Molecular-level origin of the carboxylate head group response to divalent metal ion complexation at the air-water...

We exploit gas-phase cluster ion techniques to provide insight into the local interactions underlying divalent metal ion-driven changes in the spectra of carboxylic acids at the air-water interface. This information clarifies the experimental findings that the CO stretching bands of long-chain acids appear at very similar energies when the head group is deprotonated by high subphase pH or exposed to relatively high concentrations of Ca²⁺ metal ions. To this end, we report the evolution of the vibrational spectra of size-selected [Ca²⁺·RCO₂^-]⁺·(H₂O) _to12 and RCO₂^-·(H₂O) _to14 cluster ions toward the features observed at the air-water 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 2 (symmetric and asymmetric stretching) CO bands to hydration are opposite to each other. The result is that both CO bands evolve toward their interfacial asymptotes from opposite directions. Simulations of the [Ca²⁺·RCO₂^-]⁺·(H₂O) 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 or deprotonation 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 hydrocarbon tail as the key microscopic parameter that is correlated with the observed trends.