Carboxylate Ion Availability at the Air-Water Interface

Amphiphilic organic compounds at the air-water interface play key roles in the nucleation, growth, and aging process of atmospheric aerosol. Surface-active species are expected to react preferentially with atmospheric oxidants, such as the OH radical, at the air-water interface via specific mechanisms. Establishing the relative availability of the different amphiphilic species to gas-phase oxidants at the air-water interface under atmospherically relevant conditions is, however, challenging. Here we report the interfacial availability of atmospherically relevant carboxylate ions R_n-COO^- (n = 1-7) and n-, cyclo-, aromatic-R₆-COO^- at the air-water interface via a novel application of mass spectrometry of aqueous microjets. The breakup mechanism of microjets lets us determine the relative interfacial affinities of carboxylate ions in equimolar solutions of the corresponding carboxylic acids in the 1 μM to 1 mM range under ambient conditions. We find that the interfacial affinity of R_n-COO^- increases exponentially with both chain-length and solvent-accessible surface area (SASA) except in the case of R₁-COO^-. The relative interfacial affinities for n-heptanoate (n-R₆-COO^-) > cyclohexanecarboxylate (c-R₆-COO^-) > benzoate (Ar-R₆-COO^-) are also determined. We attribute the smallest availability of Ar-R₆-COO^- at the air-water interface among the three carboxylate ions to a strong π-H bonding between the aromatic ring and water molecule. Molecular mechanisms on the availability of carboxylate ions at the air-water interface and the atmospheric implications are discussed.