Photoemission of Iodide from Aqueous Aerosol Particle Surfaces.

The photoemission of iodide from aqueous aerosol particle surfaces measures the surface concentration of iodide in predominantly supersaturated NaCl aerosol particles. Using the Langmuir model to describe the adsorption to the surface of aqueous iodide anions, the standard Gibbs free energy of adsorption is -15 kJ/mol in these systems. The presence of charged surfactants on the particle surfaces changes the adsorption behavior of iodide. The addition of sodium docecylsulfate (SDS) reduces the coverage of iodide, consistent with a competitive adsorption scenario. For surfaces coated with C-, C-, or C-trimethylammonium chloride, the addition of iodide results in the formation of iodide-surfactant ion pairs at the surface with enhanced photoemission. The adsorption free energy for iodide in these systems is -21 kJ/mol. The results demonstrate the surface enhancement of iodide in supersaturated, atmospherically relevant conditions and demonstrate important differences between single-salt solutions and mixtures in the limit of high concentration.