pH- and concentration-induced micelle-to-vesicle transitions in pyrrolidone-based Gemini surfactants

The self-assembly of novel Gemini surfactants with pyrrolidone head groups, N,N′-dialkyl-N,N′-di(ethyl-2-pyrrolidone) ethylenediamine (Di-CnP, where n = 6, 8, 10, 12), was studied systematically by employing UV–vis spectroscopy, fluorescence spectroscopy, NMR, dynamic light scattering (DLS), and cryo-transmission electronic microscopy (cryo-TEM) measurements. pH-induced spherical micelle-to-vesicle transitions (MVTs) were observed in all diluted Di-CnP aqueous solutions. Spherical micelles were formed when solution pHs were below 7.0, in which Di-CnPs were 1:2 or 1:1 type cationic surfactants, whereas vesicles were formed instead at higher pHs, e.g., pH = 11.0, when Di-CnPs were nonionic type. Apart from the pH-induced MVTs, concentration-induced MVTs were also observed in the protonated Di-CnP aqueous solutions due to counterion binding, indicating the presence of a second critical vesicle concentration (cvc) for ionic type Di-CnP. Furthermore, the cvc is decreased linearly with the number of carbon atoms n in the hydrophobic tail, following the well-known Stauff–Klevens rule, in the given protonation states.