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

• Published in: Langmuir 2012-01, Vol.28 (1), p.219-226
• Main Authors: Song, Shasha, Zheng, Qingshu, Song, Aixin, Hao, Jingcheng
• Format: Article
• Language: English
• Subjects: Chemistry; Colloidal state and disperse state; Colloids: Surfactants and Self-Assembly, Dispersions, Emulsions, Foams; Exact sciences and technology; General and physical chemistry; Membranes
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la203581m

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 (C14DMAO) in water. At a fixed C14DMAO concentration, phase transition from L1 phase to Lα phase occurs with increasing amounts of DEHPA. Moreover, in the Lα phase, with the increase in DEHPA concentration, a gradual transition process from vesicle phase (Lαv) to stacked lamellar phase (Lαl) was determined by cryo- and FF-TEM observations combining with 2H NMR measurements. The rheological data show that the viscosity increases with DEHPA amounts for Lαv phase samples because of the increase in vesicle density. At a certain molar ratio of DEHPA to C14DMAO, i.e., 80:250, the samples are with the highest viscoelasticity, indicating the existence of densely packed vesicles. While for Lαl 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 C14DMAO molecules and protonated C14DMAOH+, which induces the growth of aggregates and the decrease of aggregate curvatures.