Pressure independence of the Onsager heat of transport for a gas–liquid interface at high pressures

The Onsager heat of transport Q∗ at the liquid–vapour interface of water is constant at −24.3 ± 3.6 kJ mol−1 (two standard deviations) over the pressure range from 4 to 9.5 Torr. This result is consistent with a model in which Q∗ is determined by the temperature gradient across the Knudsen layer adjacent to the liquid, but not with one based on perturbed temperature profiles from temperature-jump theory, which predicts that the measured heat of transport should vary inversely with pressure. It implies that the Onsager heat of transport should be significant for gas–liquid exchange processes even at atmospheric pressure.