Transition from vesicle phase to lamellar phase in salt-free catanionic surfactant solution

A salt-free cationic and anionic (catanionic) surfactant system was formed by mixing a double-tailed di-(2-ethylhexyl) phosphoric acid (DEHPA, commercial name P204), which is an excellent extractant of rare earth metal ions, with a single-tailed cationic trimethyltetradecylammonium hydroxide (TTAOH) in water. With the mole ratio (r) of DEHPA to TTAOH varying from 0.9 to 1, the phase transition occurred from a densely stacked vesicle phase (L_αv) to a lamellar phase (L_α1). Macroscopic properties, such as polarization and rheology, were measured and changed greatly during the course of the phase transition. When r was 0.96 or 0.98, the steady state shear curves exhibited two yield stress values, indicating the coexistence of the L _αv phase and the L_α1 phase. The L _α1 phase formed in the salt-free and zero-charged system (r = 1.0) is defective and undulating, which was confirmed by cryogenic transmission electron microscopy (cryoTEM). The deuterium nuclear magnetic resonance spectra (²H NMR) showed that a single peak (singlet) split into two symmetric peaks (doublet) gradually, indicating the phase transition from the L _αv phase to the L_α1 phase. Correspondingly, phosphorus nuclear magnetic resonance spectra (³¹P NMR) presented changes in both the chemical shift and the peak width, indicating that these two types of bilayer structures are of different anisotropy degrees and different viscosities. When the L_α1 phase is subjected to a certain shear force, it can be reversed to a L_αV phase again, which was proved by rheological, ²H NMR, and ³¹P NMR measurements. Furthermore, a theoretical consideration about the formation of the defective and undulating L_α1 phase was taken into account from a viewpoint of energy.