Dielectric Interpretation of Specificity of Ion Pairing in Water

We present a dielectric continuum model that, at a semiquantitative level, explains why ion pair formation in water is favored by like-sized ions over unlike-sized pairs. Using both classical and ab initio continuum approaches, we show that the now well-established empirical rule, the so-called “law of matching water affinities”, can be rationalized in terms of ion solvation. Namely, pairing of differently sized ions is weakened due to a shadowing effect where the larger ion shields the smaller ion from the solvent. It is shown that this empirical law ceases to be valid for less polar solvents where strong ion−ion coulomb interactions dominate the pairing free energy. The presented model demonstrates that certain ion-specific effects, such as those connected with the Hofmeister series, can be qualitatively captured by classical continuum electrostatics, although a fully quantitative description would require explicit molecular treatment of the solvent.