Significant changes of the charge distribution at the surface of an ionic liquid due to the presence of small amounts of water

The influence of small amounts of water dissolved in 1-hexyl-3- methylimidazolium chloride ([C₆mim][Cl]) on the composition of the surface of the ionic liquid is investigated with the depth profiling technique neutral impact collision ion scattering spectroscopy. The concentration depth profiles of the elements in the sample were determined at three different water concentrations and show that small amounts of water affect the charge distribution in the ionic liquid along the surface normal. At low water concentrations (2500 ppm) the cation shows a strong presence at the surface with the alkyl chains oriented towards the gas phase, followed by a layer of anions below the alkyl chains of the cation. At higher water content (6000 to 10000 ppm) the chloride anion shows an increased concentration at the ionic liquid surface while the alkyl chains move towards the bulk showing that the surface charge becomes more negative with increasing water content. The effect is attributed to the influence of water on the hydrogen bonding network in the ionic liquid.