Equal charges can attract each other

Opposite charges attract each other, equal charges repel each other - but this does not always apply. Under certain circumstances, charged particles can break these rules, a research group led by Madhavi Krishnan from the University of Oxford has discovered. However, they need a little help, as the team now reports in the journal "Nature Nanotechnology". Particles dissolved in a liquid can interact with its molecules in such a way that like charges attract each other. Such effects can even break the symmetry between the charges: Positively charged particles attract each other, while negatively charged particles repel each other. The discovery has far-reaching consequences for phenomena such as the formation of crystals or interactions between proteins.

It has long been known, especially in biology, that molecules with the same charge or even larger structures sometimes seem to attract each other. What was missing, however, was an explanation for this: all other interactions were considered too weak to compensate for the repulsion of the charges. To solve the puzzle, the team investigated the behaviour of negatively and positively charged nanoparticles of silicate in water and ethanol. The explanation for the strange attraction of particles with the same charge lies in the behaviour of the water on the surface of the particles. Their charges impose a certain order around the charged particles on the water molecules, which also have negative and positive areas.

As the research group discovered, the spatial arrangement of the particles can become increasingly low-energy when two negatively charged particles move towards each other. Up to a certain distance, the energy that this releases is greater than the energy that has to be expended to overcome the repulsion of the same charges. This means that particles with the same charge attract each other over greater distances. As a result, the negatively charged nanoparticles in water form ordered groups due to their attraction. Positively charged particles, on the other hand, do not experience any such energy gain in water; they continue to repel each other.

However, this changes in alcohol. Its molecules arrange themselves on surfaces in such a way that they orientate their positively and negatively charged areas in the opposite direction to those of water. Accordingly, the effect of their rearrangement also has the opposite sign: positive charges attract each other, negative charges do not. The working group also shows how the decisive forces and potentials on surfaces can be measured and predicted. The effect has far-reaching consequences for a whole range of specialist areas and applications. For example, it probably influences the behaviour of many biological structures, changes the solubility of nanoparticles or chemicals or causes things to clump together that should ideally not clump together.

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