Non-volatile Impurities in the Surface

If carbon dioxide, water, or any other non-volatiles are present in solution, which may be at concentrations of 1 ppm. or considerably less, then during surface evaporation, they do not evaporate and may subsequently remain at the surface as a monomolecu-lar film. This film may be continuous, or it may consist of discontinuous floating rafts of impurity but when it forms, it will provide a significant additional impedance to the surface evaporation process and literally switch off the evaporative mass flow. 

we have another triggering mechanism whereby the superheated liquid from the wall boundary flow cannot evaporate and a superheated layer forms immediately below the surface. 

Non-volatile films may also arise from condensation of carbon dioxide, water, etc. entering the ullage vapour space during custody management operations, or from leakage of atmospheric air. 

Laboratory studies with freely evaporating cryogenic Uquids have witnessed on many occasions the switching off of evaporative mass flows for periods of many minutes, and the subsequent rapid rise in evaporation, or vapour explosions, beyond the upper limit of the flow meter. However, the introduction of carbon dioxide, either into solution, or into the ullage space, in order to stimulate, or simulate, the formation of a film and switch off the evaporation, proved to be inconclusive. 

These monomolecular films do, of course, exist in ambient temperature environments and can be used to reduce the evaporation of water stored in reservoirs. They are also responsible for the stability of smog particles in the atmosphere. 

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