Guilty as charged: Water is often modeled as a dielectric continuum, but the molecular structure of water is asymmetric. Two ions that have a virtually identical size, shape, and structure, but an opposite charge sign have been investigated to see whether charge makes a fundamental difference to water structuring. The spectroscopic data for the hydration and interface structures are found to be remarkably different for opposite charges.

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The molecular ionic surface structure and charge of the electric double layer around a nanodroplet and its structural change induced by hydrophobic effects are measured using vibrational coherent surface scattering spectroscopy, second harmonic scattering, and electrokinetic mobility measurements. Tetraalkylammonium chloride salts were added to negatively charged nanoscopic oil droplets in water. When we vary the alkyl chain length of the cation from CH3 to C4H10, both the size of the cation and its hydrophobic interaction are increased. We find that tetramethylammonium ions change the electrokinetic potential and the water structure but do not detectably adsorb to the interface. Tetrapropylammonium and tetrabutylammonium ions clearly adsorb to the interface. The corresponding (Stern) layer appears to be a mixed monolayer of anions and cations. An estimate of the amount of cations in the Stern layer is also made.