These conditional free energy curves in Fig. , show that in all chemical environments, the probe cation is sensitive to the ion pairing state of the auxiliary cation -increasing as the auxiliary cation dissociates from CO 3 2 -. In bulk water, the free energy penalty of moving a cation from the contact to solvent-separated state when the other Na is in contact with CO 3 is 4.5 k B T and increases to 5.1 k B T and 5.5 k B T when the auxiliary ion is in the solventseparated and dissociated states respectively. This net 1 k B T destabilization of the contact state relative to the solvent-separated minimum when a second cation is placed near the anion can be explained by the direct Coulomb repulsion between the two cations -agreeing with the expectation of the change in potential energy of 1 k B T due to placing an additional +1 charge on the central carbon of CO 3 .