Impact of Alkyl Chain Length on the Transition of Hexagonal Liquid Crystal-Wormlike Micelle-Gel in Ionic Liquid-Type Surfactant Aqueous Solutions without Any Additive

The search for functional supramolecular aggregations with different structure has attracted interest of chemists because they have the potential in industrial and technological application. Hydrophobic interaction has great influence on the formation of these aggregations, such as hexagonal liquid crystals, wormlike micelles, hydrogels, etc. So a systematical investigation was done to investigate the influence of alkyl chain length of surfactants on the aggregation behavior in water. The aggregation behavior of 1-hexadecyl-3-alkyl imidazolium bromide and water has been systematically investigated. These ionic liquid surfactants are denoted as C-16-C-n (n = 2, 3, 4, 6, 8, 9, 10, 12, 14, 16). The rheological behavior and microstructure were characterized via a combination of rheology, cryo-etch scanning electron microscopy, polarization optical microscopy, and X-ray crystallography. The alkyl chain has great influence on the formation of surfactant aggregates in water at the molecular level. With increasing alkyl chain length, different aggregates, such as hexagonal liquid crystals, wormlike micelles, and hydrogels can be fabricated: C-16-C-2 aqueous solution only forms hexagonal liquid crystal; C-16-C-3 aqueous solution forms wormlike micelle and hexagonal liquid crystal; C-16-C-4, C-16-C-6 and C-16-C-8 aqueous solutions only form wormlike micelle; C-16-C-9 aqueous solution experiences a transition between wormlike micelle and hydrogel; C-16-C-10, C-16-C-12, C-16-C-14 and C-16-C-16 only form hydrogel. The mechanism of the transition of different aggregation with increasing alkyl chain length was also proposed.