Deconstructing the curious behavior of carboxylate at the air-water interface with cluster ion spectroscopy

The transport of divalent metal ions (e.g., Mg²⁺ and Ca²⁺) into the troposphere is thought to arise from preferential complexation with the anionic head groups of fatty acids at the surfaces of sea-spray aerosols. Attempts to quantify the formation of these ionic complexes by monitoring the vibrational frequencies of the CO stretching vibrations has proven to be difficult, however, because the ion-driven spectral response is surprisingly similar to that of the hydrated anion. We trace the origin of this effect by studying the stepwise hydration behavior of the isolated carboxylate and contact ion pair in the gas-phase. The results reveal the critical importance of solvent coordination in the structural interpretation of surface-sensitive spectral signatures of ion complexation at the interface. Surprisingly, not only does stepwise hydration of the RCO₂^- anion and the [Ca²⁺-RCO₂^-]⁺ contact ion pair yield solvatochromic responses in opposite directions, but in both cases, the responses of the two (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that the two CO bands evolve toward their interfacial asymptotes from opposite directions. Theoretical simulations of the [Ca²⁺-RCO₂^-]⁺-(H₂O)_n clusters indicate that the metal ion remains directly bound to the head group in a contact ion pair motif as the asymmetric CO stretch converges at the interfacial value by n = 12. This establishes that direct metal complexation can account for the interfacial behavior. We discuss these effects in the context of a model that invokes the water network-dependent local electric field along the C-C bond that connects the head group to the tail as the key parameter driving the observed trends.