Puzzles solved by Van der Waals force

Attraction between uncharged molecules goes further than expected

The researchers described the van der Waals force as an interaction between waves, rather than between particles. © Fritz Haber Institute of the MPG
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The van der Waals force acting between atoms and molecules is stronger than expected. Their range is up to 100 nanometers, instead of just one nanometer. This was discovered by physicists when they first calculated the force using quantum mechanical wave functions instead of classical models. Their findings, published in the journal "Science", also clarify the hitherto enigmatic discrepancies between measurements and models.

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Van der Waals forces ensure that gases below a certain temperature condense to form liquids. They give glue its adhesive power and let a gecko hang upside down on a wall. These electrostatic forces, acting between particles, act between polar molecules, but also between electrically neutral atoms and molecules, such as methane. This works because electrons temporarily shift in the shell of the atoms, thus easily polarizing the molecule.

Although the van der Waals forces have long been known, there is a problem with their physical and mathematical description: according to the classical model according to nimtm, this force acting between individual atoms decreases with the seventh power of the distance. This means that this attraction should only be about a nanometer wide.

Waves instead of particles

Why this is so, Alberto Ambrosetti from the Fritz Haber Institute of the Max Planck Society in Berlin and his colleagues now have their theoretical reasons for the first time. Their calculations are based for the first time on a quantum mechanical model. display

The clou: The physicists considered the movement of the electrons around the atomic nucleus as wave functions, as usual in quantum physics. The Van der Waals force interprets them as an interaction of these waves in two adjacent atoms "When two molecules or nanostructures approach each other, then the waves, ie the respective electron oscillations, interact with each other, " explains senior author Alexandre Tkatchenko from Fritz-Haber -Institute.

Up to 100 nanometers wide

The new calculations show that the van der Waals force actually decreases much more slowly with distance than previously assumed. "This attraction works for up to 100 nanometers, " says Tkatchenko. Another new insight: The degree of decrease is by no means constant, but in turn varies with the distance.

For their model, the researchers calculated, among other things, which forces act between two planar graphene layers, how strongly carbon nanotubes attract and how strong the attraction between a chain of carbon atoms and a protein molecule is. Their results agreed well with the experimental findings of other researchers.

Meaning for many applications

According to scientists, this is a real paradigm shift in the description of the van der Waals interaction. However, the new findings are also of great importance for specific applications, such as adhesive research or materials technology.

"Take an airplane: today's components are increasingly based on polymer materials, " explains Tkatchenko. "The way in which the individual polymer molecules assemble upon solidification is largely determined by the van der Waals forces acting between them." Also for drug developers in the pharmaceutical industry, for example A better description of Van der Waals forces may be helpful. Because they decide how well a drug molecule binds to a target structure in the organism. (Science, 2016; doi: 10.1126 / science.aae0509)

(Max Planck Society, 11.03.2016 - NPO)