Why like-charged particles attract in monovalent salt solutions

Poisson-Boltzmann theory forbids attraction between like-charged bodies in a $1:1$ electrolyte, and correlation-driven attraction requires the coupling parameter $\Xi = 2\pi z^3 l_B^2 \sigma_s >> 1$, where $z$ is counterion valence, $l_B = e^2/(\epsilon k_B T) \sim 0.7\,\mathrm{nm}$ in water is the Bjerrum length and $\sigma_s$ is the surface charge number density; monovalent systems have $\Xi <\sim 1$. Observed attractions between like-charged microparticles in monovalent salt, near walls or in confinement, include reports that depend on the sign of the particle charge and on the solvent. An answer identifies the mechanism (interfacial solvent structure, ion correlations beyond mean field, hydrodynamic or optical artifacts) and predicts the pair potential versus salt concentration and charge sign.

Direct pair-potential measurements for isolated particle pairs far from walls, with opposite charge signs and several solvents, reproduced by a molecular-solvent calculation.