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. Here we demonstrate experimentally that the solvent plays a hitherto
unforeseen but crucial role in interparticle interactions, and importantly,
that interactions in the fluid phase can break charge-reversal symmetry. We
show that in aqueous solution, negatively charged particles can attract at long
range while positively charged particles repel. In solvents that exhibit an
inversion of the net molecular dipole at an interface, such as alcohols, we
find that the converse can be true: positively charged particles may attract
whereas negatives repel. The observations hold across a wide variety of surface
chemistries: from inorganic silica and polymeric particles to polyelectrolyte-
and polypeptide-coated surfaces in aqueous solution. A theory of interparticle
interactions that invokes solvation at an interface explains the observations.
Our study establishes a specific and unanticipated mechanism by which the
molecular solvent may give rise to a strong and long-ranged force in solution,
with immediate ramifications for a variety of particulate and molecular
processes including tailored self-assembly, gelation and crystallization, as
well as biomolecular condensation, coacervation and phase segregation. These
findings also shed light on the solvent-induced interfacial electrical
potential - an elusive quantity in electrochemistry and interface science
implicated in many natural and technological processes, such as atmospheric
chemical reactions, electrochemical energy storage and conversion, and the
conduction of ions across cell membranes.Comment: 20 pages, 6 figure