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We investigate the individual activity coefficients of pure 1:1 and 2:1electrolytes using our theory that is based on the competition of ion-ion (II)and ion-water (IW) interactions (Vincze et al., J. Chem. Phys. 133, 154507,2010). The II term is computed from Grand Canonical Monte Carlo simulations onthe basis of the implicit solvent model of electrolytes using hard sphere ionswith Pauling radii. The IW term is computed on the basis of Born's treatment ofsolvation using experimental hydration free energies. The two terms are coupledthrough the concentration-dependent dielectric constant of the electrolyte.With this approach we are able to reproduce the nonmonotonic concentrationdependence of the mean activity coefficient of pure electrolytes qualitativelywithout using adjustable parameters. In this paper, we show that the theory canprovide valuable insight into the behavior of individual activity coefficientstoo. We compare our theoretical predictions against experimental data measuredby electrochemical cells containing ion-specific electrodes. As in the case ofthe mean activity coefficients, we find good agreement for 2:1 electrolytes,while the accuracy of our model is worse for 1:1 systems. This deviation inaccuracy is explained by the fact that the two competing terms (II and IW) aremuch larger in the 2:1 case so errors in the two separate terms have lesseffects. The difference of the excess chemical potentials of cations and anions(the ratio of activity coefficients) is determined by asymmetries in theproperties of the two ions: charge, radius, and hydration free energies.

Unraveling the behavior of the individual ionic activity coefficients on the basis of the balance of ion-ion and ion-water interactions