Vibrational sum-frequency generation spectroscopy of ionic liquid 1‐butyl‐3‐methylimidazolium tris(pentafluoroethyl)trifluorophosphate at the air-water interface

•Interfacial structure of ionic liquid [bmim][fap] has been investigated by sum-frequency generation.•Orientations of butyl chain differ at the air-water and air-[bmim][fap] interfaces.•The interfacial [bmim][fap] molecules aggregate at the air-water interface as probed by BAM images.

The structure and orientation of room temperature ionic liquid (RTIL) 1‐butyl‐3‐methylimidazolium tris(pentafluoroethyl)trifluorophosphate [PF3(C2F5)3], commonly known as [bmim][fap], have been investigated at the air−[bmim][fap] and air-water interfaces, employing vibrational sum-frequency generation (VSFG) spectroscopy. The VSFG spectra in the CH stretch region suggest presence of the [bmim] cation at the interfaces. Studies reveal that the butyl chain protrudes out into air, and the imidazolium ring lies almost planar to the interface. The CH stretch intensities get enhanced at the air-water interface, mainly because of polar orientation of imidazolium cation induced by interfacial water molecules. The OH stretch intensities are also enhanced at the air-water interface due to polar orientation of interfacial water molecules induced by [bmim][fap]. The Brewster angle microscopy suggests self aggregation of [bmim][fap] in the presence of water, and the aggregation becomes extensive showing dense surface domains with time. However, the surface pressure is almost unaffected due to aggregation.

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