Aggregation Behavior of Pyrrolidinium Ionic Liquid Surfactants in-OH-Functionalized Ammonium-Based Protic Ionic Liquids

Aggregation of surfactants in ionic liquids (ILs) is poorly understood in spite of their great potential for applications in many fields. In this paper, the aggregation behavior of pyrrolidinium IL surfactants [C(n)mpyr]Br (n = 10, 12, 14, and 16) in -OH-functionalized protic ionic liquids (PILs), including ethanolammonium nitrate (EOAN), ethanol ammonium formate (EOAF), and 3-amino-1-propanol nitrate (POAN), has been studied by surface tension and H-1 NMR methods. The effect of the structure of PILs on the aggregation behavior of the IL surfactants is investigated and compared with that in ethylammonium nitrate (EAN) and water solvents. It is found that adding an -OH group in PIL cations significantly reduces the critical micelle concentration (cmc), while increasing the alkyl chain length of the PILs cations leads to an increase in the cmc values. However, the effect of anion type of the PILs on the aggregation is not pronounced for the investigated ILs. Thermodynamic analysis reveals that the aggregation behavior in EOAN is driven by enthalpy and entropy contributions. In addition, the change in the chemical shift of various protons in the IL surfactants and EOAN has been used to analyze the conformational changes of the IL surfactants in the aggregation process. The results reveal the importance of -OH-functionalized PILs in the development of designer IL-based colloidal systems for chemical reactions and materials synthesis.