Surface water 'communicates' ultrafast with underlying bulk water
Surface water and the underlying bulk water interchange energy at lightning speed. Researchers at the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam have found this by using a new technique that they developed. They discovered that the dynamic behaviour of surface water and bulk water is still the same thanks to an ultrafast energy transfer, although the structure of both surface water and bulk water differ. This has led to new fundamental views in the behaviour of water. The researchers have published their findings in Physical Review Letters on 2 March 2007.
More than half of the earth's surface consists of water. We know quite a great deal of the behaviour of bulk water thanks to all kinds of experimental techniques. But we know much less about surface water - specifically the layer one molecule thick at the air-water interface -, because it is very hard to examine the surface layer (less than one nanometre thick). The signals measured in that layer are usually lost in the much bigger bulk signal. In order to pronounce upon the structure and the behaviour of surface water the researchers at AMOLF have combined two techniques: the so-called Sum Frequency Generation (SFG) and the ultrafast 'time-resolved' measurements of water molecules. In this case ultrafast means: faster than about 100 femtoseconds. For your information: one femtosecond is to one second as one second is to about the age of the earth.
Hydrogen bonds
It is possible to map the vibrations of the outer layer of water molecules with the aid of SFG (see figure 1). This technique is surface specific: the signal can only arise from the surface water. Water molecules share the characteristic quality of having a strong interaction together and building hydrogen bonds. The hydrogen-bonded network is being interrupted at the surface. This will lead to considerable differences between the vibrational spectra of bulk water and surface water. In spite of this it is hard to pronounce upon the nature and structure of surface water. Therefore, the researchers used an ultrafast (1 femtosecond = 10–15 seconds) 'time-resolved' measurements, a kind of stroboscopic lighting of the surface with ultrashort laser pulses.
Energy dynamics
When investigating bulk water researchers has often been using 'time-resolved' measurements. With this, molecules that have an intense laser pulse are being taken from an existing situation into an energetic and higher situation. Then it is being investigated how the molecules will 'slip back' into the original situation in time. This process of energy dynamics is strongly dependent on the hydrogen bonds and provides us with detailed data on the structure of water. Thus, previous research has shown that the energy dynamics of water in the neighbourhood of ions is different from the energy dynamics of water that is being surrounded by other water molecules. Also water in little drops behaves very differently from bulk water.
Combined techniques
The researchers were able to describe the energy dynamics of, in particular, the surface water molecules by applying for the first time the ultrafast 'time-resolved' technique in combination with SFG at the surface of water and air. Surprisingly, the energy dynamics of surface water resembles very much that of bulk water, despite the fact that the hydrogen-bonded network at the surface is being interrupted. The researchers have also found an explanation for the similarity between surface water and bulk water: a continuous energy exchange is taking place between bulk water and surface water and vice versa (see figure 2). The researchers call it a continuous ultrafast communication between the molecules at the surface and the molecules in the bulk just below the surface. By discovering this communication pattern it is explained that the energy dynamics at the surface is the same to that in the bulk, despite the difference in structure. The new data provide more insight into the working of the water-air interface. At the moment the researchers use the technique in order to study the contact between water and cel membranes.
The article is entitled: 'Ultrafast vibrational energy transfer between surface and bulk water at the air-water interface' and will be published in Physical Review Letters on 2 March 2007. The authors are: Marc Smits, Avishek Ghosh, Martin Sterrer, Michiel Müller and Mischa Bonn.
For more information, please contact:
Marc Smits, FOM Institute AMOLF, Amsterdam, phone +31 (0)20 608 13 51 or Professor Mischa Bonn, FOM Institute for Atomic and Molecular Physics, Amsterdam, phone +31 (0)20 608 12 34.