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A new study has overturned a fundamental principle of physics by showing that particles with similar charges can attract each other in a solution, with the effect varying between positive and negative charges depending on the solvent. This discovery has important implications for various scientific processes, including self-assembly and crystallization. The research reveals the importance of solvent structure at the interface in determining particle-particle interactions, challenging long-held beliefs and indicating the need for a reevaluation of our understanding of electromagnetic forces. Credit: Zhang Kang
“Opposite charges attract; Like charges repel” is a fundamental principle of basic physics. However, a new study from the University of Oxford, recently published in the journal natural nanotechnology, has shown that similarly charged particles in solution can, in fact, attract each other over long distances.
Equally surprising is that the team found that the effect is different for positively and negatively charged particles, depending on the solvent.
In addition to overturning long-held beliefs, these results have immediate implications for a variety of processes involving interparticle and intermolecular interactions at various length scales, including self-assembly, crystallization, and phase separation.
The team of researchers, based in Oxford’s Department of Chemistry, discovered that negatively charged particles attract each other at large separations, while positively charged particles repel each other, while the opposite is true for solvents such as alcohols.
These findings are surprising because they appear to contradict the central electromagnetic principle that the force between charges of the same sign is repulsive at all separations.
Experimental observations
Now, using bright-field microscopy, the team tracked negatively charged silica microparticles suspended in water and found that the particles attracted each other to form hexagonally arranged clusters. However, the positively charged aminated silica particles did not form clusters in water.
Using a theory of particle interactions that considers the structure of the solvent at the interface, the team established that for negatively charged particles in water, there is an attractive force that overcomes electrostatic repulsion at large separations, leading to the formation of groups. For positively charged particles in water, this solvent-driven interaction is always repulsive and no clusters form.
This effect was found to be pH dependent: the team was able to control the formation (or not) of clusters of negatively charged particles by varying the pH. Regardless of pH, the positively charged particles did not form clusters.
Specific effects of solvents and new discoveries
Naturally, the team wondered if the effect on charged particles could be changed, so that positively charged particles form clusters and negative ones do not. By changing the solvent to alcohols, such as ethanol, which has a different interface behavior than water, this is exactly what they observed: the positively charged aminated silica particles formed hexagonal clusters, while the negatively charged silica did not.
According to the researchers, this study involves a fundamental recalibration of understanding that will influence the way we think about processes as different as the stability of pharmaceuticals and fine chemicals or the pathological malfunctions associated with molecular aggregation in human diseases. . The new findings also provide evidence of the ability to investigate properties of the interfacial electrical potential due to the solvent, such as its sign and magnitude, that were previously thought to be incommensurable.
Professor Madhavi Krishnan (Department of Chemistry, University of Oxford), who led the study, says: “I am really very proud of my two postgraduate students, as well as the undergraduates, who have worked together to drive this fundamental discovery forward. . .”
Sida Wang (Department of Chemistry, University of Oxford), first author of the study, says: “I still find it fascinating to see how these particles attract each other, even after having seen it thousands of times.”
Reference: “A charge-dependent long-range force drives a personalized assembly of matter in solution” by Sida Wang, Rowan Walker-Gibbons, Bethany Watkins, Melissa Flynn and Madhavi Krishnan, February 30, 2024, Nanotechnology from nature.
DOI: 10.1038/s41565-024-01621-5