Water on the surface moves surprisingly fast
Water molecules at the surface of a water mass rotate far faster than water molecules within it. This unexpected result was observed by researchers from the FOM Institute AMOLF and the University of Amsterdam, who filmed the rotation of water molecules at the surface in a unique manner. This new image of rapidly moving molecules complements the existing known model of the static structure of water. Even though more than half of the earth's surface is covered by water, this is the first study into the movement of molecules at the air/water surface. The researchers published their results on 29 August 2011 online in the renowned journal Physical Review Letters.
Water does not stand still
Water is a unique fluid, which is characterised by a strong interaction between the water molecules due to so-called hydrogen bonds. The molecular structure of the surface of water is well known but incomplete. The surface is just a single molecule thick and has two types of water molecules: either one or two OH groups that form hydrogen bonds (see Figure 1). Water molecules away from the surface have two OH groups that form hydrogen bonds. The presence of water molecules at the surface which have just one OH group that forms hydrogen bonds - and therefore have a 'free' OH group - is therefore unusual. This picture of water surface structure is incomplete because in reality water molecules do not remain stationary: the position and orientation of the water molecules changes on a timescale of picoseconds (0.000000000001 second). A full understanding of the surface therefore requires a sequencing of images: in other words a film of moving surface water.
Technical feat
The researchers made such a film by following the rotation of water molecules at the surface with the aid of lasers. They used an extremely short laser pulse to 'switch on' the vibration of the free OH group of the outermost layer of water molecules at the surface. With a second laser pulse they followed the movement of the molecules over the course of time. Using this technique, the researchers could specifically examine just the molecules at the water surface: a considerable technical feat. The data obtained were combined with computer simulations to produce a detailed image of the speed and mechanism of water rotation at the surface.
Quick and supple
Water at the surface was found to rotate over three times faster than water surrounded by other water. This is surprising because dissolving salts or molecules in water nearly always results in the rotational movement of water being slowed due to the disruption of the hydrogen bridge network. The water-air surface is therefore an exceptional boundary surface. The rotational movement at this surface was also found to be far more supple than that in the bulk: in the bulk, water molecules rotate by making irregular large rotational steps, whereas water molecules at the surface rotate in a supple and continuous manner.
The water surface is therefore more dynamic than had been expected. This could be important for many different areas, such as the biophysics of membranes, electrochemistry and atmospheric chemistry. The dynamic nature of the water-air surface plays a major role in processes that take place at similar hydrophobic water surfaces, such as the water-protein surface, in which water appears to play a critical role in the correct folding of the protein.
Contact
For more information please contact AMOLF groupleader Professor Mischa Bonn, bonn@amolf.nl, (020) 754 71 15
Reference
Ultrafast reorientation of dangling OH groups at the air-water interface using femtosecond vibrational spectroscopy
Cho-Shuen Hsieh1, R. Kramer Campen1, Ana Celia Vila Verde2, Peter Bolhuis2, Han-Kwang Nienhuys1, and Mischa Bonn1,3, Physical Review Letters, 2011, Vol. 107.
1 FOM Institute AMOLF.
2 Van 't Hoff Institute for Molecular Science, University of Amsterdam.
3 Max Planck Institute for Polymer Research, Mainz, Germany.