Adsorption of Atmospheric Gases at the Air−Water Interface. I. NH₃

Abstract

Several water-soluble vapors of atmospheric importance adsorb at the air−water interface. This paper supplies a thermodynamic and kinetic framework for analyzing this phenomenon. As an example, temperature- and time-dependent surface-tension measurements of aqueous ammonia solutions are used to extract the interfacial binding energies and evaporation rates. The standard Gibb's energy of adsorption of vapor-phase ammonia to the water surface is −(19.1 ± 0.5) kJ mol-1 at 298 K; the saturated coverage is (1.2 ± 0.2) × 1014 molecules cm-2. The Gibb's energy of activation for ammonia evaporation from the water surface lies in the range 13−18 kJ mol-1 at 298 K. Ab initio calculations of the NH3−H2O and NH3−(H2O)2 complexes have also been performed to further understand the nature of the surface-bound species. The experimental and ab initio results, taken together, suggest that ammonia is bound by a small number (two or three) of water molecules at the surface; this complex species represents a “critical cluster” which is easily transferred into the bulk solution.