Restructuring of Hydration Shell Water due to Solvent-Shared Ion Pairing (SSIP): A Case Study of Aqueous MgCl and LaCl Solutions.

Hydration of ions plays a crucial role in interionic interactions and associated processes in aqueous media, but selective probing of the hydration shell water is nontrivial. Here, we introduce Raman difference with simultaneous curve fitting (RD-SCF) analysis to extract the OH-stretch spectrum of hydration shell water, not only for the fully hydrated ions (Mg, La, and Cl) but also for the ion pairs. RD-SCF analyses of diluted MgCl (0.18 M) and LaCl (0.12 M) solutions relative to aqueous NaCl of equivalent Cl concentrations provide the OH-stretch spectra of water in the hydration shells of fully hydrated Mg and La cations relative to that of Na. Integrated intensities of the hydration shell spectra of Mg and La ions increase linearly with the salt concentration (up to 2.0 M MgCl and 1.3 M LaCl), which suggests no contact ion pair (CIP) formation in the MgCl and LaCl solutions. Nevertheless, the band shapes of the cation hydration shell spectra show a growing signature of Cl-associated water with the rising salt concentration, which is a manifestation of the formation of a solvent-shared ion pair (SSIP). The OH-stretch spectrum of the shared/intervening water in the SSIP, retrieved by second-round RD-SCF analysis (2RD-SCF), shows that the average H-bonding of the shared water is weaker than that of the hydration water of the fully hydrated cation (Mg or La) but stronger than that of the anion (Cl). The shared water displays an overall second-order dependence on the concentration of the interacting ions, unveiling 1:1 stoichiometry of the SSIP formed between Mg and Cl as well as La and Cl.