Temperature measured close to the interface of an evaporating liquid

Recent measurements of the temperature profile across the interface of an evaporating liquid are in strong disagreement with the predictions from classical kinetic theory or nonequilibrium thermodynamics. However, these previous measurements in the vapor were made within a minimum of 27 mean free paths of the interface. Since classical kinetic theory indicates that sharp changes in the temperature can occur near the interface of an evaporating liquid, a series of experiments were performed to determine if the disagreement could be resolved by measurements of the temperature closer to the interface. The measurements reported herein were performed as close as one mean free path of the interface of an evaporating liquid. The results indicate that it is the higher-energy molecules that escape the liquid during evaporation. Their temperature is greater than that in the liquid phase at the interface and as a result there is a discontinuity in temperature across the interface that is much larger in magnitude (up to 7.8 °C in our experiments) and in the opposite direction to that predicted by classical kinetic theory or nonequilibrium thermodynamics. The measurements reported herein support the previous ones.