Thermal transport and non-equilibrium coupling effects in molecular liquids and solutions
File(s)
Author(s)
Zhao, Guansen
Type
Thesis
Abstract
Thermal gradients in molecular liquids and solutions generate coupled transport phenomena such as thermodiffusion, thermoelectricity, and thermal polarization, where temperature differences induce concentration gradients or electric fields. This thesis investigates these effects using non-equilibrium molecular dynamics (NEMD) simulations of water, aqueous electrolyte solutions, and ethanol across a wide range of thermodynamic conditions.
First, thermal transport and polarization in supercooled water are examined using the TIP4P/2005 model. A minimum in thermal conductivity coincides with extrema in the isobaric thermal expansion coefficient, isothermal compressibility, and speed of sound, linking anomalous heat transport to the hypothesized liquid–liquid phase transition (LLPT). Thermal gradients also induce polarization coefficients of several mV/K, with a non-monotonic temperature dependence arising from competing dipolar and quadrupolar contributions.
The study is then extended to supercooled NaCl and LiCl aqueous solutions. Complex thermodiffusive behavior emerges, including multiple thermophilic–thermophobic transitions and Soret coefficient minima between 220 and 250 K. These anomalies correlate with structural transformations between low- and high-density liquid forms of water. Increasing salt concentration disrupts the hydrogen-bond network and weakens these signatures while preserving the underlying thermodynamic anomalies. The influence of pressure up to 2 GPa is subsequently explored for 1 m NaCl and LiCl solutions. Thermal conductivity increases with pressure, whereas the Soret coefficient transitions from thermophilic to thermophobic behavior as the hydrogen-bond network collapses.
Finally, simulations of liquid and supercritical ethanol reproduce experimental thermal conductivity trends and reveal thermally induced polarization with a sign inversion near 400 K. Near the critical point, polarization is significantly enhanced, similar to the behavior observed in water.
Overall, this thesis provides a unified molecular-level understanding of how temperature gradients couple to mass, charge, and polarization fluxes in liquids, connecting transport properties to structural order, hydrogen bonding, and thermodynamic state.
First, thermal transport and polarization in supercooled water are examined using the TIP4P/2005 model. A minimum in thermal conductivity coincides with extrema in the isobaric thermal expansion coefficient, isothermal compressibility, and speed of sound, linking anomalous heat transport to the hypothesized liquid–liquid phase transition (LLPT). Thermal gradients also induce polarization coefficients of several mV/K, with a non-monotonic temperature dependence arising from competing dipolar and quadrupolar contributions.
The study is then extended to supercooled NaCl and LiCl aqueous solutions. Complex thermodiffusive behavior emerges, including multiple thermophilic–thermophobic transitions and Soret coefficient minima between 220 and 250 K. These anomalies correlate with structural transformations between low- and high-density liquid forms of water. Increasing salt concentration disrupts the hydrogen-bond network and weakens these signatures while preserving the underlying thermodynamic anomalies. The influence of pressure up to 2 GPa is subsequently explored for 1 m NaCl and LiCl solutions. Thermal conductivity increases with pressure, whereas the Soret coefficient transitions from thermophilic to thermophobic behavior as the hydrogen-bond network collapses.
Finally, simulations of liquid and supercritical ethanol reproduce experimental thermal conductivity trends and reveal thermally induced polarization with a sign inversion near 400 K. Near the critical point, polarization is significantly enhanced, similar to the behavior observed in water.
Overall, this thesis provides a unified molecular-level understanding of how temperature gradients couple to mass, charge, and polarization fluxes in liquids, connecting transport properties to structural order, hydrogen bonding, and thermodynamic state.
Version
Open Access
Date Issued
2025-11-28
Date Awarded
2026-03-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Bresme, Fernando
Publisher Department
Department of Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)