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Reversible micelle-vesicle conversion of oleyldimethylamine oxide by pH changes
Hiroshi Maeda1, Shimon Tanaka, Yousuke Ono
1Department of Chemistry, Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan. h.maescc@mbox.nc.kyushu-u.ac.jp
This study extends research on oleyldimethylamine oxide surfactant solutions, revealing a salt-induced micelle-to-vesicle conversion sequence. Viscoelastic properties change significantly with ionization degree, contrasting with salt-free conditions.
Area of Science:
- Colloid and Surface Chemistry
- Physical Chemistry
- Materials Science
Background:
- Extends preliminary study on oleyldimethylamine oxide reversible micelle-vesicle conversion.
- Investigates the influence of ionic strength and degree of ionization on surfactant aggregation.
Purpose of the Study:
- To elucidate the phase sequence and viscoelastic behavior during micelle-to-vesicle conversion.
- To compare the effects of added salt versus no added salt on surfactant structures.
- To examine the kinetics and shear-dependent properties of the conversion process.
Main Methods:
- Rheological measurements (viscoelasticity, shear moduli, viscosity).
- Varying surfactant concentration (0.05 M and 0.15 M) and degree of ionization (alpha).
- Conducting experiments with and without added salt (0.01 M NaCl).
Main Results:
- A distinct phase sequence (fibrous micelle, fused network, vesicles, lamellae) observed with 0.01 M NaCl and increasing alpha.
- Viscoelasticity transitions from Maxwell-type to gel-like behavior.
- Synergistic effects between headgroups observed even at low ionic strengths.
- Shear thickening behavior noted at specific ionization degrees with salt.
Conclusions:
- Added salt significantly alters the aggregation pathway and final structures compared to salt-free conditions.
- Vesicle formation from micelles is time-dependent, with rheological methods suggesting different timescales.
- The study highlights complex self-assembly behavior driven by ionization and ionic strength.
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