Pressure-independence of the Onsager heat of transport at a gas–liquid interface: A theoretical problem

Recent measurements of the Onsager heat of transport Q∗ for ammonia at the surface of water have found Q∗ to be constant for gas pressures over the range 10–900 Torr. This result cannot be understood in terms of a molecular-scale model for the origin of the heat of transport at a gas–liquid interface, which predicts that Q∗ should decrease with increasing pressure, and must be regarded as a direct consequence of Onsager’s reciprocal relation for coupled heat and matter fluxes.