The
binding of group II metal cations such as Ca2+ and
Mg2+ has been largely categorized as electrostatic or ionic
using carboxylate symmetric and asymmetric stretching frequency assignments
that have been historically used with little regard for the solvation
environment of aqueous solutions. However, given the importance of
these cations and their binding mechanisms related to biological function
and in revealing surface enrichment factors for ocean to marine aerosol
transfer, it is imperative that a deeper understanding be sought to
include hydration effects. Here, infrared reflection–absorption
and Raman spectra for surface and solution phase carboxylate binding
information, respectively, are compared against bare (unbound) carboxylate
and bidentate Zn2+:carboxylate spectral signatures. Spectral
non-coincidence effect analysis, temperature studies, and spectral
and potential of mean force calculations result in a concise interpretation
of binding motifs that include the role of mediating water molecules,
that is, contact and solvent-shared ion pairs. Calcium directly binds
to the carboxylate group in contact ion pairs where magnesium rarely
does. Moreover, we reveal the dominance of the solvent-shared ion
pair of magnesium with carboxylate at the air–water interface
and in solution.