Air bubbles in water glue dirt particles together.© Punchstock

Detergent-free cleaning is being developed by researchers in Australia. Oil and grease can be washed away with nothing but pure water, they say.

Oily dirt does not normally dissolve in water. But Richard Pashley and colleagues at the Australian National University in Canberra have shown that water is much better at dispersing oily substances if any air dissolved in it is removed, in a process called degassing.

Water can be degassed cheaply and efficiently by pumping it through a porous membrane, the researchers say, and then sprayed on to surfaces and fabrics. This could reduce our use of detergents, which create environmental problems when they are washed into the water system: detergents can fertilize algal growth so much that animals in swamps and lakes are harmed.

Pashley and his team tested normal distilled water and degassed water by filling oily test tubes with water and shaking them for several seconds.

The tubes of degassed water became much more turbid, the team reports in the Journal of Physical Chemistry B¹. This shows that the oil was dispersed throughout the water as tiny droplets, and suggests that degassed water could lift oily stains off clothing.

Bubble trouble

Normal water contains tiny nitrogen and oxygen bubbles. These accumulate on the surface of water-repellent, or hydrophobic, materials that are in contact with the water.

This layer of gas molecules causes larger bubbles to form between hydrophobic surfaces. And the surface tension of any bubble bridging two surfaces pulls the particles together.

In effect, air bubbles act like a glue that prevents oily substances from breaking up. This makes it harder for an oil droplet to detach from a greasy stain and become dispersed in water. But if the tiny bubbles are removed, that happens more easily.

Conventional detergents surround grease droplets with a layer of detergent molecules, giving them a water-soluble coating.

The team removes gas from water samples using several cycles of freezing and vacuum pumping. But they say that "industrially, other methods may be more appropriate", such as passing the water through a membrane made from hydrophobic material, which would absorb dissolved gas.

Degassed water reabsorbs gases when exposed to the air, but the researchers say that this would happen so slowly that it would not harm the cleaning power of freshly degassed water sprayed on dirty material. In fact, as air seeps into the used water, it could cause dirt particles in the water to stick together, making them easy to filter out. 

• References

  1. Pashley, R. M., Rzechowicz, M., Pashley, L. R. & Francis, M. J. J. Phys. Chem. B 109, 1231−1238 (2005). | Article | ChemPort |