Raman evidence for a solvation-heterogeneity-mediated precipitation pathway of sodium sulfate in water-DMSO mixtures

Salt precipitation in water-organic solvent mixtures is generally interpreted in terms of enhanced ion association driven by reduced water activity. Here, Raman spectroscopy and molecular dynamics (MD) simulations reveal a different precipitation mechanism for Na₂SO₄ in water-DMSO mixtures. Increasing DMSO content causes a systematic redshift of the sulfate Raman band and the emergence of a low-wavenumber sulfate population near saturation. Simulations show that DMSO is largely excluded from the first solvation shell of SO₄^2-, preserving a locally water-rich hydration structure even as the bulk water network becomes strongly disrupted. Consistent with this picture, neither contact ion pairs nor large ionic aggregates become dominant prior to precipitation. Instead, the results support a solvation-heterogeneity-mediated pathway in which distinct hydrated sulfate environments persist up to the precipitation threshold. Crystallization is subsequently triggered by the collapse of hydration stability rather than by extensive ion aggregation. These findings provide molecular-level insight into ion-specific solvation effects in mixed-solvent crystallization.