  Ionic solutions are often regarded as fully dissociated ions dispersed in a
polar solvent. While this picture holds for dilute solutions, at higher ionic
concentrations, oppositely charged ions can associate into dimers, referred to
as Bjerrum pairs. We consider the formation of such pairs within the nonlinear
Poisson-Boltzmann framework, and investigate their effects on bulk and
interfacial properties of electrolytes. Our findings show that pairs can reduce
the magnitude of the dielectric decrement of ionic solutions as the ionic
concentration increases. We describe the effect of pairs on the Debye screening
length, and relate our results to recent surface-force experiments.
Furthermore, we show that Bjerrum pairs reduce the ionic concentration in bulk
electrolyte and at the proximity of charged surfaces, while they enhance the
attraction between oppositely charged surfaces.
