The Rate of Evaporation of Water through Duplex Films

Abstract

The stability of duplex films of hydrocarbon oils containing spreaders is discussed in this paper. Duplex films containing spreaders of high molecular weight and of complex nature, e.g. stand oil, eucalyptus residue, polymerized oleic acid, may remain spread for long periods but are thermodynamically metastable. The penetration of water molecules from a water surface through a duplex film has been discussed in terms of the evaporation resistances of the interfacial layer of spreader, the bulk layer of oil and the stagnant layer of air above the film. Evaporation through duplex films of thickness 1-100 μ has been studied in conditions of " still air " at atmospheric pressure and in a vacuum, and in non-turbulent winds at several controlled wind velocities. The efficiency of duplex films in reducing evaporation is greater in wind than in still air and Increases with the wind velocity. When duplex films thicker than 10 μ are placed on water surfaces, even a wind at 8 miles per hour does not increase the rate of evaporation. Thus with such films the resistance of the stagnant layer of air above the film is small compared with that of the oil layer and of the interfacial film of spreader. With all duplex films thicker than 10 μ the total evaporation resistance is proportional to the film thickness and depends on the nature of the spreader. An analysis of the experimental results suggests that the interfacial layer of spreader makes an appreciable contribution to the evaporation resistance of the duplex films under investigation. Approximate relative figures characterizing the magnitude of this contribution are calculated for several spreaders.