Local order in liquids
Long suspected molecular ordering confirmed by ultrafast laser spectroscopy
In liquids there is, contrary to the common perception, no complete disorder at the molecular level. This is shown by Matthijs Panman and his colleagues at the Van 't Hoff Institute for Molecular Sciences (HIMS) in Amsterdam, who recently detected a molecular ordering in liquid alcohol. By means of picosecond laser spectroscopy the researchers established that the angle between the oxygen-hydrogen bonds of two neighbouring molecules is usually about 120 degrees. This week Physical Review Letters published their work, sponsored by the Dutch Foundation for Fundamental Research on Matter FOM.
In school we learn that molecules in a liquid are randomly arranged. But is that completely correct? Based on computer simulations scientists have been suspecting for many years that molecules in many liquids are locally ordered, especially in liquids such as water or alcohol where the molecules show a strong interaction with each other.
The HIMS researchers have now confirmed this assumption by establishing a local orientational order in alcohol (ethanol). They show that nearby molecules are ordered at a fairly well-defined angle with respect to each other - although there is no long-distance ordering like in crystals.
Light and vibrations
In collaboration with the Amsterdam Center for Multiscale Modeling, Matthijs Panman, Sander Woutersen and Bernd Ensing devised an experiment to observe the local ordering. They illuminated alcohol with polarised infrared light pulses with a duration of about one picosecond (one trillionth (10-12) of a second).
Each alcohol molecule contains a bond between its oxygen atom (O) and hydrogen atom (H). If the infrared light has the right frequency, these O-H bonds in the alcohol molecules can start to vibrate. They resonate, lengthening and shortening in-sync with the light wave.
In the polarised laser beam the electric field of the light has a fixed direction and only the O-H bonds parallel to this its electrical field are affected. However, when these bonds start to vibrate they can pass on their vibration to O-H bonds in neighbouring molecules, since all O-H bonds have the same resonance frequency.
The transfer of the vibration is orientation independent which means that also in the O-H bonds which are not parallel to the electrical field a vibration is induced. As a result of this there is a loss in vibrational directionality, which can be detected in the experimental set-up. The strength of this loss in directionality depends on the angle between the O-H bonds of the neighbouring molecules.
By comparing the results of the laser measurements with computer simulations of the liquid, the researchers were able to estimate the average angle between neighbouring molecules. In this way they discovered that neighbouring O-H bonds in alcohol are at an angle of about 120 degrees with respect to each other.
In other liquids other angles were established: for example in the liquid N-methylacetamide the bonds between the nitrogen atom and the hydrogen atom of neighbouring molecules were shown to be roughly parallel with each other.
The researchers explain the local order in both liquids through the interactions (hydrogen bonds) between the neighbouring molecules. The local order only exists for several trillionths (10-12) of a second, because the network of hydrogen bonds is highly dynamic: hydrogen bonds between molecules constantly break and form.
The research not only provides a better understanding of liquids and the behaviour of their molecules. A good knowledge of liquids is vital to understanding how chemical reactions take place, so that the efficiency of these reactions can be increased.
M. R. Panman, D. J. Shaw, B. Ensing, and S. Woutersen: Local Orientational Order in Liquids Revealed by Resonant Vibrational Energy Transfer Phys. Rev. Lett. 113, 207801 – Published 12 November 2014.