Researchers from the Indian Institute of Science Education and Research (IISER) Pune, the Central Salt & Marine Chemicals Research Institute (CSMCRI), Bhavnagar and National Chemical Laboratory (NCL), Pune have developed a membrane with exceptional hydrophobic and extremely high oil-loving (oleophilic) properties. The superior water-repellent and oleophilic membrane can potentially be used for tackling the globally challenging issue of marine oil spills.

Such is the hydrophobic nature of the synthesised metal-organic framework (MOF) that the water contact angle is nearly 176 degrees — the “first example of an ultrahydrophobic MOF.” At the same time, the MOF membrane has a superior affinity for oil (oleophilicity) with an oil contact angle of nearly zero degree. The results were published in the journal Chemistry - A European Journal.

“The membrane was fabricated by mixing bulk MOF material with a binder and solvent and spray coated on to an inexpensive polypropylene substrate. The substrate is stable in organic solvent,” says Ankit M. Kansara from CSMCRI and one of the authors of paper. “The membrane is 100-120 micrometre in thickness.”

“The MOF is inherently ultra-microporous in nature and the porosity is retained when the thin film-like membrane is formed on the matrix. Because of the highly porous nature of the material, the coated MOF’s surface area is as high as 1,000 metre sq per gram,” says Soumya Mukherjee from IISER and the first author of the paper.

The ultrahydrophobicity was achieved by synthesising the MOF with a high density of fluorine; fluorine is inherently hydrophobic in nature and any material that is fluorine-rich becomes hydrophobic. By virtue of being highly hydrophobic, the MOF membrane, by default, becomes distinctly oleophilic or oil-loving in nature. The marked influence of fluorines was strongly supported by theoretical insights provided by Dr. Arnab Mukherjee from IISER and a coauthor of the paper.

“The use of more fluorine makes the MOF water stable. Water stability is a prima facie criterion for industrial applications and being environmentally benign,” says Mr. Mukherjee.

When water-oil mixture is passed through the membrane the oil permeates by rapid absorption, while water is retained above the membrane. “The oil permeation was 100 per cent in the case of an oil-water mixture,” says Prof. Sujit K. Ghosh from IISER and the corresponding author of the paper. “So if you put the membrane in an oil-water mixture, it can perfectly separate oil from water. The membrane acts like a filter.”

Water-oil emulsification takes place in the sea when water gets mixed with oil under high water current conditions. “It is very difficult to separate oil and water from an emulsion. So in another experiment, the oil was completely separated from water when we passed the water-oil emulsion through the membrane,” says Mr. Mukherjee. The emulsion droplets demulsified at the very instant they touched the membrane, and oil passed through while water was wholly retained above the membrane. The separation of water-oil emulsion was totally driven by gravity with no external force applied. The best part is the recyclability traits of the membrane. When external mechanical force in the form of ultrasonic waves is applied in the presence of a hydrophobic organic solvent, the constituents of oil come out of the pores of the oil-saturated membrane. “The oil tends to come out due to the presence of competing hydrophobic molecules during the ultrasonification process lasting 30-60 minutes, depending on the size of the membrane and volume of oil absorbed,” says Mr. Mukherjee.

The membrane is then heated at 70 degree C to remove the organic solvent and quickly regenerate the MOF. The organic solvent tends to evaporate after some time, even if the membrane is not heated. “There was 100 per cent removal of oil from the membrane. We were able to get back all the oil used,” Mr. Mukherjee adds.