Aggregation Behavior of Imidazolium-Based Surface-Active Ionic Liquids with Photoresponsive Cinnamate Counterions in the Aqueous Solution

Two imidazolium-based surface active ionic liquids (SAILs) with photoresponsive cinnamate aromatic counterions, viz. 1-dodecy1-3-methylimidazolium cinnamate ([C(12)mim] [CA]) and 1-dodecyl-3-methylimidazolium para-hydroxy-cinnamate ([C(12)mim][PCA]), were newly synthesized, and their self-assembly behaviors in aqueous solutions were systematically explored. Results of surface tension and conductivity measurements show that both [C(12)mim] [CA] and [C(12)mim] [PCA] display a superior surface activity in aqueous solutions compared to the common imidazolium-based SAIL, 1-dodecy1-3-methylimidazolium bromide (C(12)mimBr), which implies the incorporation of cinnamate aromatic counterions can promote the micellar formation. Furthermore, [C(12)mim] [CA] shows higher surface activity due to the higher hydrophobicity of its counterion in comparison to [C(12)mim][PCA] that has a hydroxyl group. Both hexagonal liquid-crystalline phase (H-1 and cubic liquid-crystalline phase (V-2) were constructed in the [C(12)mim] [CA] aqueous solutions. In contrast, the [C(12)rnim][PCA]/H2O system only exhibits a single hexagonal liquid-crystalline phase (H-1) in a broad concentration region. These lyotropic liquid crystal (LLC) phases were comprehensively characterized by polarized optical microscopy (POM), small-angle X-ray scattering (SAXS), and rheometer. Investigation on the temperature-dependent self-assembly nanostructures demonstrates that the higher temperature leads to a looser arrangement. Under UV irradiation, trans-cis photoisomerization of the phenylalkene group results in inferior surface activity of the prepared SAILs in aqueous solution with higher cmc values. Moreover, UV light irradiation induces obvious change of the structural parameters without altering the LLC phases. This work is expected to enrich the investigations of phase behaviors formed in SAILs systems and receive particular attention due to their unique properties and potential applications in drug delivery, biochemistry, materials science, etc.