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

• Published in: The Journal of chemical physics 2018-05, Vol.148 (19)
• Main Authors: Belchior, Diana C V, Sintra, Tânia E, Carvalho, Pedro J, Soromenho, Mário R C, Esperança, José M S S, Ventura, Sónia P M, Rogers, Robin D, Coutinho, João A P, Freire, Mara G
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.5012020

This work provides a comprehensive evaluation of the effect of the cation alkyl side chain length of the 1-alkyl-3-methylimidazolium chloride series ([C C im]Cl, = 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 [C C im]Cl, water, and K PO or K CO and the respective Setschenow salting-out coefficients ( ), a quantitative measure of the two-phase formation ability, were determined. An odd-even effect in the values along the number of methylene groups of the longest IL cation alkyl side chain was identified for the ABS formed by K CO , 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 values is, however, more significant in ILs up to = 6, where the nanostructuration/nanosegregation of ILs plays a less relevant role. Still, with the [C C im]Cl ( = 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.