Odd-even effect on the formation of aqueous biphasic systems formed by 1-alkyl-3-methylimidazolium chloride ionic liquids and salts

Abstract

This work provides a comprehensive evaluation of the effect of the cation alkyl side chain length of the 1-alkyl-3-methylimidazolium chloride series ([CnC1im]Cl, n = 2-14) of ionic liquids (ILs) on their capability to form aqueous biphasic systems (ABSs) with salts and self-aggregation derived properties. The liquid-liquid phase behavior of ternary systems composed of [CnC1im]Cl, water, and K3PO4 or K2CO3 and the respective Setschenow salting-out coefficients (ks), a quantitative measure of the two-phase formation ability, were determined. An odd-even effect in the ks values along the number of methylene groups of the longest IL cation alkyl side chain was identified for the ABS formed by K2CO3, a weaker salting-out agent where the phenomenon is clearly identified. In general, cations with even alkyl side chains, being likely to display higher molar volumes, are more easily salted-out and thus more prone to undergo phase separation. The odd-even effect in the ks values is, however, more significant in ILs up to n = 6, where the nanostructuration/nanosegregation of ILs plays a less relevant role. Still, with the [CnC1im]Cl (n = 7-14) series of ILs, an odd-even effect was also identified in the ILs’ ionization degree, molar conductivity, and conductivity at infinite dilution. In summary, it is shown here that the ILs’ odd-even effect occurs in IL aqueous solutions and not just in neat ILs, an already well-established phenomenon occurring in a series of ILs’ properties described as a result of the orientation of the terminal methyl groups to the imidazolium ring cation and consequent effect in the ILs’ cohesive energy.

Liquid–Liquid Equilibrium and Extraction Performance of Aqueous Biphasic Systems Composed of Water, Cholinium Carboxylate Ionic Liquids and K2CO3

“…Although this order shows that the capability to form ABS (or of the IL to be salted-out) increases with the increase of the alkyl chain length of the IL anion, this is only observed up to cholinium pentanoate; for higher anion alkyl chain lengths the self-aggregation of the ILs occurs, decreasing their ability to create ABS. , This trend in ABS formation is in agreement with what has been recently observed for cholinium-based ILs + K 2 HPO 4 ABS and seems to be independent of the salt used . Moreover, the same trend was previously reported for the IL cation effect in ABS formation …”

“…40 Moreover, the same trend was previously reported for the IL cation effect in ABS formation. 41 Three mixture compositions at the biphasic region of each ABS were prepared, and eqs 1 to 7 were applied to determine the respective TLs, along with their respective length (TLLs), which are reported in Table 3. The critical points determined for the studied systems are reported in Figure 3.…”

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

“…Very recently, it has been shown (Belchior et al, 2018) that diluted solutions of [C n C 1 im]Cl ( n = 2–14) used to form aqueous biphasic systems (ABS) with salts also exhibit such odd/even effects. The odd/even effect was observed in the Setschenow salting-out coefficients ( ks ) for systems containing water, [C n C 1 im]Cl and K 2 CO 3 (Figure 9A).…”

“… (A) Setschenow constant, ks , as a function of the carbon number at the longest alkyl chain, n , in [C n C 1 im]Cl, for the ABS formed with the salt K 2 CO 3 . (Reprinted from Belchior et al, 2018 with the permission of AIP Publishing) (B) Degree of ionization as a function of the carbon number at the cation alkyl chain, n , in [C n C 1 im]Cl. (Reprinted from Belchior et al, 2018 with the permission of AIP Publishing).…”

“…(Reprinted from Belchior et al, 2018 with the permission of AIP Publishing) (B) Degree of ionization as a function of the carbon number at the cation alkyl chain, n , in [C n C 1 im]Cl. (Reprinted from Belchior et al, 2018 with the permission of AIP Publishing).…”