Non-Equilibrium Thermodynamics of the Gas-Liquid Interface: Measurement of the Onsager Heat of Transport for Ammonia at the Surface of Water

Abstract

The Onsager heat of transport Q* for ammonia at the surface of water was measured at ammonia partial pressures from 12 to 875 Torr and found to be independent of pressure over that range, with an average value of –7.7 ± 2.8 kJ mol^–1 (95% confidence limits). Plots of ΔP versus ΔT initially showed a change of slope, or “knee”, at ΔT ~ 0.5 °C. After the stainless-steel Onsager cell had been conditioned by exposure to several hundred Torr of ammonia, plots of ΔP versus ΔT were linear up to ΔT ~ 6 °C. The conditioning is reversible, and is attributed to the effect of chemisorbed ammonia on thermal accommodation coefficients at the metal surface. The results are discussed in relation to the free-energy barrier to evaporation at the surface of a liquid, and in relation to field measurements of the rates of air–sea exchange of greenhouse gases.