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

• Published in: Journal of molecular liquids Vol. 370; p. 121028
• Main Authors: Taßler, Stephanie, Bonatout, Nathalie, Eusébio, Tiago M., Valente, Raquel M., Rego, Tomás, Ibrahim, Helen, Morgado, Pedro, Filipe, Eduardo J.M., Goldmann, Michel, Fontaine, Philippe
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.01.2023; Elsevier
• Subjects: 1-Alkyl-3-methylimidazolium ionic liquids; Chemical Sciences; Condensed Matter; GIXD; IRRAS; Langmuir monolayer; Material chemistry; MD Simulations; Physics; XRR; N(Tf)2; PF6; ESP; Cnmim; C18mim; 1-Alkyl-3-methylimidazolium ionic liquids; MD Simulations; C16mim; Cl; C14mim; XRD; π-A-Isotherm; BAM; GIXD; IRRAS; arb. units; C12mim; LC; cmc; XRR; LE; Langmuir monolayer; C20mim
• DOI: 10.1016/j.molliq.2022.121028
• ISSN: 0167-7322

Molecular dynamics simulations snapshots and electron density determined by X-ray reflectivity on layers of C20mimNTf2 ionic liquid at the air water interface undergoing a monolayer to trilayer transition with increase of surface density. [Display omitted] •1-alkyl-3-methylimidazolium ionic liquids with chain longer than 16 carbons and with NTF2 cation forms Langmuir monolayers.•For [C20mim][NTf2], X-ray diffraction reveal a transition from an expanded monolayer to a trilayer organized structure.•The monolayer to trilayer transition was reproduced by atomistic molecular dynamics simulations. The ability of 1-alkyl-3-methylimidazolium ionic liquids, [Cnmim][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.