Effect of molar ratio and alkyl chain length on the aggregation behavior of imidazolium-based ionic liquid surfactants/SDS systems

In our previous publication (https://doi.org/10.1016/j.jcis.2019.04.023), the volumes and concentrations of syringe and sample cell in Isothermal titration calorimetry (ITC) limit the molar ratios and concentrations during the titration process. The molar ratios and concentrations of sodium dodecylsulfate (SDS) are almost higher than those of the ionic liquids (ILs) throughout the titration process. From the perspective of SDS molar ratios and concentrations, when the molar ratios and concentrations of the imidazolium-based ILs exceed those of SDS, the research remains unexplored. This aspect of the research has been consistently neglected, yet it is crucial for a fundamental understanding of self-assembly structure transitions. For these mixed systems, the phase behavior, zero-shear viscosity, and dynamic viscoelasticity of the system do not increase with the increasing carbon chain length of imidazolium ILs. The variation in carbon chain length exhibits an optimal value (1-octyl-3-methyl-imidazolium chloride ([OMIM]Cl)/SDS system), which may be related to the balance between the electrostatic attraction among headgroups and the hydrophobic interactions among hydrophobic chains. To enhance a deeper comprehension of the variations in macroscopic properties across the entire range of mole ratios, enthalpy changes and cryogenic Transmission Electron Microscopy (cryo-TEM) were employed to study the mixed systems. When 100.0 mM imidazolium-based ILs were titrated into 12.5 mM SDS solution, the titration curves consistently exhibited a plateau. During this plateau, SDS micelles undergo disassembly and reassemble into vesicle structures of different sizes. The formation of vesicles is a continuous process until the imidazolium ILs is completely consumed. We propose a straightforward and feasible method to determine the molecular ratios of surfactants responsible for constructing vesicle structures in different alkyl chain lengths systems by fitting the enthalpy changes at different stages. Through this approach, the molecular ratios of SDS and 1-Hexyl-3-methyl-imidazolium chloride ([HMIM]Cl), 1-Octyl-3-methylimidazolium chloride ([OMIM]Cl), and 1-Dodecyl-3-methyl-imidazolium chloride ([DMIM]Cl) in the vesicle structures are 1:1, 4:5 and 7:10, respectively.