Thermophoresis in Liquids and its Connection to Equilibrium Quantities

Abstract

Thermophoresis is the process of particles moving along a temperature gradient in thermodynamic systems. Even though it has been studied for over 150 years, there is neither a complete theoretical description of thermophoresis in liquids nor of the Soret coefficient, the quantifiying measure of the Soret equilibrium. Recent studies connect its nature to equilibrium properties of the system, namely the excess enthalpy and the excess entropy, while there is still a debate over which of both describes the Soret coefficient more accurately and whether it can even be represented using those quantities. In this work I present a theoretical description for both cases based on density analysis by means of Brownian motion and dynamical density functional theory, where I assume local equilibrium. The interpretation of the Brownian stochastic differential equation (SDE) as an Ito or Stratonovich SDE is shown to have an influence on the outcome of the density. I argue that a Soret coefficient proportional to the excess enthalpy is connected to a system in a thermal gradient following its equation of state. Furthermore, I derive that the Boltzmann distribution law for external potentials does not hold for systems in thermal gradients but has to be replaced by a more general quantity. The theoretical predictions are consequently tested by means of BD simulations, where several systems are shown to follow their equation of state. The Soret coefficient of a solute in a toy model solvent is shown to be proportional to the solute's equilibrium solvation enthalpy. I further attempt to verify the theoretic derivations by means of MD simulations of noble gas solutes in SPC/E water. While the Soret equilibrium SPC/E water density follows its equation of state, neither Soret coefficient nor Soret equilibrium solute densitie entail a coherent agreement with any of the theoretical predictions.