N-Alkylimidazolium carboxylates as a new type of catanionic surface active ionic liquid: synthesis, thermotropic behavior and micellization in water. Issue 40 (30th September 2022)

• Record Type: Journal Article
• Authors: Wachsmann, Sebastian B.; Bauhof, Jessica; Raab, Aileen Rebecca; Zens, Anna; Sottmann, Thomas; Laschat, Sabine
• Is Part Of: Soft matter. Volume 18:Issue 40(2022)
• Page Start: 7773
• Page End: 7781
• Publication Date: 2022-09-30
• Subjects: Soft condensed matter -- Periodicals; 530.413
• DOI: 10.1039/d2sm00854h
• Languages: English
• ISSNs: 1744-683X

Abstract

Abstract : Salt-free 1-alkyl-3-methylimidazolium alkylcarboxylates [C n mim][C m −1 COO] CASAILs improve bulk thermal stability, maintain broad-lamellar (SmA) mesophases, self-organize into micelles in water and exhibit a remarkable high surface activity. Abstract : Aiming at a new type of salt-free CASAIL (Catanionic Surface Active IL) for electrochemical applications or emulsifiers, dispersants, and foaming or antifoaming agents, we combined mesogenic anions (carboxylate) and cations (imidazolium) of similar shape and size to synthesize a series of congruent ion pairs of 1-alkyl-3-methylimidazolium alkylcarboxylates [C n mim][C m −1 COO] ( n = 10–16, m = 10–16). With particular focus on alkyl chain length varieties in both, imidazolium cations and carboxylate anions ( n / m ), the self-assembly in the bulk phase and in solution was investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), X-ray diffraction (XRD) experiments and surface tension measurements. Our results revealed that the presence of long alkyl chains on both the cation n and anion m leads to improved thermal stability of the bulk material while maintaining broad lamellar (SmA) mesophases. In water, we observed a strong and linear decrease of log(cmc) for increasing both the carboxylate anion and imidazolium cation chain length due to the increasing hydrophobic effect. Surprisingly, for both thermotropic behavior and micellization, the chain length of the carboxylate anion had a stronger impact than the chain length of the imidazolium cation, indicating its greater surface activity and tendency to form micelles.