Self-Assembled Aggregates Originated from the Balance of Hydrogen-Bonding, Electrostatic, and Hydrophobic Interactions

Rich phase behavior was observed in salt-free cationic and anionic (catanionic) mixtures of a double-tailed surfactant, di(2-ethylhexyl)phosphoric acid (abbreviated as DEHPA), and tetradecyldimethylamine oxide (C(14)DMAO) in water. At a fixed C(14)DMAO concentration, phase transition from L(1) phase to L(α) phase occurs with increasing amounts of DEHPA. Moreover, in the L(α) phase, with the increase in DEHPA concentration, a gradual transition process from vesicle phase (L(αv)) to stacked lamellar phase (L(αl)) was determined by cryo- and FF-TEM observations combining with (2)H NMR measurements. The rheological data show that the viscosity increases with DEHPA amounts for L(αv) phase samples because of the increase in vesicle density. At a certain molar ratio of DEHPA to C(14)DMAO, i.e., 80:250, the samples are with the highest viscoelasticity, indicating the existence of densely packed vesicles. While for L(αl) phase samples, with increasing DEHPA amount, a decrease of bilayer curvature was induced, leading to a decrease of viscosity obviously. Compared with general catanionic surfactant mxitures, in addition to the electrostatic interaction of ion pairs, the transition of the microstructures is also ascribed to the formation of the hydrogen bonding (-N(+)-O-H···O-N-) between C(14)DMAO molecules and protonated C(14)DMAOH(+), which induces the growth of aggregates and the decrease of aggregate curvatures.