A comprehensive study on micellization of dissymmetric pyrrolidinium headgroup-based gemini surfactants

Abstract

Three groups of pyrrolidinium headgroup-based gemini surfactants of 1,1′-(propane-1,3-diyl)bis(1-alkyl pyrrolidinium) bromide, in categories of symmetric C_mC₃C_mPB (m = 10, 12, 14), dissymmetric C_mC₃C₁₄PB (m = 10, 12, 14) and C_mC₃C_nPB (m = 8, 10, 12, m + n = 24) surfactants, are studied using equilibrium surface tension, conductivity, fluorescence, and NMR techniques. The importance of the dissymmetry on the micellization has been revealed in detail. The increase in the hydrophobic chain length m for C_mC₃C_mPB and C_mC₃C₁₄PB or in the dissymmetry (n/m) for C_mC₃C_nPB can strengthen the aggregation ability and surface activity of the surfactants significantly, i.e., a lower critical micelle concentration (cmc) and a lower surface tension at cmc (γ_cmc). However, the aggregation number at cmc (N*) obeys the opposite variation tendency and it becomes smaller upon increasing m or n/m, due to the formation of premicelles. Thermodynamic results reveal that the contribution of enthalpy (ΔH⁰_m) to the Gibbs free energy (ΔG⁰_m) is strengthened by increasing m or n/m during the spontaneous micellization process. Moreover, ¹H NMR results confirm the microenvironment change of the surfactants from polar water to micelles during the micellization, and 2D Noesy NMR spectra suggest that the methylene groups in the ring should adopt a conformation toward the nonpolar micellar core rather than in the polar water.