By tweaking the composition, the researchers were able to make the coating highly oil-absorbing as well.
Inspired by fish scales that exhibit excellent property to remain oil-free even when the water is contaminated with oil, scientists at the Indian Institute of Technology (IIT) Guwahati have developed a special kind of superior oil-repulsive (oleophobic) coating. When applied to any material — wood, glass or metal objects — the coating keeps the surface free of oil contamination under water.
By tweaking the composition of the coating, the team led by Dr. Uttam Manna from the Department of Chemistry at IIT Guwahati, has been able to make the coating also extremely oil-loving (oleophilic) under water. The oleophilic coating applied on objects will be helpful in removing oil from oil-contaminated surfaces or for absorbing oil from ocean oil-spills.
One gram of cotton ball coated with the highly oleophilic material was able to absorb over 1,000 grams of oil, irrespective of the density of the oil, the researchers found. The results were published in the journal Chemical Science.
Researchers have so far tried to achieve oleophobic property under water by using hydrogel and metal oxide coating that mimics the physical structure of fish scale. But such substances do not have long-term stability under water as the oleophobic property is lost once the topography of the coating gets damaged.
In contrast, the coating developed by the IIT researchers was stable even at extreme temperatures — up to -15 degree C and 100 degree C. It was also found to be stable for the duration of test (30 days) when the pH of water was reduced to 2 (highly acidic) and increased to 11 (highly alkaline). The property remained intact even when kept in artificial sea water for 80 days. The property was unaffected when the top surface of the coating was physically removed using sand paper, scratched with hand, and even after a sand-drop test.
“All these tests showed the physical durability and chemical stability of the coating. The oleophilic and oleophobic property was unaffected when subjected to various physical and chemical tests,” says Dr. Manna.
Preparing the coating
Two reactants (branched polyethylene and dipentaerythritol penta-/hexa-acrylate or 5Acl) are mixed together to get a nanocomplex. The nanocomplex has residual reactivity and so the functional groups react with primary amine-containing molecule. To impart more physically durability and chemically stability, the researchers coated the nanocomplex with a porous, durable polymer coating.
“The polymer is covalently cross-linked with the nanocomplex and this imparts chemical stability, while the porosity of the polymer renders the coating physical stability,” says Dr. Manna. “Because of the porosity, the coating maintains the desired property even when abraded, which is extremely difficult to achieve with the current approaches.”
Since the coating is still reactive in nature, it can be exploited strategically to make it extremely oleophilic or oleophobic under water. When treated with octadecylamine small molecule, the material becomes extremely oleophilic and completely absorbs oil. And when treated with glucamine small molecule, the material becomes highly oleophobic with a contact angle of about 171 degrees.
Contact angles
“The oil-contact angle under water is about 64 degree before treating with the small molecules. When treated with octadecylamine small molecule, the oil-contact angle under water drops to zero and oil is completely absorbed. And when treated with glucamine small molecule, the oil-contact angle increases to nearly 171 degrees,” says Dibyangana Parbat from the Department of Chemistry at IIT Guwahati and the first author of the paper.
As a result of the extreme oleophobic property of the coating, the oil bounces when it comes in contact with the coating, while it gets absorbed in the case of the highly oleophilic coating. “The oil gets absorbed and the air trapped in the pores of the polymer coating comes out as bubbles in the case of the super oleophilic coating,” Parbat says.
“We tested the super oleophilicity by applying this coating on a cotton ball. The coated cotton ball was able to instantaneously soak oil under water. The oil does not spill from the cotton ball and can be recovered by physically compressing the cotton ball,” he says. Similarly, the super oleophilic property was tested by coating glass, wood and metal surfaces. The treated surfaces remained oil-free for the entire duration of testing — 90 days.
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