Thermal Transport Anomalies of Electrolyte Solutions in the Water Supercooled Regime: Signatures of the Liquid-Liquid Water Phase Transition

Abstract: Water exhibits remarkable anomalies when supercooled, attributed to a hypothesized liquid-liquid phase transition (LLPT) between low-density (LDL) and high-density (HDL) liquid 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 a striking behavior. Initially thermophobic at high temperatures (> 280 K), becoming thermophilic upon cooling, then reverting to thermophobic below 220 K. This behavior correlates with structural changes in the hydrogen-bond network of water. Specifically, we find that electrolyte solutions dominated by HDL structures, which are characterized by lower tetrahedral order, exhibit thermophobic behavior, whereas thermodynamic states dominated by LDL structures, with higher tetrahedral order, display thermophilic behavior. 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.