Effect of the aliphatic chain length on electrical double layer formation at the liquid/vacuum interface in the [Cnmim][BF4] ionic liquid series

Neutral impact collision ion scattering spectroscopy (NICISS) was used to determine the surface structure of three ionic liquids, 1-hexyl-3- methylimidazolium [C₆mim], 1-octyl-3-methylimidazolium [C ₈mim], 1-decyl-3-methylimidazolium [C₁₀mim] tetrafluoroborates [BF₄]. Concentration depth profiles of the elements in an ionic liquid (IL) homologous series with a common anion were obtained. We show that separation between the oppositely charged ions is seen for all three ionic liquids, resulting in an electrical double layer formation. The surface charge shifts from more positive to more negative with increasing aliphatic chain length.