It Hates Water

>Water is supposed to be the universal solvent. True, water and oil don't mix. This is because most oils are hydrophobic to one degree or another. (No, that doesn't mean they have rabies. That's a whole different hydrophobia.) That's because, at the molecular level, hydrophobic substances (or hydrophobes) are not electrically polarized, while water is, and because all matter tends toward a low-energy state, the two bond with one another poorly. With this simple rule from chemistry in mind, researchers in Massachusetts have discovered that a substance known as MeSiCl3 can, under the right conditions, form a coating on silicon wafers that is absolutely repellant to water.

We're not talking about your garden variety water-repellant spray coating here. Even those will allow water and the coating to touch each other at varying degrees of what is called their contact angle. On a typical water-repellant coated surface, water will simply rest on the surface at a contact angle greater than 0 degrees and less than 180 degrees—the higher the number, the higher the repellant property. However, when properly prepared, MeSiCl3 coating allows a contact angle of 180 degrees for water. That is to say, they are not in contact with each other at all—achieving superhydrophobicity.

To accomplish this astonishing feat, the Massachusetts scientists had to put MeSiCl3 through some tricky procedures. They had to dip a silicon wafer into a mixture of MeSiCl3 and toluene, and then rinse it with toluene, ethanol, and water, which produced a toluene-swollen, three-dimensional methylsiloxane network that collapsed to form a hydrophobic coating of nanoscale cylindrical fibers when rinsed again with ethanol.

The research, by Lichao Gao and Thomas J. McCarthy, of Polymer Science and Engineering Department of the University of Massachusetts, in Amherst, appears in the current issue of the Journal of the American Chemical Society online. Gao and McCarthy believe their new coating substance could be used in applications that demand perfect water-repelling properties.

An expert in the field of surface chemistry, Manoj Chaudhury, of Lehigh University, in Lehigh, Pa., told British-based Chemistry World magazine: "This is a real breakthrough in the research of superhydrophobicity. The method is very appealing because of its simplicity, and it is expected to find real technological applications in various coatings industries."

So, the next time you're designing something that needs to be perfectly free of the consequences of contact with water, MeSiCl3 might just be the right chemical for you.

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