Thermal transport anomalies of electrolyte solutions in the water supercooled regime: Signatures of the liquid–liquid water phase transition

Water exhibits remarkable anomalies when supercooled, attributed to a hypothesized liquid-liquid phase transition (LLPT) between low-density and high-density liquid (HDL) phases. Using non-equilibrium molecular dynamics simulations, we explore thermal transport and coupled effects in supercooled NaCl and LiCl solutions (1-4 m, 200-300 K). At 1 m, thermal conductivity exhibits a pronounced minimum near 220 K, coinciding with maxima in isothermal compressibility and minima in the speed of sound, both of which are signatures of critical fluctuations. The anomalies progressively diminish with increasing salt concentration and vanish at 4 m, suggesting suppression of the LLPT. The Soret coefficient exhibits striking behavior, which is initially thermophobic at high temperatures (>280 K, solute migrates toward the cold side), becomes thermophilic upon cooling (solute migrates toward the hot side), and then reverts to thermophobic below 220 K. This behavior correlates with structural changes in the hydrogen-bond network of water. In particular, we find that the deep minima in the Soret coefficient, corresponding to the strongest thermophilic response in NaCl and LiCl, occur in thermodynamic states characterized by a low fraction of HDL-like structures, indicating a predominance of highly tetrahedrally ordered water environments. Furthermore, Seebeck coefficients exhibit sign reversals near 220-230 K, highlighting the thermoelectric sensitivity to structural transformations and temperature. These findings establish thermal transport as a sensitive probe of supercooled water, revealing that electrolyte solutions preserve the water's anomalies deep into the supercooled regime.