Microscopic mechanism of thermomolecular orientation and polarization

TL;DR

This paper develops a microscopic theory linking thermomolecular orientation and polarization in molecules under temperature gradients, matching simulations and enabling control through molecular design.

Contribution

It introduces an analytically solvable microscopic model that connects molecular properties to thermomolecular effects, a previously unknown link.

Findings

Theory quantitatively matches molecular dynamics simulations.

Thermomolecular orientation depends on molecular volume, size anisotropy, and dipole moment.

Orientation can be maximized by tuning molecular properties.

Abstract

Recent molecular dynamics simulations show that thermal gradients can induce electric fields in water that are comparable in magnitude to electric fields seen in ionic thin films and biomembranes. This surprising non-equilibrium phenomenon of thermomolecular orientation is also observed more generally in simulations of polar and non-polar size-asymmetric dumbbell fluids. However, a microscopic theory linking thermomolecular orientation and polarization to molecular properties is yet unknown. Here, we formulate an analytically solvable microscopic model of size-asymmetric dumbbell molecules in a temperature gradient using a mean-field, local equilibrium approach. Our theory reveals the relationship between the extent of thermomolecular orientation and polarization, and molecular volume, size anisotropy and dipole moment. Predictions of the theory agree quantitatively with molecular dynamics simulations. Crucially, our framework shows how thermomolecular orientation can be controlled and maximized by tuning microscopic molecular properties.