Issue Date: December 8, 2014
A Better Measure Of Van der Waals Forces
Van der Waals interactions are attractive forces that arise from fluctuations in electron distribution. Such forces are weak and difficult to study experimentally. Nevertheless, they are critical to many areas of chemistry, such as in molecular self-assembly and material adhesion. A team led by Christian Wagner of Germany’s Jülich Research Center has measured for the first time the long-range van der Waals force between molecules and a surface (Nat. Commun. 2014, DOI: 10.1038/ncomms6568). They did the experiments by using an atomic force microscope tip to bind to a single π-conjugated polynaphthalene molecule lying flat on a gold surface, then using the tip to drag the molecule into a vertical position and away from the surface. The interaction between the molecule and the surface was reflected in the oscillation frequency of the AFM tip. Investigating three different polynaphthalenes, Wagner and colleagues found that the van der Waals interactions increased nonlinearly with size such that larger molecules had about 10% greater attraction to the surface than expected from their number of atoms. The researchers think that’s likely because greater electron delocalization makes the molecules easier to polarize. The findings confirm results from computationally expensive modeling and point to a need to incorporate the effects in more routine computations, Wagner says.
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