The interaction of metals with fatty acids at the air-water interface is important in determining the interfacial chemistry of sea-spray aerosols. A particular issue of interest in this work is the non-classical behavior of Ca2+ binding to fatty acids (i.e., does not correspond to Gouy-Chapman behavior) and the observation that Ca2+ is enriched in sea spray aerosols. This implies that there is an unusually strong interaction between Ca2+ ions and the carboxylate head group, which has been addressed at the air water interface using SFG and IR reflectance spectroscopies, which yield spectroscopic signatures of the Ca2+ induced species at the surface. These are remarkably similar to those of a hydrated carboxylate group by itself, raising the question of where the metal ions actually are and how they affect C-O stretching bands. To address this, we report the spectra of the isolated M2+RCO- contact ion pairs (M=Mg, Ca, Zn) using cold-ion vibrational pre-dissociation spectroscopy (CIVP) with He, H2, and D2 tagging. Although the resulting spectra are indeed strongly metal dependent, these signatures are strongly dependent on the degree of microhydration, bringing the symmetric and asymmetric CO2 stretches closer to those in the bare ion. Moreover, hydration of either the –CO- head group alone or the contact ion pairs yields surprisingly similar spectra after the completion of the first hydration shell. We discuss these observations in the context of electronic structure predictions and classical solvation theory. We report CIVP spectra of [M(C2D5COO)(H2O)n]- (M = Mg2+, Ca2+, Zn2+, n=0-5) and IR reflection-absorption spectra of d31-palmitate in MCl2 aqueous solutions and track the binding motif from the isolated metal-carboxylate dimer to bulk solvation.

Unraveling the spectroscopic signatures of divalent metal ion binding to carboxylate head groups at the air-water interface with cold gas-phase clusters

255th National Meeting (2018)

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