Gases, interaction with ionic liquids

For two reasons, the interaction of ionic liquids with gas molecules is under constant investigation First, it is surprising to note that some gases like C02 dissolve very readily in many common ILs while others (H2, 02,...) do not [48]. Second, from a technical point of view ILs are interesting solvents for typical industrial applications like hydrations, hydroformylations and so on. 

Reactions in solution proceed in a similar manner, by elementary steps, to those in the gas phase. Many of the concepts, such as reaction coordinates and energy barriers, are the same. The two theories for elementary reactions have also been extended to liquid-phase reactions. The TST naturally extends to the liquid phase, since the transition state is treated as a thermodynamic entity. Features not present in gas-phase reactions, such as solvent effects and activity coefficients of ionic species in polar media, are treated as for stable species. Molecules in a liquid are in an almost constant state of collision so that the collision-based rate theories require modification to be used quantitatively. The energy distributions in the jostling motion in a liquid are similar to those in gas-phase collisions, but any reaction trajectory is modified by interaction with neighboring molecules. Furthermore, the frequency with which reaction partners approach each other is governed by diffusion rather than by random collisions, and, once together, multiple encounters between a reactant pair occur in this molecular traffic jam. This can modify the rate constants for individual reaction steps significantly. Thus, several aspects of reaction in a condensed phase differ from those in the gas phase ... 

Armstrong, D.W., He, L., and Liu, Y.-S., Examination of ionic liquids and their interaction with molecules, when used as stationary phases in gas chromatography, Anal. Chem., 71, 3873,1999. 

The characteristic effect of surfactants is their ability to adsorb onto surfaces and to modify the surface properties. Both at gas/liquid and at liquid/liquid interfaces, this leads to a reduction of the surface tension and the interfacial tension, respectively. Generally, nonionic surfactants have a lower surface tension than ionic surfactants for the same alkyl chain length and concentration. The reason for this is the repulsive interaction of ionic surfactants within the charged adsorption layer which leads to a lower surface coverage than for the non-ionic surfactants. In detergent formulations, this repulsive interaction can be reduced by the presence of electrolytes which compress the electrical double layer and therefore increase the adsorption density of the anionic surfactants. Beyond a certain concentration, termed the critical micelle concentration (cmc), the formation of thermodynamically stable micellar aggregates can be observed in the bulk phase. These micelles are thermodynamically stable and in equilibrium with the monomers in the solution. They are characteristic of the ability of surfactants to solubilise hydrophobic substances. 

The studies discussed above have been concerned with ionic systems in the gas phase or in small clusters. This work has provided information about the structural and energetic properties of these systems as well as their reactions. However, the practical interest in these materials is in condensed phases. The inclusion of intermolecular interactions is essential for realistic descriptions of these materials. It is important to consider the electrostatic, long-range interactions. A practical way to consider these interactions is to perform simulations of solids or liquids with periodic boundary conditions. 

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