Effect of head group on the aggregation behavior of surface active ionic liquids with aromatic counterions in aqueous solutions

Abstract

Aggregate behavior of N-dodecyl-N-methylpyrrolidinium salicylate ([C12mpyrr][Sal]) and N-dodecyl-N-methylpiperidinium salicylate ([C12mpip][Sal]) in aqueous solution was systematically explored, respectively. Integrated the results of this work and relevant report on 1-dodecyl-3-methylimidazolium salicylate ([C12mim][Sal]), regulatory effect of head group in surface active ionic liquids (SAILs) with aromatic counterion on their self-assembly behavior was demonstrated. Both [C12mim][Sal] and [C12mpip][Sal] display lower cmc value compared to [C12mpyrr][Sal] which was verified by surface tension and electrical conductivity measurements. The reasons behind the observed differences may lie in the intramolecular π-π interaction between unsaturated ring head groups and aromatic counterions for the [C12mim][Sal] and more positive interaction energy between [C12mpip][Sal] and water molecules than [C12mpyrr][Sal] calculated from density function theory (DFT). Entropy was regarded as the thermodynamic driving force during micellization process of the two SAILs within the range of temperature studied. Similar to [C12mim][Sal], a hexagonal liquid crystalline phase (H1) and a cubic liquid crystalline phase (V2) appear sequentially with the increasing concentration of [C12mpyrr][Sal], whereas for [C12mpip][Sal], only a broad region of H1 phase was observed. Small angle X-ray scattering (SAXS) patterns suggest that analogous to [C12mim][Sal], higher concentrations of [C12mpyrr][Sal], and [C12mpip][Sal] and lower temperature can lead to a closer alignment of SAIL molecules in the H1 phase. The present work is expected to enrich the research contents on self-assembly behavior of SAILs and illuminate wide potential applications in nanomaterials, drug delivery and medicine release.