Interfacial Structure at the Quaternary Ammonium-Based Ionic Liquids|Gold Electrode Interface Probed by Surface-Enhanced Infrared Absorption Spectroscopy: Anion Dependence of the Cationic Behavior

The interfacial structure at the quaternary ammonium-based ionic liquids­(ILs)|gold­(Au) electrode interface has been studied using surface-enhanced infrared absorption spectroscopy (SEIRAS). Four anions, bis­(perfluoroalkanesulfonyl)­amide (C_nC_nN^–; n = 0, 1, 2, 4), have been combined with a quaternary ammonium cation, trioctylmethylammonium (N₈₈₈₁⁺), to investigate the influence of the perfluoroalkyl chain length of the anion on the behavior of the quaternary ammonium cation at the interface. In addition, to investigate the effect of the alkyl chain length of the quaternary ammonium cations on the cationic behavior, we have also combined a cation with a shorter alkyl chain, tributylmethylammonium (N₄₄₄₁⁺) with C₁C₁N^–. Thus, we have performed SEIRAS measurements at the Au interface of five ILs: [N₈₈₈₁⁺]­[C_nC_nN^–] (n = 0, 1, 2, 4) and [N₄₄₄₁⁺]­[C₁C₁N^–]. The four CH stretching bands originating from the quaternary ammonium cations have been individually analyzed, enabling us to reveal the behavior of the quaternary ammonium cations at the interface. The cationic behavior is found to dramatically depend not only on the alkyl chain length but also on the perfluoroalkyl chain length of the counterion. For [N₈₈₈₁⁺]­[C₄C₄N^–] and [N₈₈₈₁⁺]­[C₂C₂N^–], octyl chains of N₈₈₈₁⁺ cannot reach the Au electrode surface at positive potentials because the bulky anions in the first ionic layer on the electrode surface block the approach. Conversely, for ILs with the smaller anions ([N₈₈₈₁⁺]­[C₁C₁N^–] and [N₈₈₈₁⁺]­[C₀C₀N^–]), octyl chains of N₈₈₈₁⁺ can penetrate into a space in the first ionic layer of small anions. The butyl chains of N₄₄₄₁⁺ in [N₄₄₄₁⁺]­[C₁C₁N^–] cannot reach the surface at positive potentials even across the first ionic layer of the small anions because of their relatively short alkyl chain length.