Non-equilibrium thermodynamics of the gas-liquid interface: Measurement of the Onsager heat of transport for carbon dioxide at the surface of water

The Onsager heat of transport Q* for carbon dioxide at the surface of water has been determined by measuring the total gas pressure as a function of the temperature difference across a 6-mm vapour gap above the water surface, and subtracting previously measured water-vapour pressures. The metal surface above the vapour gap was conditioned with carbon dioxide prior to the measurements, to enable efficient thermal accommodation for CO₂ molecules striking that surface, so that plots of ΔP against ΔT were linear for ΔT values up to at least 6 K. For liquid surface temperatures ranging from 0 to 5 °C, we find Q* = -6:9 ± 1.7 kJ mol^-1. Within experimental error, the results were independent of pH over the range from 6.50 to 9.04, and of CO2 partial pressure between 13 and 46 Torr. The factor Q*=RT is -3, which implies that the fractional temperature gradient is at least three times as important as the fractional partial-pressure gradient in controlling the magnitude and direction of the steady-state CO₂ flux through a water surface.