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Title

Role of Hydration in Magnesium Versus Calcium Ion Pairing with Carboxylate: Solution and the Aqueous Interface

Document Type

Article

Publication Date

10-2-2021

Publication Title

The Journal of Physical Chemistry B

Volume Number

125

Issue Number

40

DOI

https://doi.org/10.1021/acs.jpcb.1c06108

Abstract

The binding of group II metal cations such as Ca(2+) and Mg(2+) 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 Zn(2+) : 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.

First Page

11308

Last Page

11319

Link Out URL

https://doi.org/10.1021/acs.jpcb.1c06108

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