Year: 2026
Type: article
Abstract: Ion association in aqueous electrolyte solutions depends critically on the interplay between ion hydration and electrostatic attraction such that different ion-pair species have been detected in aqueous 2:2 electrolytes with sulfate as the prototypical divalent anion. Dielectric relaxation (DR) spectroscopy has the potential to resolve and quantify the various ion-pair species, yet literature studies are too scarce to systematically address the effect of the nature of the cation on ion association. Here, we report a DR study of aqueous solutions of MnSO4 and FeSO4 at 25 °C, at frequencies 0.2 ≤ ν/GHz ≤ 115 and concentrations up to near-saturation (∼2.7 and ∼0.8 mol·L–1 respectively). The observed spectra were consistent with the presence of five Debye modes: two at higher frequencies (ν > ca. 10 GHz) that correspond to bulk-water processes, and three solute-related modes at lower frequencies. The slowest mode, centered at ν ≈ 0.3 GHz, was consistent with overlapping contributions from the rotation of double-solvent-separated ion pairs (2SIPs) and hydrated triple ions (M2SO42+, T+) as has been found for some other divalent metal sulfate solutions. The other lower frequency modes, centered at ν ≈ 1.5 GHz and ∼8 GHz, were assigned (respectively) to the rotation of solvent-shared (SIPs) and contact (CIPs) ion pairs, with the latter mode possibly also incorporating an unresolved contribution from “slow” bulk-water molecules. Effective total cation solvation numbers, Zt(M2+, aq), derived from the water-related modes, indicated that both Mn2+ and Fe2+ immobilize approximately 11 water molecules on the DR time scale, independent of solute concentration. Combination of the present results with literature data revealed a strong correlation between the types of ion aggregates detected by DR spectroscopy and the hydration enthalpy of the divalent metal cations (excepting Zn2+). Concentrations of the various species derived from the solute-related modes were used to calculate formation constants for each species that were, where comparisons were possible, in reasonable agreement with literature values obtained by other techniques.
Authors: Johannes Hunger, Glenn Hefter, Richard Buchner
Language: English
Subfield: Atomic and Molecular Physics, and Optics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: hybrid
OpenAlex ID: W7225804921