Surface behaviour of 1-alkyl-3-methylimidazolium ionic liquids at the air-water interface

The ability of 1-alkyl-3-methylimidazolium ionic liquids, [C_nmim][X], to form layers at the air–water interface has been investigated systematically for the first time: five different chain lengths of the cation, n = 12, 14, 16, 18, 20 were studied, combined with three different anions, namely [N(Tf)₂]⁻, [PF₆]⁻ and [Cl]⁻. The results show that only cations with chains longer than n = 16, combined with the [N(Tf)₂]⁻ anion form stable monolayers at the air/water interface, allowing measurable surface pressure vs area per molecule (π-A) isotherms immediately after spreading. The films of [C₂₀mim][N(Tf)₂] on water have been characterised using Brewster angle microscopy (BAM), Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS), specular X-ray reflectivity (XRR) and grazing incidence X-ray diffraction (GIXD). Additionally, atomistic molecular dynamics simulations were also performed to obtain a molecular level interpretation of the experimental results. The results are compatible with a transition from an expanded monolayer to a trilayer structure, followed by the 3D collapse of the film.