Van der Waals Interactions Measured for the First Time
Physicists from the University of Basel were able to measure van der Waals interactions between single atoms. These interactions are the weakest among chemical bonds, but can be seen in our daily lives.
You might remember them from your high school’s chemistry and physics lectures. Van der Waals interactions are one of the weakest interactions known to our world. Nevertheless, they are very important and you can find them literally everywhere around you.
They influence the way molecules behave, how they interact with each other and create highly ordered complicated structures. Therefore, they can also affect the functionality of such complicated structures.
Scientists from the Swiss Nanoscience Institute and the University of Basel managed to measure these interactions between individual atoms — something that has never been done before.
Their results were published in the highly prestigious journal Nature Communications.
Everywhere around us
Van der Waals interactions are weaker than any other type of chemical bond. Inspite of being so weak they influence the three dimensional structure of proteins or even the overall shape of polymer structures. They can be found between water molecules, or individual sheets of graphite, which are really soft thanks to very low strength of these interactions.
All these examples might be quite hard to imagine. However, Van der Waals forces can be seen on a bigger scale too. One of the most common examples is the gecko — a lizard which is able to climb on very smooth surfaces.
The gecko owes its Spiderman-like abilities to nothing else than van der Waals interactions. Combined strength of these interactions between the surface and microscopic hair on its legs allow the gecko to climb even on glass. The amount of hair is quite astonishing too; each square centimeter of a gecko’s foot contains about 150'000 of them.
Noble gases
In order to measure the van der Waals forces between individual atoms, Basel scientists had to choose an adequate material for measurements. They decided to measure atoms of inert gases.
According to Shikegi Kawai, former postdoc at the University of Basel and leading author of the study, “the inert gases have a closed shell. If we would have used a different type of atom, another interactions could be formed, like covalent bonds. Then we could not directly measure van der Waals interactions only. This is the reason why we chose them.”
The atoms of various inert gases were trapped into a two-dimensional layer of organic material with a copper layer on top. These copper atoms created structures similar to bird nests. The copper layer contained equally distributed nano-hollows.
Far from being far
“This technology was developed in collaboration with another University of Basel research group. They actually sit right next door. It was not far to get there,” added Kawai with a smile. When they deposited the noble gases onto the copper layer they found out that they go directly into the copper nests and become stable.
If they afterwards positioned an AFM tip containing a single xenon atom right above these atoms and moved it repeatedly up and down, they measured weak interactions being formed between them.
Confirmed calculations
The scientists have performed a lot of calculations prior to these experiments. These helped them to predict the intensity of van der Waals interactions. Their experiments indeed confirmed the calculated values; the values even dropped radically with increasing distance between the measured atoms.
“We got a quite good match between the calculated data and the actual experiment,” Kawai affirmed. “But when we used xenon –xenon interactions, we got up to two times higher values than expected.”
Authors have explained this phenomenon in their article by a formation of a weak covalent type interaction. That is a very interesting hypothesis, as these types of interactions are not expected to be formed between these atoms.
“Scientists in Basel have been trying to measure the van der Waals interactions for over 15 years. It was actually one of the fundamental research goals of using AFM,” added Kawai.
Scientists can now begin to understand the properties of van der Waals interactions. These understandings can be transferred to other fields of study or technologies, which produce materials relying on weak interactions, such as diffusion, adsorption or 3D arrangement.
Available from doi: 10.1038/ncomms11559.
The University of Basel has an international reputation of outstanding achievements in research and teaching. Founded in 1460, the University of Basel is the oldest university in Switzerland and has a history of success going back over 550 years. Learn more
