The binding of divalent cations to negatively charged functional groups significantly contributes to the structure and function of biological and atmospheric systems. The binding of Ca²⁺ and Mg²⁺ ions to carboxylate headgroups is commonly characterized using asymmetric and symmetric stretch vibrational frequency data markers. Here we investigate both solution and interfacial regimes using Raman and infrared reflection absorption spectroscopy (IRRAS), respectively, to obtain a more detailed molecular picture of the binding motifs of Ca²⁺ versus Mg²⁺ to carboxylate headgroups beyond the simple picture of ionic binding through electrostatics. Our Raman results show a significant decrease in intensity of the symmetric COO^- mode upon salt addition with Ca²⁺showing a slightly larger intensity change than Mg²⁺ which is attributed to the differences in binding environments. Through deconvolution methods, we identify a new band that emerges. Also in the solution phase, Raman temperature studies reveal an unusual behavior at 60 ^oC showing a change in the isosbestic point due to anomalous properties of liquid water. In addition, at 80^oC, the frequency and intensity spectral responses of Mg²⁺approach that of Ca²⁺ indicating that at this temperature the metal cations have a similar binding coordination to the carboxylate. This is highly consistent with our calculations showing a larger energy threshold of Mg²⁺ for removing a water molecule from the hydration shell. The IRRAS surface spectra of the asymmetric COO^- mode reveals unique M²⁺:COO^- spectral signatures, more distinct and differentiable than the analogous solution spectra. Surface spectra along with our calculations and insights from the solution studies reveal that Ca²⁺interacts with carboxylate groups in two distinct coordination geometries, direct-ion pairing and solvent shared, whereas the Mg²⁺ mostly coordinate in a fully solvent shared motif.

Bethany Rudd

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