Anions, ionic liquid structure

A complementary study of steel/steel contacts (100Cr6) [47] used an oscillation wear and friction tester to screen ionic liquids in the boundary lubrication regime at 30, 100 and 170°C at 200 N load. Results are summarized as follows (i) some promising ionic liquids gave lower friction and wear at all temperatures, compared to reference lubricants, (ii) chemical structures of ionic liquids have a high impact on their physico-chemical as well as on tribological properties and (iii) careful selection of both cation and anion, as a tailor-made ionic liquid, enables adaptation of ionic liquid structures to specific applications. 

Nucleophilic substitution reactions in ionic liquids have recently been the subject of both synthetic [33] and kinetics and mechanistic studies [34]. Ionic liquids may be efficient promoting media for nucleophilic displacement reactions and important information about the ionic liquid properties has been obtained from the study of these reactions. The kinetic investigation of bimolecular substitution reactions of the halides on methyl p-nitrobenzenesulfonate, recently carried out by Wdton et al. [34] in several ionic media characterized by the same anion, [Tf2N] , and different cations ([BMIM]+, [BMMIMJ+ and [BMPY]+) or by the same cation, [BMIM] and different anions ([BF4] , [PFs] , [SbF ] and [Tf2N] ), has shovrathat the halides nucleophilicities depend on the ionic liquid structure (Scheme 5.1-7). 

Ionic Liquids, Structures, Fig. 2 Trans-cis isomerism of the bis(trifluoromethanesulfonyl)imide anion... 

Some (ionic liquid + water) systems do not exhibit the usual UCST behaviour, but show a lower critical solution temperature (LCST) behaviour instead. Here, the phase separation takes place at higher temperatures and mutual solubilities increase upon cooling until a homogeneous solution is reached. Only a small change in ionic liquid structure can already change the type of phase behaviour (UCST or LCST). For example, the tetrabutyl-phos-phonium-based ionic liquid with trans 2-butenedione (fumarate) as anion shows UCST behaviour, whereas the similar ionic liquid with the cis 2-bute-nedione (maleate) shows LCST behaviour after mixing with water. ... 

As discussed above, many ionic liquid structures seem to be persistent, or at least are only slowly degradable in the environment. This tendency of persistency, combined with the mostly relatively high hydrophobicity of ionic liquid cations and anions, bears the hazard of bioaccumulation. Thus, we suggest that a greater research effort be made to elucidate the bioaccumulation potential of ionic liquids and the underl3dng mechanisms involved. Since ionic liquids can be both hydrophobic and ionic, special mechanisms and distribution effects may come into play in their bioaccumulation behaviour. And, as already mentioned above, the possibly high persistency and slow degradation rates require more and detailed chronic effect studies in order to complete the hazard picture of at least the most frequently used ionic liquid structures. 

Figure 3 Small sample of the (a) cations and (b) anions that can be combined to make an ionic liquid. It has been estimated that there are well over 10 different compounds that could be made into ionic liquids. (Structures provided courtesy of Robin Rogers.)...

The following discussion concerns the thermal liquidus ranges available in different ionic liquids, as functions of cation and anion structure and composition. In particular, those structural features of cation and anion that promote these properties (while providing other desirable, and sometimes conflicting characteristics of the liquid, such as low viscosity, chemical stability, etc.) and variations in liquidus ranges and stabilities are the focus of this chapter. 

Another ionic liquid, containing a nonyl-rather than a butyl-side chain, is shown in Figure 4.2-2. There is little difference between the basic structures of these two ion-pairs (Figures 4.2-1 and 4.2-2) with respect to the non-bonded interactions (hydrogen bonds) occurring between the F atoms on the anion and the C-H moieties on the imidazolium cation. 

Ionic liquids have been described as designer solvents [11]. Properties such as solubility, density, refractive index, and viscosity can be adjusted to suit requirements simply by making changes to the structure of either the anion, or the cation, or both [12, 13]. This degree of control can be of substantial benefit when carrying out solvent extractions or product separations, as the relative solubilities of the ionic and extraction phases can be adjusted to assist with the separation [14]. Also, separation of the products can be achieved by other means such as, distillation (usually under vacuum), steam distillation, and supercritical fluid extraction (CO2). 

The disordered nature of the a-AgI structure may be regarded as intermediate between that of a typical ionic solid in which every lattice site is occupied and a typical ionic liquid in which both anions and cations are disordered. This is borne out by entropy calculations based on heat capacity data (O Keeffe and Hyde, 1976). These show a large increase in entropy at the P (or y) to a transition and a similar increase in entropy on melting of the a polymorph at 557 °C ... 

In catalytic reactions, ionic liquids with controllable coordinating strengths and/or reactivities toward the catalyst are particularly important. The coordinating strength of the ionic liquids depends on the nature of the anions and on the metal complex involved in the catalysis. With a large selection of available anions, it is possible to tailor the ionic liquid to suit specific chemical reactions, and variation of the structure and the composition of the cations and anions can alter the solubility of organic reactants in the ionic liquid. 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

Work with supported ionic liquids was extended to a cationic polymer, poly (diallyldimethylammonium chloride), which has quaternary ammonium functional groups (Fig. 16) 268). The extra-structural counter anion is Cl . The polymer was applied to simultaneously incorporate an ionic liquid and a transition-metal catalyst via a simple mixing of the components. Wilkinson s catalyst and [BMIM]PF6 were... 

Fig. 1 Structures of cations and anions found in the most common ionic liquids...

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