Consider the structure of the Na 2 CO 3 ion complex near a liquid interface. Since two Na + cations associate with a single CO 3 2anion, two cation-anion distances, r 1 and r 2 are required to describe the complex. Moreover, the cations may become correlated through direct cationcation interactions or through solvent mediated interactions where some configurations of the Na 2 CO 3 complex are more favorably solvated than others, requiring the specification of the cation angular displacement, θ NaCNa = arccos (r 1 • r1 ), where r is the unit vector associated with the vector r. The CO 3 2anion is planar and may exhibit preferential alignment with respect to the interface. This alignment can be captured by the angle θ CO 3 between the normal axis to the carbonate plane and the normal to the interface. These coordinates are show in Fig. .A.