Surface active SNS-based dicationic ionic liquids containing amphiphilic anions: Experimental and theoretical studies of their structures and organization in solution

Abstract

Surface active ionic liquids (SAILs) have been reported as new media that collectively offer the advantages of the aqueous and oily phases. In particular, dicationic ionic liquids (DILs) have attracted much interests because their tunable physicochemical properties allow them to act as sustainable active catalysts in chemical reactions (CO2 conversion, esterification) and also as extraction media to remove drugs/pollutants from aqueous systems. In order to better understand this class of ILs, this work describes new strategies for the synthesis of SNS-based dicationic ILs containing amphiphilic anions ([C12SO4]-, [C12ESO4]-, [C12BSO3]- and [C12SAR]-) and the evaluation of their structural organization and aggregation level in solution. The results obtained by experimental techniques (FTIR, TGA, DSC, POM, ESI-MS, DLS and NMR) combined with those achieved by theoretical DFT calculations revealed that the anion has an important function to modulate the properties of the SNS-based ILs in solution, while the presence of a methyl group at the C2 position of the imidazolium ring seems to be not sufficient to change such physicochemical properties. The ILs containing the anion [C12BSO3]- showed a superior ionic organization in solution due to the cationic aggregates observed in the ESI(+) mode and the large size of aggregates observed by DLS. This behavior may be assigned to a close proximity of the cationic imidazolium ring and the aromatic ring in the anion (π-π interaction), and by NMR analysis (ROESY and DOSY) it was possible to confirm interactions between cation and anion. Therefore, the theoretical and experimental results obtained for the SNS-based dicationic ILs containing amphiphilic anions indicate that these ILs can be applied as media in both pure and/or solution systems for many sustainable applications.