Effect of humidity on monolayer desorption at the air–water interface

Abstract

THE effect of atmospheric humidity on the physicochemical properties of lipid monomolecular films at the air–water interface has been thought to be of secondary importance, and has never been studied experimentally. Yet, the physical situation encountered at the air–water interface may be strongly dependent on the humidity in the gas phase1,2. Harkins3, while not specifically investigating this effect, took painstaking measures to ensure saturation of the air above the interface in an effort to suppress any temperature difference between the interior of the aqueous phase and its free surface. Obviously, below the saturation point of the air, the free surface of water is expected to be at a lower temperature than the interior by virtue of the evaporation of liquid. Thus, in some instances, the temperature of the outermost layer of the free water surface was reported to drop by 15°C or more relative to the temperature of the bulk from measurements of the surface tension during evaporation4. In other investigations the mean temperature, measured by thermistors, of a thin region immediately below the free surface of a water pool was reported to be several degrees lower than that in the interior during evaporation1. The data presented here constitute a clear indication of the decisive effect of atmospheric humidity on the kinetics of monolayer desorption from the air–water interface. Figure 1 illustrates the dramatic difference in the observed decrease of the surface pressure of myristic acid monolayers in three different conditions of atmospheric humidity.