Spectroscopic Insights into Ion Pairing in Sulfate Electrolytes: Implications for Sorption in Aqueous Systems

Ion pairing, including contact, solvent-shared, and solvent-separated ion pairs (CIPs, SIPs, and SSIPs, respectively), affects aqueous reaction chemistry, sorption, and precipitation processes. Accurate characterization of ion pairing in aqueous sulfate electrolytes is essential for understanding their thermodynamics and hydration structure, yet comprehensive quantification across aqueous and mixed-solvent systems remains limited. We combine Raman spectroscopy with dielectric spectroscopy, using waveguide-based setups (WR-62 and WR-28, applied for the first time) and a broadband probe station (10 MHz–50 GHz), to characterize ion pairing and solvent–solute interactions in monovalent cation sulfate salts in water and water/methanol solutions. Dielectric spectroscopy resolved SSIP and SIP fractions, with SSIPs dominating Li2SO4 and Na2SO4. Raman spectroscopy quantified CIPs (K2SO4 > Na2SO4 ≈ Li2SO4), consistent with cation hydration and polarizability. Methanol increased SIP and CIP concentrations. Ion association stability constants and mean activity coefficients were calculated with Archer’s form of the Pitzer model, incorporating short-range binary and ternary ion–ion interactions. This approach predicts mean activity coefficients at elevated ionic strengths without double-counting ion-pairing contributions, enabling determination of stability constants and aqueous speciation. Resolving ion-pair speciation is important for accurate thermodynamic models of sulfate-bearing waters relevant to membrane separations and resource recovery.