A qualitatively new understanding of the nature of ions at the liquid water surface is emerging. Traditionally, the characterization of liquid surfaces has been limited to macroscopic experimental techniques such as surface tension and electrostatic potential measurements, wherein the microscopic picture then has to be inferred by applying theoretical models. Because the surface tension of electrolyte solutions generally increases with ion concentration, all inorganic ions have been thought to be repelled from the air-water interface, leaving the outermost surface layer essentially devoid of ions. This oversimplified picture has recently been challenged: first by chemical kinetics measurements, then by theoretical molecular dynamics simulations using polarizable models, and most recently by new surface sensitive experimental observations. Here we present an overview of the nature of the interfacial structure of electrolyte solutions and give a detailed description of the new picture that is emerging.

The recent demonstration of enhanced surface anion concentrations for aqueous electrolyte solutions strongly contrasts current textbook descriptions. Small cations are still expected to be repelled from the surface, but recent simulations predict that hydronium (H3O+) cations are instead preferentially adsorbed at the interface. Here we describe a comparative second harmonic generation (SHG) study of aqueous solutions of hydriodic acid (HI) and alkali iodides (NaI and KI), which establish lower limits of 55% and 34% larger surface iodide concentrations for HI solutions relative to NaI and KI solutions, respectively. This result implies that hydronium ions must exist in much higher densities near the liquid surface than do the alkali ions, in support of the theoretical predictions.

Ions induce changes in the H-bond network of water that extend by >20 nm, vary for H2O and D2O, and lead to surface tension anomalies.

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The nonlinear optical response of semiconductor nanowires has potential application for frequency conversion in nanoscale optical circuitry. Here, second-and third-harmonic generation (SHG, THG) are imaged on single zinc oxide (ZnO) nanowires using near-field scanning optical microscopy (NSOM). The absolute magnitudes of the two independent χ (2) elements of a single wire are determined, and the nanowire SHG and THG emission patterns as a function of incident polarization are attributed to the hexagonal nanowire geometry and χ (2) tensor symmetry.