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Stealth Molecules Reveal Hydrophobic Force

By Stephen Luntz

Monash University researchers believe they have provided the first accurate measurement of the force that shapes biological molecules and allows non-stick frying pans.

The hydrophobic force causes water molecules to clump together while avoiding oily or waxy surfaces if possible. However, 30 years of attempted measurements have produced highly varied figures, both for how strong the force is and for the distance over which it operates. Dr Rico Tabor says the problem has been that surface roughness and other forces, such as the van der Waals force that allows geckos to stick to ceilings, get mixed up in the measurements, and estimates of these then need to be subtracted.

Tabor has come up with a new way to measure the hydrophobic force on its own by introducing carefully designed droplets of oil into water in such a way that all other forces are eliminated.

In order to do this, Tabor says it is necessary to have droplets with the same refractive index as water. No such pure oil exists, but Tabor combined one oil with a higher refractive index with one with a lower index in such a ratio that the mixed index is identical to water.

Using these “stealth” molecules, Tabor found that the force exists over a distance of just 0.3 nm – about the size of a water molecule when part of a liquid. This distance is in line with theoretical modelling, but much smaller than many past measurements.

“The value of knowing this is that it gives more understanding of the true nature of hydrophobic behaviour, and also gives us the opportunity to better model protein folding,” Tabor says. “It could also be used for microfluidics, which is becoming important with lab-on-chip analysis.

The finding was published in the Journal of Physical Chemistry Letters.

“The origin of the force is that water molecules like to be surrounded by other water molecules,” Tabor says. “Hydrophobic surfaces disrupt the hydrogen bonds to create a structure so that water molecules readjust to minimise contact with the surface.”

An oil droplet with the same refractive index as water has been used to measure the strength and distance of repulsion between water molecules and substances with which they cannot bond.