  We investigate the individual activity coefficients of pure 1:1 and 2:1
electrolytes 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 on
the basis of the implicit solvent model of electrolytes using hard sphere ions
with Pauling radii. The IW term is computed on the basis of Born's treatment of
solvation using experimental hydration free energies. The two terms are coupled
through the concentration-dependent dielectric constant of the electrolyte.
With this approach we are able to reproduce the nonmonotonic concentration
dependence of the mean activity coefficient of pure electrolytes qualitatively
without using adjustable parameters. In this paper, we show that the theory can
provide valuable insight into the behavior of individual activity coefficients
too. We compare our theoretical predictions against experimental data measured
by electrochemical cells containing ion-specific electrodes. As in the case of
the 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 in
accuracy is explained by the fact that the two competing terms (II and IW) are
much larger in the 2:1 case so errors in the two separate terms have less
effects. The difference of the excess chemical potentials of cations and anions
(the ratio of activity coefficients) is determined by asymmetries in the
properties of the two ions: charge, radius, and hydration free energies.
