Agglomeration Drives the Reversed Fractionation of Aqueous Carbonate and Bicarbonate at the Air–Water Interface

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 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 a 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 (CO3 2– ) and bicarbonate (HCO3 – ) 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. Extensive computer simulations reveal that strong ion pairing of CO3 2– with the Na+ countercation in the interfacial region results in the formation of near-neutral agglomerate clusters, consistent with a theory of interfacial ion adsorption based on hydration free energy and capillary waves. Simulated X-ray photoelectron spectra predict a 1 eV shift in the carbonate spectra compared to that of bicarbonate, further confirming our experiments. These findings not only advance our fundamental understanding of ion adsorption chemistry but also impact important practical processes such as ocean acidification, sea-spray aerosol chemistry, and mammalian respiration physiology.

A bicarbonate-rich liquid condensed phase in non-saturated solutions in the absence of divalent cations

“…In parallel, present evidence of density/viscosity, composition, and refractive index contrasts in the sodium bicarbonate/chloride system to suggest that bicarbonate-rich LCP coexists with a bulk solution phase, presumably through the same water polymorphism mechanism of HDL and LDL local water structure stabilization. It is not clear from the presented data whether the bicarbonate-rich LCP separates through a liquid-liquid transition or through a macroscopic liquid-liquid separation, however, other works indicate these liquid-liquid phase transitions occur frequently in aqueous solutions ( Mergo and Seto, 2020 ; Devlin et al, 2023 ).…”

Adsorption, Orientation, and Speciation of Amino Acids at Air–Aqueous Interfaces for the Direct Air Capture of CO2

“…We did not include dianionic carbamate species in simulations given that for the concentration range of AAs studied here hydrolysis leads to the formation of HCO 3 – as the primary capture product. In future investigations, we plan to explore AAs with greater solubility and explicitly account for the presence of dianionic carbamates that might partition to the surface in the presence of coordinating counterions . This will require a comparison of the performance of nonpolarizable and polarizable force fields, which is beyond the scope of the current work.…”

“…In future investigations, we plan to explore AAs with greater solubility and explicitly account for the presence of dianionic carbamates that might partition to the surface in the presence of coordinating counterions. 61 This will require a comparison of the performance of nonpolarizable and polarizable force fields, which is beyond the scope of the current work.…”

Real-Space Pseudopotential Method for the Calculation of Third-Row Elements X-ray Photoelectron Spectroscopic Signatures

“…The PP methods using PBE functional and B3LYP-refining step are denoted as PP−PBE and PP−PBE(B3LYP), respectively, adopting a consistent nomenclature with our previous work. 50,74 All-Electron Approach. AE calculations, together with experimental data, 1 serve as valuable benchmarks for the developed PP approach.…”