Finding a surprising degree of order on the surface of miniscule water droplets

Nanoscopic and microscopic water droplets in hydrophobic (water repellent) environments surround us. Additionally, biological processes depend on the interactions of water droplets with other interfaces – indeed, 60% of the human body itself is made up of water. The EU-funded project WII (Water, Ions, Interfaces) has now helped shed more light on the quantum effects in water and on the range of interfaces with which water droplets interact.

WII was set up to add to the stock of existing knowledge about how the structural, dynamic, and biological properties of water contribute to the functioning of living systems, as well as in the hope of sparking further technological innovations which could harness nature’s design. Members of the WII project team recently reported in Nature Communications their discovery that molecules on the surface of water droplets were surprisingly more organised than had been previously thought.

Water as a stage for molecular dramas

The published study set out to better understand the properties and behavior of water droplets when surrounded by a hydrophobic substance such as oil, as these have implications for the functioning of the whole the aqueous system. Knowledge about water droplets is usually inferred from data gleaned from studies looking at macroscopic air/water interfaces or from aqueous solutions of solvated hydrophobes. As the authors point out, this is an inexact methodology due to the differences in size, chemical composition and temperature dependence when compared to the environments of actual miniscule droplets, typically only one thousandth of a hair thick.

The WII researchers developed a unique method for examining the surface of these miniscule droplets. According to the WII project coordinator Professor Sylvie Roke, ‘The method involves overlapping ultrashort laser pulses in a mixture of water droplets in liquid oil and detecting photons that are scattered only from the interface.’ She goes on to explain that, ‘These photons have the sum frequency of the incoming photons and are thus of a different colour. With this newly generated colour, we can determine the structure of the interface.’

The researchers found that the water droplet surface is much more ordered than that of normal water, being somewhat comparable to that of ice, with molecules which have very strong hydrogen bonds, with a stable tetrahedral (pyramid shaped) configuration surrounding each molecule. Unexpectedly, this arrangement also appeared on the surface of the miniscule droplets even at room temperature, that is 50°C higher than the temperature at which the phenomenon would have been expected.

It is known that water’s 3D hydrogen bonded network while being cooperative, also rearranges itself every few femtoseconds (one quadrillionth of a second) with quantum level interactions determining its properties. One of WII’s key contributions has been to focus on this quantum level/femtosecond time scale going up to the macroscopic level/time scale. These findings have the potential to shed light on a range of atmospheric, biological and geological processes.

For more information, please see:
CORDIS project webpage