Enhanced concentration of polarizable anions at the liquid water surface: SHG spectroscopy and MD simulations of sodium thiocyanide

Contrasting current textbook descriptions, a consistent picture of substantial concentration enhancement of highly polarizable anions at the surface of aqueous electrolyte solutions is emerging. Such enhancement may have important implications for chemistry occurring on aqueous aerosols and ocean surfaces. Here we present a combined experimental and theoretical investigation of the liquid/air interface of aqueous sodium thiocyanide at varying salt concentrations. Normalized second harmonic generation intensities fitted to Langmuir isotherms yield a Gibbs free energy of adsorption of -1.80 kcal/mol. These results are in accord with molecular dynamics simulations in slab geometry, which predict an appreciable surface enhancement of SCN-.