Effect of Surface Active Ions on the Rate of Water Evaporation

Current understanding of the vapor-liquid exchange kinetics of liquid water is incomplete, leading to uncertainties in modeling the climatic effects of clouds and aerosol. Initial studies of atmospherically relevant solutes (ammonium sulfate, sodium chloride) indicate that their effect on the evaporation kinetics of water is minimal, but all those constituent ions are also expected to be depleted in concentration at the air-water interface. We present measurements of the evaporation kinetics of water from 4 M sodium perchlorate solution, which is expected to have an enhanced concentration of perchlorate in the surface layer, using Raman thermometry of liquid microdroplets in a free evaporation regime. We determine the evaporation coefficient γe to be 0.47 ( 0.02, ca. 25% smaller than our measured value for pure water (0.62 ( 0.09). This change, while small, indicates that direct interactions between perchlorate ions and evaporating water molecules are affecting the evaporation mechanism and kinetics and suggests that other solutes with high surface affinities may also produce a similar influence in the atmosphere and elsewhere.