anchored hydrocarbon chains on other materials. Remarkably, the water contact angle was also not increased by perfluorination of the hydrocarbon chains, i.e. replacing the hydrogen atoms with fluorine. In materials such as Teflon, perfluorination leads to particularly water-repellent properties. However, the team reports that the water contact angle of the newly developed material has actually been significantly reduced. Further analyses in computer simulations have confirmed that the perfluorinated molecules - unlike the hydrocarbon chains - cannot assume the energetically favorable state of high entropy.
Adhesion further reduced
The research team also varied the surface roughness of their SAM@SURMOF systems in the nanometer range. This made it possible to further reduce adhesion. Water droplets then begin to roll off at extremely small angles of inclination, and the water-repellent or self-cleaning properties are significantly increased once again. "Our work also provides a comprehensive theoretical analysis linking unexpected experimental behaviors to the high entropy state of the 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."
The study was published in the journal Materials Horizons.