Surface of ionic liquids

Rodopoulosb, T. (2010) Orientation and mutual location of ions at the surface of ionic liquids. Phys. Chem. Chem. 

Structure of ionic liquids at interfaces, another fascinating and important subject, was studied by Baldelli and co-workers [63] and by Ouchi [64] using surface spectroscopic techniques. Non-polar alkyl chains are segregated towards vacuum at the free surface of ionic liquids, as a result of the balance between coulombic and van der Waals forces [63, 64]. 

Mbrational spectroscopy is well suited to elucidating the details on the surface of ionic liquids. This is due to the high degree of chemical information that is provided in the vibrational spectrum The various techniques mentioned earlier each have advantages and limitations, but the level of information is clearly unique and highly informative. From the vibrational spectrum, information as to the identity of the molecules is available. However, more specifically, the chemical functional groups are identified as they each have unique... 

Overall, vibrational spectroscopy provides very useM chemical information on the surface of ionic liquids, and some specific exanples are given in the following. 

Many of the important chemical applications of ILs will occur at solid surfaces, including electrochemical processes at IL-electrode interfaces, lubrication of ILs, fabrication of IL solid electrolytes and IL solid catalysts, etc. When a solid interface is present, molecules near the interface are subject to diflferent interactions than in the bulk phase, and the free energy of a surface can often be reduced by local changes in molecular orientation, aggregation, density, or composition. Familiar examples include surface adsorption, wetting and the electrochemical double-layer structure, where dipole moments usually lie at the interface. The surfaces of ionic liquids at the solid surface show dramatic changes in local structure, which can be demonstrated using simulations and probed by a number of experimental techniques. Due to a wide variety of experimental, theoretical, and... 

Meiss, S. A., Rohnke, M., Kienle, L., Abedin, S. Z. E., Endres, F. Janek, J. (2007). Employing Plasmas as Gaseous Electrodes at the Free Surface of Ionic Liquids Deposition of NanocrystaUine Silver Particles, ChemPhysChem 8 50-53. 

Viewed in conjunction with the solid-like, nonvolatile nature of ionic liquids, it is apparent that TSILs can be thought of as liquid versions of solid-supported reagents. Unlike solid-supported reagents, however, TSILs possess the added advantages of kinetic mobility of the grafted functionality and an enormous operational surface area (Figure 2.3-1). It is this combination of features that makes TSILs an aspect of ionic liquids chemistry that is poised for explosive growth. 

Although this technique has not been used extensively, it does allow structures of adsorbed layers on solid substrates to be studied. Liquid reflectivity may also be performed with a similar set-up, which relies on a liquid-liquid interface acting as the reflective surface and measures the reflectivity of a thin supported liquid film. This technique has recently been used to investigate water-alkane interfaces [55] and is potentially useful in understanding the interaction of ionic liquids with molecular solvents in which they are immiscible. 

The surface structures of ionic liquids have been studied by direct recoil spectrometry. In this experiment, a pulsed beam of 2-3 keV inert gas ions is scattered from a liquid surface, and the energies and intensities of the scattered and sputtered (recoiled) ions are measured as a function of the incident angle, a, of the ions. Figure 4.1-16 shows a scheme of the process for both the scattered and sputtered ions. 

The incident ions cause recoil in the surface atoms. In studies of ionic liquids, only direct recoil - that is, motion in the forward direction - was measured. Watson and co-workers [56, 57] used time-of-flight analysis with a pulsed ion beam to measure the kinetic energies of the scattered and sputtered ions and therefore determine the masses of the recoiled surface atoms. By relating the measured intensities of the... 

The charged species were in all cases found to concentrate at the surface of the liquid under vacuum conditions. Little surface separation of the anions and cations was observed. For the [PFg] and [BFJ ions, the cation ring was found to prefer a perpendicular orientation to the surface, with the nitrogen atoms closest to the surface. An increase in the alkyl chain length caused the cation to rotate so that the alkyl chain moved into the bulk liquid, away from the surface, forcing the methyl group closer to the surface. For halide ionic liquids, the data were less clear and the cation could be fitted to a number of orientations. 

These are typical of ionic liquids and are familiar in simulations and theories of molten salts. The indications of structure in the first peak show that the local packing is complex. There are 5 to 6 nearest neighbors contributing to this peak. More details can be seen in Figure 4.3-3, which shows a contour surface of the three-dimensional probability distribution of chloride ions seen from above the plane of the molecular ion. The shaded regions are places at which there is a high probability of finding the chloride ions relative to any imidazolium ion. 

In conclusion, many methods are available for the immobihzation of metal complexes with diazaligands. The biphasic liquid methods seem to be more versatile - especially in the case of ionic liquids, where modification of the ligands is not needed. Very good results can also be obtained with solid supports, although the surface effects, which can be positive or negative. 

Some other studies showed that the combination of the three polymorphs with reduced crystallite size and high surface area can lead to the best photocatalysts for 4-chlorophenol degradation [37], or that particles in the dimension range 25-40 nm give the best performances [38]. Therefore, many elements contribute to the final photocatalytic activity and sometimes the increased contribution of one parameter can compensate for the decrease of another one. For example, better photocatalytic activity can be obtained even if the surface area decreases, with a concomitant increase in the crystallinity of the sample, which finally results in a higher number of electron-hole pairs formed on the surface by UV illumination and in their increased lifetime (slower recombination) [39]. Better crystallinity can be obtained with the use of ionic liquids during the synthesis [39], with a consequent increase of activity. 

In 2002 Mehnert and co-workers were the first to apply SILP-catalysis to Rh-catalysed hydroformylation [74], They described in detail the preparation of a surface modified silica gel with a covalently anchored ionic liquid fragment (Scheme 7.7). The complex N-3-(3-triethoxysilylpropyl)-4,5-dihydroimidazole was reacted with 1-chlorobutane to give the complex l-butyl-3-(3-triethoxysilylpropyl)- 4,5-dihydroimidazolium chloride. The latter was further treated with either sodium tetrafluoroborate or sodium hexafluorophosphate in acetonitrile to introduce the desired anion. In the immobilisation step, pre-treated silica gel was refluxed with a chloroform solution of the functionalised ionic liquid to undergo a condensation reaction giving the modified support material. Treatment of the obtained monolayer of ionic liquid with additional ionic liquid resulted in a multiple layer of free ionic liquid on the support. 

Scheme 7.7. Preparation of surface anchored ionic liquid phases...

A rather new concept for biphasic reactions with ionic liquids is the supported ionic liquid phase (SILP) concept [115]. The SILP catalyst consists of a dissolved homogeneous catalyst in ionic liquid, which covers a highly porous support material (Fig. 41.13). Based on the surface area of the solid support and the amount of the ionic liquid medium, an average ionic liquid layer thickness of between 2 and 10 A can be estimated. This means that the mass transfer limitations in the fluid/ionic liquid system are greatly reduced. Furthermore, the amount of ionic liquid required in these systems is very small, and the reaction can be carried in classical fixed-bed reactors. 

Fig. 15. Preparation of surface-anchored ionic liquid phases. Reproduced with permission from Me-hnert et al. (97).

Figure 3.2 Signal patterns from 80% organic vapor samples at 120°C. Responses are normalized to the surface loading of ionic liquids and the concentrations of vapors in the carrier gas. (Reprinted from Jin, X., Yu, L., Garcia, D., Ren, R.X., and Zeng, X., Anal. Chem., 78,6980-6989,2006. Copyright 2006 American Chemical Society. With permission.)...

Determination of void volumes for surface-confined ionic liquids as stationary phases in high-performance liquid chromatography... 

Liu, S.-J, Zhou, R, Xiao, X.-H., Zhao, L., Liu, X., and Jiang, S.-X., Surface confined ionic liquid—A new stationary phase for the separation of ephedrines in high performance liquid chromatography. Chin. Chem. Lett., 15, 1060-1062, 2004. 

Berkman and Egloff explain that some additives increase the flexibility or toughness of bubble walls, rather than their viscosity, to render them more durable. They cite as illustrations the addition of small quantities of soap to saponin solutions or of glycerin to soap solution to yield much more stable foam. The increased stability with ionic additives is probably due to electrostatic repulsion between charged, nearly parallel surfaces of the liquid film, which acts to retard draining and hence rupture. 

Hence the ionic liquids with the lowest viscosity tend to have highly fluorinated anions as these shield the charge density and result in low surface tensions. The cation also affects the viscosity of ionic liquids. For imidazolium cations, the viscosity initially decreases as the length of the R group increases, as the ion-ion interactions decrease and hence the surface tension decreases. However, as the alkyl group increases in size its mobility will decrease due to a lack of suitably sized voids for the cations to move into. This can be seen in the data presented by Tokuda ct al. who showed a minimum in viscosity for ethyl methyl imidazolium salts [129]. 

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