Onsager Heat of Transport Measured at the n-Heptanol Liquid−Vapor Interface

The Onsager heat of transport Q* has been determined by measuring the effect of a temperature gradient at the interface on the apparent vapor pressure of the liquid. Results for n-heptanol extend to pressures that are low enough for the separation between the liquid and the warmer surface above it to be less than one mean free path. At low pressure, the dependence of Δ P on Δ T is found to be linear, in contrast to the curves obtained when the separation is greater than a mean free path. As with aniline, the heat of transport is negative and its magnitude approaches the latent heat of vaporization at low pressures. The results are discussed in relation to three anomalous effects that have been predicted or observed during steady-state evaporation and also in relation to the problem of determining rates of air−sea exchange for atmospheric gases.