APXPS is a surface-sensitive technique that allows atom-specific characterization of chemical states as well as depth profiling of the overall system. Because electrons emitted with higher energies have greater escape depths (effective attenuation lengths), they provide information about species deeper into the solution than electrons with lower energies. Thus, increases in the incident photon energy correlate to increases in the probe depth.

Analysis of the data showed that, as depth increases, the peak-area ratios approach unity, as would be expected in bulk regions of 50:50 mixtures. At shallower depths, however, both carbonic acid (H2CO3) and carbonate (CO32-) have higher concentrations than bicarbonate (HCO3-). This is not surprising for neutral H2CO3 relative to charged ions (because electrostatic effects tend to repel ions from interfaces). On the other hand, the enhancement of CO32- (doubly charged and strongly hydrated—i.e., attracting many water molecules) over HCO3- (singly charged and less strongly hydrated) is surprising and appears to conflict with recent models for interfacial ion adsorption.

We suggest that the conflict could be resolved if CO32- adsorbs to the interface as an ion pair with sodium (i.e., Na+:CO32-), neutralizing some of the charge effects mentioned earlier. Further theoretical modeling will be needed to gain greater insight into this unexpected reversal in the depth profile of interfacial carbonate systems.