Figure shows the two-dimensional the free energy as a function of the two anion-cation separation distances F (r 1 , r 2 ), of the Na 2 CO 3 complex in the bulk and interface chemical environments. Similar to the dissociation of simple ion pairs, each cation shows two minima. The most stable configuration is the contact ion pair (CP) at r 1/2 < 4.5 Å, where the sodium ion is in direct contact with the anion. The second minimum is the metastable solventseparated ion pair (SSP) at 4.5 < r 1/2 < 7 Å, where the sodium remains near the carbonate anion, with a water molecule separating them. Finally, beyond r 1/2 > 7 Å, the free energy surface plateaus, yielding the dissociated (D) state where the two ions are no longer correlated. These three ion-pairing states are shown schematically in Fig. .B.