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

Abstract

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.

Phase State and Thermodynamic Properties of Proxy Sea Spray Aerosol Interfaces Derived from Temperature-Dependent Equilibrium Spreading Pressure

“…25 For the 100 mM NaCl solution, metal ions have been shown via infrared reflectionabsorption spectroscopy to alter the hydration of the carboxylate headgroups. 2,27,30,39 The PA headgroup can undergo two types of Na + coordination, either binding to one oxygen or both oxygen atoms of the PA headgroup. Spatial distribution functions were reported in literature explaining these potential sodium interactions with PA. 25 It has been shown that PA interacts with one or two Na + , and these differences in coordination could elucidate the anomalous thermodynamic values of 3D to 2D spreading we are observing in the 100 mM NaCl system.…”

“…54 Studies that followed showed that the PA−Na + interactions help stabilize the air−water interface. 2,34 Additionally, fatty acids like PA have been abundantly sourced from marine algae, 63 undergo temporal fluctuations in enrichment at the air−seawater interface, 64 and contribute to ice nucleating particle (INP) counts. 35,65,66 65 Additionally, ESP has been used as a tool for thermodynamic consideration pertaining to ice nucleation.…”

“…Sea spray aerosols (SSAs) present highly dynamic microenvironments in which their interfacial properties are strongly driven by chemical and physical interactions such as carboxylate−metal binding, 1,2 acid−base equilibrium, 3−5 and ion hydration. 6−8 Each of these parameters unlocks important thermodynamic inputs for global climate models, which provide better insights into atmospheric phenomena like ice nucleation and cold cloud formation.…”

“…Carboxylate−metal interactions under oceanic and atmospheric conditions have been well documented in the literature using various spectroscopic techniques. [1][2][3]23 Exposure to high concentrations of NaCl drives the interfacial equilibrium for certain semisoluble acids, demonstrating a need to expand upon Na + studies, such as their thermodynamically driven interactions, at SSA interfaces. 3 However, the thermodynamic properties of these carboxylate−metal interactions persist as nontrivial measurements.…”

“…Sea spray aerosols (SSAs) present highly dynamic microenvironments in which their interfacial properties are strongly driven by chemical and physical interactions such as carboxylate–metal binding, , acid–base equilibrium, − and ion hydration. − Each of these parameters unlocks important thermodynamic inputs for global climate models, which provide better insights into atmospheric phenomena like ice nucleation and cold cloud formation. The key to unraveling these thermodynamically driven interactions is equilibrium-based measurements and calculations. , Herein, we use equilibrium spreading pressure (ESP) measurements to calculate the thermodynamic equilibrium between the three-dimensional (3D) solid phase and a two-dimensional (2D) film of a fatty acid monolayer to inform on the interfacial chemistry of SSA organic coatings…”