Confirmation of enhanced anion concentration at the liquid water surface

The textbook view of the liquid electrolyte surfaces as being devoid of ions have recently been challenged by molecular dynamics simulations, which predict a surface enhancement of highly polarizable anions. Here we present the first direct experimental verification of this prediction. Enhanced azide ( N 3 - ) concentrations were measured at the liquid surface by femtosecond second harmonic generation (SHG) experiments exploiting the charge-transfer-to-solvent (CTTS) resonance of N 3 - , yielding a surface excess free energy of −9.9 ± 0.3 kJ/mole. Such surface-enhanced concentrations of anions could have important consequences for the chemical reactions taking place on atmospheric aerosols and at the ocean-air interface.