Reversible micelle-vesicle conversion of oleyldimethylamine oxide by pH changes.
A preliminary study on the reversible micelle-vesicle conversion of oleyldimethylamine oxide [Kawasaki, H. et al. J. Phys. Chem. B. 2002, 106, 1524 ] is extended in the present study. In the presence of 0.01 M NaCl at a surfactant concentration of 0.05 M, a micelle-to-vesicle conversion with increasing degree of ionization alpha takes place in the following sequence: growth of fibrous micelle (alpha < 0.2), a fused network (alpha approximately 0.3), fibrous micelles + (perforated) vesicles (alpha = 0.4), and vesicles + lamellae (alpha = 0.5). Viscoelasticity correspondingly varies from the Maxwell-type behavior of the entangled network of fibrous micelles to the gel-like behavior of vesicle suspensions, via a fluid solution-like behavior of the fused network. This phase sequence is in contrast with the case of no added salt where no branching of micelles is observed, and long micelles and bilayers (vesicles + lamellae) coexist at alpha = 0.5. In water, a state of the lowest viscoelasticity occurs around alpha = 0.2 for both surfactant concentrations 0.05 and 0.15 M. Synergism between protonated and nonprotonated amine oxide headgroups is observed despite low ionic strengths. From the time course of the reversible micelle-vesicle conversion, vesicles seem to be formed from threadlike micelles within 25 h according to the shear moduli, while a longer conversion time is suggested by a flow property (viscosity). Shear thickening behavior is observed at alpha = 0.2 and 0.4 in 0.01 M NaCl but not in water.
Maeda
Hiroshi
H
Department of Chemistry, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan. h.maescc@mbox.nc.kyushu-u.ac.jp
Tanaka
Shimon
S
Ono
Yousuke
Y
Miyahara
Masahiko
M
Kawasaki
Hideya
H
Nemoto
Norio
N
Almgren
Mats
M