A natural biopolymer, chitosan (a kind of polysaccharide obtained from a chitin shell such as the shrimp’s), which is water-soluble, has been chemically modified by researchers at the Indian Institute of Technology (IIT) Guwahati to selectively remove either an oil or water phase from an oil-water mixture. This becomes possible by making the chitosan-based material, also biodegradeable, to exhibit either an extremely water-repelling property in air (like the lotus leaf) or an extremely oil-repelling property under water (like a fish scale).
In a breakthrough, the researchers have also made it possible to switch the chitosan-based material’s property — from being extremely water-repelling to extremely oil-repelling and vice-versa — by treating it with certain chemicals. It is also possible to repeatedly switch from one property to another.
Fabrication
To prepare the water or oil repelling chitosan, a team led by Dr. Uttam Manna from the institute’s Department of Chemistry and Centre for Nanotechnology first converted the material into nanoparticles and then to a stable gel material by treating it with a chemical (5Acl). This gel was found to have chemically active residues (amines and acrylate), which when treated with a small amine resulted in optimisation of the two very different properties in the same material.
Says Nirban Jana from the institute’s Department of Chemistry and first author of a paper published in the journal Chemistry of Materials , “This is the first time that the liquid repellency property of the material is made switchable, from superhydrophobic to superoleophobic under water and back to superhydrophobic by treating the material at low pH and ethanol, respectively.”
The chitosan — which is converted into a stable gel — allows the researchers to selectively remove the oil or water phase from an oil-water mixture by making the material either superhydrophobic or superoleophobic, respectively. For example, if the oil spill (in water) is less, the material can be made water-repelling to remove or collect the oil. In case the spill is huge and the water phase relatively less, the material can be made extremely oil-repelling to collect or remove water.
Property switch
By treating the material with acid (pH 1) for about 15 minutes, the team (led by Dr. Uttam Manna from the institute’s Department of Chemistry) was able to completely switch the property of the material — from being extremely water-repelling to becoming extremely oil-repelling under water.
Similarly, by treating the biopolymer with ethanol for 10 minutes followed by air drying, the team was able to switch the property from being oil-repelling to becoming water-repelling.
Says Dr. Manna, “The water contact angle of the superhydrophobic biopolymer is over 152º and the oil contact angle under water is nearly 159º.” The higher the contact angle the greater is the liquid repellency of the material.
Superior performance
The researchers tested the ability of the biopolymer to separate oils — kerosene, motor oil, olive oil and even crude oil — of different densities from water. Says Dr. Manna, “Under water, we were able to completely remove even crude oil from the water phase. The selective separation efficiency for both oil and water phases was above 95% immaterial of the viscosity of the oil.”
The biopolymer’s superhydrophobic property remained intact under diverse chemical conditions such as extreme pH (pH 1 and pH 13), sea and river water for seven days, and high (100º C) and low (10º C) temperatures.
The material was found to retain both hydrophobicity and oleophobicity even when the top surface of the biopolymeric material was physically abraded using sand paper. Despite the abraded surface being cleaved through manual peeling using an adhesive, the liquid repellence property remained intact. No change in this was seen after the mechanically damaged material was subjected to even a continuous stream of sand grains. Exposure to UV light for a month too did not destroy this repellence property.
