Structural Study at the Gas‐Liquid Interface of 1‐Alkyl‐3‐Methylimidazolium Alkylsulfates Using Surface Potential Measurements

Abstract

Sum frequency generation, surface potential, and surface tension measurements have been combined on the pure ionic-liquid-gas interface for 1-alkyl-3-methylimidazolium alkylsulfate ionic liquids. The results show that surface potential of the ionic liquid generally increases as the alkyl chain on the cation or anion increases in length. This is due to the increased ordering of the surface dipole, mostly coming from the terminal methyl group of the alkyl chain. Both sum frequency generation spectroscopy and surface potential measurements suggest that the charged components, that is, the aromatic ring and the sulfate, occupy nearly the same plane at the surface for all ionic liquids studied herein.

The ionic liquid–vacuum outer atomic surface: a low-energy ion scattering study

“…Martinez et al have measured the ionic liquid-vacuum surface potential for 10 [C n C 1 Im][C n OSO 3 ] ionic liquids (where n ¼ 1 to 8). 42 These results were interpreted using a model of an alkyl overlayer and an ionic underlayer. It should be noted that we have detected the OSO 3 group at the ionic liquid-vacuum outer atomic surface for [C 4 C 1 Im][C 8 OSO 3 ].…”

“…24 Ionic liquids have sufficiently low vapour pressure that they can be studied at room temperature using standard UHV apparatus, opening up the possibility of using a wide range of surface science techniques to probe the molecular level structure of these liquid systems. 13,[25][26][27] The techniques that have been used to investigate the ionic liquid-vacuum surface (the region within which the properties of the ionic liquid are signicantly different from that of the bulk ionic liquid) can be separated into four different areas: spectroscopy, [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] scattering, [46][47][48][49][50][51][52][53][54][55][56][57][58][59] sticking probability/temperature programmed desorption, [60][61][62] and molecular dynamics (MD) simulations. 45,[63][64][65][66]…”

“…SFG spectroscopy studies on an extensive range of ionic liquids by Baldelli and co-workers have concluded that the imidazolium ring plane is oriented parallel to the ionic liquidvacuum surface plane. 1,41,42 Reports from MD simulations have concluded that the most probable imidazolium ring orientation is parallel 66,67,70 or perpendicular [64][65][66][71][72][73][74]77,78 to the ionic liquidvacuum surface plane; it is unclear at this stage if the different orientations reported are due simulation effects or ionic liquids.…”

Surface structure evolution in a homologous series of ionic liquids

“…Thus, the evolution of the RTIL-air interface structure as the main interactions change gradually from mixed Coulomb-vdW to vdW dominance by varying the chain length, and that only, has not been hitherto determined by high-resolution X-ray methods for even a single long homologous RTIL series. Related issues were, however, addressed by simulations (10)(11)(12), and macroscopic (9,25,26) and compositional (27)(28)(29)(30)(31) methods. Thus, a quantitative determination of the structure's characteristics (layering periodicity and decay length) for a broad n range in a single RTIL series, as done here, should also provide an important yardstick for testing theoretical and simulation studies.…”

Probing Liquid Interfaces with Room-Temperature Ionic Liquids Using the Excited-State Dynamics of a Cationic Dye

“…IL interfaces have been mostly investigated by SSFG. [32][33][34][35][36][37][38][39][40][41] For example, ILs at various interfaces have been thoroughly studied by Baldelli and coworkers. 40 Their studies show that the non-polar part of the ions, e.g.…”