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 CO(2) molecules striking that surface, so that plots of Delta P against Delta T were linear for Delta T values up to at least 6 K. For liquid surface temperatures ranging from 0 to 5 degrees 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 CO(2) 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 CO(2) flux through a water surface.