Interfacial water generates a long-ranged force between objects in solution Sida Wang, Rowan Walker-Gibbons, Bethany Watkins, Melissa Flynn, Madhavi Krishnan University of Oxford, Oxford, UK The interaction between charged objects in solution is generally expected to recapitulate two central principles of electromagnetics: (i) like-charged objects repel, and (ii) they do so regardless of the sign of their electrical charge. We have demonstrated experimentally that the solvent plays a hitherto unforeseen but crucial role in interparticle interactions 1 , and importantly, that interactions in the liquid phase can break charge-reversal symmetry 2-4 . We have shown that in aqueous solution, negatively charged particles can attract at long range while positively charged particles repel. We have further found that this ‘charge-asymmetry’ in the interaction can be inverted by solvents whose average interfacial orientation at a surface is different from that of water. For example, in alcohols we have found that that positively charged particles may attract whereas negative particles repel. We have further demonstrated that the magnitude of this solvent-induced attractive force depends on a change in magnitude of the charge carried by the object, reflected in the p H dependence of this solvent-generated force. The observations hold across a wide variety of surface chemistries: from inorganic silica and polymeric particles to polyelectrolyte- and polypeptide-coated surfaces in solutions. A theory of interparticle interactions that invokes solvation at an interface explains the observations 2-4 . Our study establishes a specific and unanticipated mechanism by which the molecular solvent may give rise to strong and long ranged forces in solution, with immediate ramifications for a variety of particulate and molecular processes including tailored self-assembly, gelation and crystallisation, as well as biomolecular condensation, coacervation and phase segregation.
Interparticle interactions in solution depend on solvent interfacial arrangement and can break charge- reversal asymmetry. ( a ) Schematic representation of two interacting spheres at an intersurface separation x and the dominant orientation of water molecules at the solid-liquid interface. On average, a water molecule close to a neutral surface point its dipole moment (green arrow) away from the interface and towards the bulk 2 . The symmetry-broken average orientation of a water molecule at a surface gives rise to a non-zero interfacial (or solvation) potential, , even at a neutral surface 1,2,4 . ( b ) Negatively charged carboxylic acid particles (COOH, blue) form slowly reorganizing hexagonally close packed structure in water, whereas positively charged aminated particles (NH 2 , pink) repel. In alcohols such as Isopropanol, the sign of the long-ranged force flips sign in each case, such that positively charged NH 2 particles attract and negatively charged COOH particles repel. Pair interaction potentials, , inferred from Brownian Dynamic simulations are shown below each digitized experimental snapshot. Scale bar: 20 μm. References 1. S. Wang, R. W.-G., B. Watkins, M. Flynn, M. Krishnan. https://doi.org/10.48550/arXiv.2212.12894 2. Walker-Gibbons, R., Kubincová, A., H nenberger, P. H. & Krishnan, M. The Journal of Physical Chemistry B 126 , 4697-4710 (2022). 3. Behjatian, A., Walker-Gibbons, R., Schekochihin, A. A. & Krishnan, M. Langmuir 38 , 786-800 (2022). 4. 4. Kubincová, A., H nenberger, P. H. & Krishnan, M. Journal of Chemical Physics 152 , 104713 (2020).
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