How two water molecules dance together
The team led by Professor Martina Havenith from Ruhr-Universität Bochum and Professor Joel Bowman from Emory University in Atlanta, together with colleagues from Radboud University in Nijmegen and Université de Montpellier, describe the work in the journal Angewandte Chemie International Edition on 27 July 2019.
Water is the most important solvent in chemistry and biology and possesses an array of strange properties — for instance, it reaches its highest density at four degrees Celsius. This is due to the special interactions between the water molecules. “Describing these interactions has posed a challenge for research for decades,” says Martina Havenith, head of the Bochum-based Chair of Physical Chemistry II and spokesperson for the Ruhr Explores Solvation (Resolv) Cluster of Excellence.
Experiments at extremely low temperatures
The team investigated the simplest conceivable interaction, namely between precisely two individual water molecules, using terahertz spectroscopy. The researchers send short pulses of radiation in the terahertz range through the sample, which absorbs part of the radiation. The absorption pattern reveals information about the attractive interactions between the molecules. A laser with especially high brightness, as is available in Nijmegen, was needed for the experiments. The researchers analysed the water molecules at extremely low temperatures. To do this, they successively stored individual water molecules in a tiny droplet of superfluid helium, which is as cold as 0.4 Kelvin. The droplets work like a vacuum cleaner that captures individual water molecules. Due to the low temperature, a stable bond occurs between two water molecules, which would not be stable at room temperature.
This experimental setup allowed the group to record a spectrum of the hindered rotations of two water molecules for the first time. “Water molecules are moving constantly,” explains Martina Havenith. “They rotate, open and close.” However, a water molecule that has a second water molecule in its vicinity cannot rotate freely — this is why it is referred to as a hindered rotation.
A multidimensional energy map
The interaction of the water molecules can also be represented in the form of what is known as water potential. “This is a kind of multidimensional map that notes how the energy of the water molecules changes when the distances or angles between the molecules change,” explains Martina Havenith. All the properties, such as density, conductivity or evaporation temperature, can be derived from the water potential. “Our measurements now allow the best possible test of all potentials developed to date,” summarises the researcher.