Retardation of Water Drop Evaporation with Monomolecular Surface Films

Abstract

The influence of a variety of water-insoluble monomolecular films on the evaporation rate of 50 to 500 μ diameter water drops was determined at 25C and 50% relative humidity. The results indicate that only linear polar molecules capable of close packing can form monomolecular films which inhibit the gas transport of water molecules across an air–water interface. Slight deviations from straight-chain linearity due to substituents on the hydrocarbon backbone of the surface-active molecule prevent molecular adlineation, alter the physical state of the monolayer, and sharply reduce its ability to retard evaporation. The influence of impurities which do not affect evaporation was evaluated by studying mixed films which contained both linear and nonlinear molecules. For each of the chemical systems studied, the rate of evaporation decreased to a constant value as the surface film was compacted on the shrinking drop surface. Evaporation rates were decreased up to 17-fold by the most effective fatty alcohol and ester monolayers. Literature claims of much greater retardation effectiveness are attributed to evaporation measurements of the film-forming organic material rather than that of the water drop. The implications of these chemical effects to fog and haze stabilization and to the possible modification of atmospheric processes are discussed.