Comparing the charge distribution along the surface normal in the [C6mim]+ ionic liquid with different anions

Concentration depth profiles of the elements in [C6mim]X (X = [BF4]− and [Cl]−) have been determined using neutral impact collision ion scattering spectroscopy in order to investigate how the choice of the anion influences the surface structure of the [C6mim]+ ionic liquids. The results show that the more polarisable, fluorinated anion [BF4]−, has a stronger tendency to adsorb to the surface compared to [Cl]−. While the [BF4]− intermingles with the cation aliphatic chains, the decreased surface propensity of the [Cl]− allows for the preferential orientation of the cation aliphatic chains pointing towards the vapour phase.