Ionic liquids cations and anions

Figure 13.1 Ionic liquid cations and anions discussed in this chapter.

Poly(pyrrole) films grown by constant potential in the ionic liquid tri(hexyl) (tetradecyl) phosphoniumbis(trifluoromethanesulfonyl)amide, [P6,6,6,u][NTf2], and subsequently cycled 100 times and reduced in the monomer-free ionic liquid, showed the presence of both ionic liquid cations and anions within the film. Interestingly, poly(pyrrole) films grown by constant potential in the ionic liquid but with no subsequent electrochemical cycling also contained both ionic liquid cation and anion within the film (Figure 7.19). 

Ionic liquids differ from classical molten salts by being liquids at room temperature, or their melting points are below 100 °C. They have an ionic structure and usually consist of an organic cation and an inorganic or organic anion. The most common cations found in ionic liquids are the tetraalkylammonium, tetraalkyl-phosphonium, N-alkylpyridinium, 1,3-dialkylimidazolium, and trialkylsulfonium moieties [11]. Some common ionic liquid cations and anions are presented in Fig. 7.1. 

Tsuzuki S et al (2009) Molecular dynamics simulations of ionic liquids cation and anion dependence of self-diffusion coefficients of ions. J Phys Chem B 113 10641... 

Castiglione and co-workers have extensively utilised HOESY and NOESY experiments for investigating the interactions between ionic liquid cation and anion ( H- ) [32, 37] and, more recently, between the ionic liquid and the lithium cation ( H- Li) primarily in the... 

On the basis of the above publications, we now know that some ionic liquid cations and anions are biodegradable under certain conditions, but most of them are recalcitrant towards biodegradation when test procedures according to ready biodegradability test conditions are performed. With respect to structure-biodegradability relationships, the following rules of thumb can be applied to ionic liquids ... 

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. 

Fig. 10. Separation of a mixture of ionic liquid cations and anions (concentration 0.5 mM each) with use of tandem ion exchange columns. 1, DMIM 2, NMIM 3, OMIM 4, HMIM 5, TfzN- 6, AMIM 7, PFs- 8, BF4- 11, Br 12, Cl- 13, BMIM signals 9,10 unidentified. Conductivity detection. Mobile phase 5 mM potassium hydrogen phthalate/lSmMphthalicacid, 45% (v/v) acetonitrile pH 3.05. Absolute conductivity of mobile phase, 476 pS/ cm flow rate, 1 mL/ min injection volume, 100 pL. Columns in the order strong anion exchange Phenosphere SAX, strong cation exchange Luna SCX. (Markowska Stepmowski, 2008).

Markowska, A. Stepnowski, P. (2008). Simultaneous determination of ionic liquid cations and anions using ion chromatography with tandem ion exchange columns a preliminary assessment. Analytical Sciences, 24 (No.l0) 1359-1361. 

These results indicate that ionic liquid cations and anions cannot be discussed separately the mutual influence of the counter ion has to be considered. That means that the acceptor (donor) properties are not only a function of the cation (anion), as it is usually suggested for the Kamlet-Taft parameters a and p, but also a function of the anion (cation). Increasing the basicity, e.g., from [BFJ to [Cl]" the accepting ability is reduced. Increasing the acidity, e.g., from [S2 2 2] to [chol]" the donating ability of [NTf2]" is reduced. 

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