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  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. 

 Reference: J. Chem. Phys. 146, 194904 (2017).