Ionic liquids aggregate formation

Blesic, M., Marques, M.H., Plechkova, N.V., Seddon, K.R., Rebelo, L.RN., and Lopes, A., Self-aggregation of ionic liquids Micelle formation in aqueous solution. Green Chem., 9, 481-490, 2007. 

In this article, we have discussed the use of ionic liquids as solvent for the seE-assembly of surfactants. In ionic liquids, the formation of the same types of amphiphile self-assembly phases as aqueous systems (micelles, vesicles and microemulsions) was evidenced. These aggregates were characterized by surface tension measurements, differential scanning calorimetry, scattering experiments (DLS and SANS), pulse field gradient spin-echo NMR, electrical conductivity measurements, electron microscopy. 

Self-aggregation of ionic liquids micelle formation in aqueous solution. Green Chem., 9,481-490. 

A typical special feature of the melts of ionic crystals (ionic liquids) are their high concentrations of free ions, of about 25 M. Because of the short interionic distances, considerable electrostatic forces act between the ions, so that melts exhibit pronounced tendencies for the formation of different ionic aggregates ion pairs, triplets, complex ions, and so on. 

Intermolecular hydroamination or hydroarylation reactions of norbornene and cyclo-hexadiene carried out with catalytic amounts of Brpnsted or Lewis acid in ionic liquids have been found to provide higher selectivity and yields than those performed in classical organic solvents. This effect was attributed to the increases of the acidity of the medium and stabilization of ionic intermediates through the formation of supramolec-ular aggregates with the ionic liquid.38... 

Doibritz S, Ruth W, Kragl U (2005) Investigation on aggregate formation of ionic liquids. Adv Synth Catal 347 1273-1279... 

Other important biphasic concepts are based on the use of room-temperature ionic liquids (cf. Section 7.3) and, more recently, supercritical C02 (cf. Section 7.4) [31]. In addition, the second phase needs not necessarily to be another solvent. An amphiphilic approach can also be based on the use of micellar systems or vesicles formed by surfactants (cf. Section 4.5). A properly functionalized ligand itself can also function as the surfactant (cf. Section 3.2.4), which can even result in the formation of very stable aggregates [32]. 

In particular, it has been suggested that the highly ordered structure of these salts may contain voids, and that these voids can accommodate small solute molecules. Furthermore, since the chains present on the cations are flexible they can move more rapidly than the whole cation, permitting a rapid diffusion of solutes from one void to another [18]. The formation of cavities (voids) in ionic liquids has been recently studied via Monte Carlo simulations [19]. Analysis of cavity size distribution functions shows that ionic liquids exhibit a large tendency to form cavities, a property which seems to be correlated to the attractive interactions between ions and, particularly, to the tendency of the ions to associate into ion aggregates. 

It is, however, notable that the strong attractive interactions between ions, which are at the origin of the tendency of ions to associate in ion aggregates, may determine the formation of cavities inside the IL. The presence of cavities may favor the motion of small, uncharged species and render ionic liquids in this situation much more similar to polymeric matrixes than to true solvents. 

It should be noted here that MS techniques have been used with great success in the recent past to study species dissolved in ionic liquids [78] as well as the aggregate formation of ionic liquid ions in different ion mixtures [79] and in the presence of organic solvents [80], impressively underlining the potential of mass spectrometry for ionic liquid investigation in a more general respect. 

The Walden plot of the log(equivalent conductivity) versus log(fluidity) can be used to show how good an ionic liquid is (see section 3.2). Using this method, PILs have generally been shown as poor ionic liquids, though it is not possible to differentiate whether this is due to incomplete proton transfer, aggregation, or the formation of ion complexes. 

Dorbritz, S., Ruth, W. Kragl, U. (2005). Investigation on Aggregate Formation of Ionic Liquids. Adv. Synth. Catal., 347, pp. 1273-1279... 

Moreover, metal nanoparticles dispersed in these fluids are stable and can serve as active catalysts for reactions in multiphase conditions. The catalytic properties (activity and selectivity) of these soluble metal nanoparticles indicate that they possess a pronounced surface like (multi-site) rather than single site like catalytic properties. In other cases the metal nanoparticles are not stable and tend to aggregate or alternatively they serve as simple reservoirs of mononuclear catalytically active species. However, the ionic liquid provides a favourable environment for the formation of metal nanopartides with, in most cases, a small diameter and size distribution under very mild conditions. 

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