In the right solution, like charges can attract like opposites

The principle that opposite charges attract and like charges repel has some exceptions, chemists have shown, which complicates what is considered a fundamental law of physics. Few things make chemists happier than convincing physicists, so the discovery must have caused great joy for that reason alone, and there may be significant biological implications as well.

The observation that opposites attract and repel each other has found its way into electrostatics and magnetism. Applied to human interactions, it’s so familiar that we sing about it. However, we know that for psychology these patterns are highly conditional, and it turns out the same is true at the molecular level.

“Because like-charged objects in vacuum are expected to repel regardless of whether the sign of the charge they carry is positive or negative, it is expected that like-charged particles in solution should also repel monotonically,” says a team led by Professor at the University of Oxford. Madhavi Krishnan writes. However, they have proven this to be wrong.

Led by Krishnan, two graduate students discovered that negatively charged particles can attract each other in slightly acidic water with low salt concentrations.

Even stranger: Positively charged particles continue to repel each other, at least until you get them drunk. Once the charges are placed in an alcohol solution, the positive charges exhibit long-range attraction, while the negative ones return to normal repulsion.

Krishnan and colleagues applied charges to silica microparticles 5 micrometers (0.0002 inches) wide and suspended them in water or two types of alcohol. Electrostatic repulsion pushed particles with the same charge apart, according to laws established in the 19th centurye century. Nevertheless, negatively charged silica particles in water formed themselves into hexagonal clusters, although the effect disappeared when the pH of the water was outside a narrow window, about 5-6.5. The clusters contain voids, proving that the attractions can occur even when they are not in direct contact. In ethanol or isopropanol it was the negatively charged particles that came together.

Something else, which they call the “electrosolvation force,” overcame normal repulsion, but only in the right liquid.

We know that electrorepulsion can be overcome, otherwise atoms could not have more than one proton in a nucleus. However, the strong force that makes large nuclei possible only works on a scale of 10-15 meters. The van der Waals force, best known for allowing geckos to cling to the ceiling, acts on a much larger scale than the strong force, but is still too weak at the range studied here to be responsible .

Although the effect is highly dependent on the fluid in which the charges are distributed, the particles themselves do not seem to matter as long as they carry the charge. The team repeated their results with polypeptide- and polyelectrolyte-coated surfaces, and even reversed the effect by successively coating particles in layers carrying a positive or negative charge.

Experiments like this rarely come out of nowhere: it’s difficult to get funding for research that you have no reason to believe will happen. The authors note: “Over the decades, consistent reports have been made of the attraction between similarly charged particles from the nanometer to the micrometer scale.” Many theoretical studies have attempted to explain these observations, without complete success.

Previous examples involved more complex materials, including nucleic acids, the building blocks of life. The Oxford team tried to simplify, which should make it easier to find the cause. They predicted in an earlier paper that a solvent “can make a substantial contribution to the total interaction free energy of two approaching objects carrying electric charges,” and this could be sufficient to attract similar charges. Now they have their proof.

Aggregations of silica microparticles may not be important in themselves, but the authors note that proteins with net negative charges have been reported to clump together, likely caused by the same effects. The authors even suggest that the clustering and self-assembly we see here may have been relevant to the coming together of molecules that gave rise to the first life.

First author Sida Wang said in a statement: “I still find it fascinating to see how these particles attract each other, even after seeing this a thousand times.”

The research has been published open access in Nature Nanotechnology.

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