Terahertz fingerprint reveals the effect of alcohols on sodium ions hydration shell.

This article investigates the influence of alcohol polarity on the formation of sodium chloride (NaCl) hydrate crystals in ternary systems composed of ions, water, and alcohols at low temperatures. Using terahertz time-domain spectroscopy (THz-TDS) and low-temperature x-ray diffraction (XRD), the study finds that strongly polar alcohols like methanol (MeOH) and ethanol (EtOH) inhibit NaCl · 2H₂O hydrate formation by entering the first solvation shell of Na⁺ ions, disrupting typical hydration. In contrast, weakly polar alcohols such as tert-butanol (TBA) do not penetrate this solvation shell and instead promote hydrate crystallization. Isopropanol (IPA), with intermediate polarity, exhibits complex effects that vary with concentration. These findings highlight the dominant role of molecular polarity over size in modulating ion hydration and salt hydrate crystallization in biologically and environmentally relevant ternary systems.