Abstract: We show that thermal gradients polarize liquid and supercritical acetonitrile. The polarization results in a stationary electrostatic potential that builds up between hot and cold regions. The strength of the field increases with the static dielectric constant or with decreasing temperature. At near standard conditions, the thermal polarization coefficient is ??0.6 mV/K, making it possible to induce significant electrostatic fields, ?103 V/m, with thermal gradients ?1 K/?m. At supercritical conditions, ?600 K and 0.249 g/cm3 (the critical isochore), the electrostatic field is of the same order, despite the low dielectric constant of the fluid. In this case, the electrostatic field is determined by the enhanced rotational diffusion of the molecules and stronger cross-coupling between heat and polarization fluxes. We show that the coupling between the heat and polarization fluxes influences the thermal conductivity of acetonitrile, which becomes a worse heat conductor. For the thermodynamic states investigated in this work, the thermal polarization effect leads to a ?2%?5% reduction in thermal conductivity.

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