The interfaces of water/room temperature ionic liquids (RTIL) (1-alkyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)amide ([C(n)mim]TFSA): n = 4, 8) are investigated by infrared-visible sum frequency generation (IV-SFG) vibrational spectroscopy and molecular dynamics (MD) simulation. SFG spectra taken within the SO stretch region drastically differ between air/RTIL and water/RTIL interfaces. When a RTIL surface is in contact with water, a broadened and blue-shifted SO2-ss mode peak is observed in the SFG spectra, indicating an inhomogeneous intermolecular interaction due to hydrogen bonding of the [TFSA]- anions and water molecules at the water/[C(n)mim]TFSA interface. MD simulations show the SO2 groups of the anion are preferentially orientated toward the water phase, which is consistent with the SFG spectral features. Polar orientation of the [TFSA] anion originates from the ordered structure of the alkyl chains of [C(n)mim]+ cations.

We demonstrate for the first time the formation of a non-polar alkyl-chain dividing layer between a room-temperature ionic liquid (RTIL) and an n-alcohol. This newly described non-polar interfacial layer, which should be more hydrophobic than both RTIL and alcohol phases, might find applications in liquid/liquid reaction systems, or serve as a soft nano-functional space.

The effect of Li + addition at the interface of a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) room-temperature ionic liquid (RTIL) and a Pt electrode is investigated by infraredvisible sum-frequency generation (IV-SFG) vibrational spectroscopy. Addition of Li + to the Pt|[C2mim][FSA] system results in the extension of the electrochemical window (EW) by more than 1.0 V at its negative edge. The potential dependence of the SF signal reveals that the [FSA] − anion of neat [C2mim][FSA] is desorbed at −1.5 V while it remains in place even at −2.0 V when Li + is added. The SFG spectra indicate that the [FSA] − anion at the Pt|[C2mim][FSA] interface interacts with Li + at the interface with the negatively-charged Pt electrode. This [FSA] − anion layer anchored through Li + suppresses [C2mim] + cation adsorption on the negatively-charged Pt electrode, resulting in a wider electrochemical window.

IR-visible sum-frequency generation (IV-SFG) vibrational spectroscopy and a molecular dynamics (MD) simulation were used to study the local layering order at the interface of 1-butanol-d9 and 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]PF6), a room-temperature ionic liquid (RTIL). The presence of a local non-polar layer at the interface of the two polar liquids was successfully demonstrated. In the SFG spectra of 1-butanol-d9, we observed significant reduction and enhancement in the strength of the CD3 symmetric stretching (r(+)) mode and the antisymmetric stretching (r(-)) mode peaks, respectively. The results can be well explained by the presence of an oppositely oriented quasi-bilayer structure of butanol molecules, where the bottom layer is strongly bound by hydrogen-bonding with the PF6(-) anion. MD simulations reveal that the hydrogen-bonding of butanol with the PF6(-) anion causes the preferential orientation of the butanols; the restriction on the rotational distribution of the terminal methyl group along their C3 axis enhances the r(-) mode. As for the [bmim](+) cations, the SFG spectra taken within the CH stretch region indicate that the butyl chain of [bmim](+) points away from the bulk RTIL phase to the butanol phase at the interface. Combining the SFG spectroscopy and MD simulation results, we propose an interfacial model structure of layering, in which the butyl chains of the butanol molecules form a non-polar interfacial layer with the butyl chains of the [bmim](+) cations at the interface.

IV-SFG vibrational spectroscopy and MD simulation studies successfully demonstrate the presence of a “head-to-head” bi-layer structure at ionic liquid surfaces.