For the first time, scientists have succeeded in measuring the strength of the very weak van der Waals forces passing between individual atoms.
Noble gas atoms deposited on molecular network are investigated with a probing tip, which is decorated with a Xe atom. The measurements give information about the very weak van der Waals forces between these individual atoms. Image credit: University of Basel.
According to physicists, van der Waals forces are among the weakest, yet most decisive interactions governing condensation and aggregation processes and the phase behavior of atomic and molecular matter.
Van der Waals interactions arise due to a temporary redistribution of electrons in the atoms and molecules. This results in the occasional formation of dipoles, which in turn induce a redistribution of electrons in closely neighboring molecules.
Due to the formation of dipoles, the two molecules experience a mutual attraction, which is referred to as a van der Waals interaction. This only exists temporarily but is repeatedly re-formed.
The individual forces are the weakest binding forces that exist in nature, but they add up to reach magnitudes that we can perceive very clearly on the macroscopic scale.
To measure the van der Waals forces, a team of physicists led by the University of Basel and Aalto University used a low-temperature atomic force microscope with a single xenon (Xe) atom on the tip.
The scientists then fixed the individual argon (Ar), krypton (Kr) and Xe atoms in a molecular network.
This network, which is self-organizing under certain experimental conditions, contains so-called nano-beakers of copper atoms in which the noble gas atoms are held in place like a bird egg. Only with this experimental set-up it was possible to measure the tiny forces between microscope tip and noble gas atom, as a pure metal surface would allow the noble gas atoms to slide around.
“We measure the paradigmatic van der Waals interactions represented by the noble gas atom pairs Ar–Xe, Kr–Xe and Xe–Xe with a Xe-functionalized tip of an atomic force microscope at low temperature. Individual noble gas atoms were fixed at node sites of a surface-confined two-dimensional metal–organic framework,” the physicists said.
They found that the magnitude of the van der Waals interaction scales with the size of the noble gas atom on the surface, that is, in order of Xe–Xe > Kr–Xe > Ar–Xe.
“Systematic force measurements established the depths of the potential curves for Xe–Ar, Xe–Kr and Xe–Xe at 18.1, 26.1 and 35.9 meV, respectively, which is in agreement with the expected trend,” they said.
“The tails of the interactions of Xe–Ar and Xe–Kr were found to display van der Waals r−6 limiting behavior; however, the effective strength of the interaction was found to depend on the influence of the tip and the substrate on the atomic polarizability, leading to a deviation from the behavior of isolated noble gas atom.”
“For Xe–Xe interaction, we find a deviation from the van der Waals power law, which we interpret as an effect of stronger covalent interaction between the Xe atoms.”
The team is working on the assumption that, even in the noble gases, charge transfer occurs and therefore weak covalent bonds are occasionally formed, which would explain the higher values.
The findings were published May 13, 2016 in the journal Nature Communications.
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Shigeki Kawai et al. 2016. Van der Waals interactions and the limits of isolated atom models at interfaces. Nature Communications 7, article number: 11559; doi: 10.1038/ncomms11559
