  In the course of our investigations of the adsorption of ions to the
air-water interface, we previously reported the surprising result that
doubly-charged carbonate anions exhibit a stronger surface affinity than do
singly-charged bicarbonate anions. In contrast to monovalent, weakly hydrated
anions, which generally show enhanced concentrations in the interfacial region,
multivalent (and strongly hydrated) anions are expected to show much weaker
surface propensity. In the present work, we use resonantly enhanced deep-UV
second harmonic generation spectroscopy to measure the Gibbs free energy of
adsorption of both carbonate ($CO_3^{2-}$) and bicarbonate $(HCO_3^-)$ anions
to the air-water interface. Contrasting the predictions of classical
electrostatic theory, and in support of our previous findings from X-ray
photoelectron spectroscopy, we find that carbonate anions do indeed exhibit
much stronger surface affinity than do the bicarbonate anions. Molecular
dynamics simulation reveals that strong ion pairing of $CO_3^{2-}$ with the
$Na^+$ counter-cation in the interfacial region, resulting in formation of
near-neutral agglomerates of $Na^+$ and $CO_3^{2-}$ clusters, is responsible
for this counterintuitive behavior. These findings not only advance our
fundamental understanding of ion adsorption chemistry, but will also impact
important practical processes such as ocean acidification, sea-spray aerosol
chemistry, and mammalian respiration physiology.
