  Over the past few decades the experimental literature has consistently
reported observations of attraction between like-charged colloidal particles
and macromolecules in solution. Examples include nucleic acids and colloidal
particles in bulk solution and under confinement, and biological liquid-liquid
phase separation. This observation is at odds with the intuitive expectation of
an interparticle repulsion that decays monotonically with distance. Although
attraction between like-charged particles can be theoretically rationalised in
the strong-coupling regime, for example, in the presence of multivalent
counterions, recurring accounts of long-range attraction in aqueous solution
containing monovalent ions at low ionic strength have posed an open conundrum.
Here we show that the behaviour of molecular water at an interface -
traditionally disregarded in the continuum electrostatics picture, provides a
mechanism to explain attraction between like-charged objects in a broad
spectrum of experiments. This basic principle will have important ramifications
in the ongoing quest to better understand intermolecular interactions in
solution.
