Agglomeration Drives the Reversed Fractionation of Aqueous Carbonate and Bicarbonate at the Air-water Interface

TL;DR

This study reveals that carbonate ions have a stronger affinity for the air-water interface than bicarbonate ions due to ion pairing and agglomeration, challenging classical electrostatic expectations.

Contribution

It demonstrates the role of ion pairing and agglomeration in reversed ion fractionation at the air-water interface, supported by spectroscopy and molecular dynamics simulations.

Findings

Carbonate exhibits stronger surface affinity than bicarbonate.

Ion pairing leads to near-neutral clusters at the interface.

Results challenge classical electrostatic theory predictions.

Abstract

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.