Effect of Surfactants on the Evaporation of Water Close to 100° C

In a continuation of studies on the superheat temperature of highly purified water evaporating close to 100°C, reductions in the evaporation rate were observed after additions of surfactants whose concentrations were as low as 9200 Ų/molecule of surface water. For surfactants above their melting points, at low concentrations, it appears that the changes in evaporation are influenced only by the amount of surfactant and whether it is nonionic or ionic. At higher concentrations, the chemical nature of the surfactant becomes important in reducing evaporation. For the highest concentrations studied, the reduction produced was not chemically specific. A decrease in evaporation rate was also observed when surfactants were added to water boiling from a single nucleation site at low heat fluxes. In both quiescent and ebullient evaporation at low heat fluxes, these reductions in evaporation are attributed to the surfactant-produced decrease in the surface-tension-driven covection occurring at the liquid-vapor interface.