Dissolving carbon dioxide (CO2) in water produces carbonate systems that are key to many processes essential to life, from the buffer system that regulates pH levels in blood, to the carbon cycle that governs CO2 uptake by Earth’s oceans. A detailed understanding of such systems is complicated by the presence of an interface--a cell membrane or the ocean surface, for example--that the CO2 must cross and that could affect the behavior of the various carbonate species (molecules containing the carbonate ion, CO32-). At the ALS, we have used ambient-pressure x-ray photoemission spectroscopy (APXPS) to probe the relative concentrations of carbonates near an interface, finding a surprising reversal in the expected abundances of carbonate (CO32-) and bicarbonate (HCO3-) as a function of depth. The results raise important questions about what is really happening at interfacial regions, with relevance to topics ranging from carbon sequestration to biomedical research.
Above: Measured X-ray photoemission spectra of 50:50 Na2CO3:NaHCO3 mixtures at various incident photon energies corresponding to electron kinetic energies (eKE) of 200 eV, 400 eV, 600 eV, and 800 eV, respectively. The measured spectra were fit with two Gaussian peaks with the same parameters (width, center) as those measured for the pure components. The decrease in absolute signal as the photon energy is increased is a result of the reduction in photoemission cross section.
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.
Above: Peak area ratios vs. eKE for 50:50 Na2CO3:NaHCO3 mixtures and 50:50 H2CO3:NaHCO3 mixtures.
416. Lam, R. K., Smith, J. W., Rizzuto, A. M., Karslıoğlu, O., Bluhm, H., Saykally, R. J. "Reversed interfacial fractionation of carbonate and bicarbonate evidenced by X-ray photoemission spectroscopy" J. Chem. Phys., 146, 094703 (2017)