Micelle-Vesicle Transition of Fatty Acid Based Ion-Pair Surfactants: Interfacial Evidence and Influence of the Ammonium Counterion Structure

We describe the synthesis and the physicochemical study of new ion‐pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface‐tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show...

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Veröffentlicht in:Chemphyschem 2007-09, Vol.8 (13), p.2013-2018
Hauptverfasser: Bordes, Romain, Vedrenne, Marc, Coppel, Yannick, Franceschi, Sophie, Perez, Emile, Rico-Lattes, Isabelle
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Sprache:eng
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Zusammenfassung:We describe the synthesis and the physicochemical study of new ion‐pair amphiphiles from a mixture of bicyclic, cyclic, linear or branched amines and fatty acids of three chain lengths. Surface‐tension measurements of bicyclic, cyclic and branched structures of ammonium/alkanoate acid ion pairs show a phase transition, with two plateaux in the plot of surface tension versus log(c) (c=concentration). Such behaviour is related to the structure of the counterion, the alkyl chain length and the temperature. Pulsed gradient spin echo NMR spectroscopy experiments were performed to demonstrate the existence of micelles on the first plateau and to confirm the phase transition. The existence of vesicles on the second plateau of the surface tension was proved by CryoTEM observation and dynamic light scattering (DLS) measurements. Mainly, according to the structure of the counterion, there is either a strong association and a positioning along the chain leading to vesicles or a less strong association leading to external positioning and the formation of micelles at low concentrations or vesicles at higher concentrations. Micelles or Vesicles? Single‐tailed surfactants have been associated with various organic counterions. Depending on the structure of the ion‐paired surfactant, a micelle‐to‐vesicle transition is observed (see picture), thus demonstrating the key role played by the positioning of the counterion in supramolecular self‐assembly.
ISSN:1439-4235
1439-7641
DOI:10.1002/cphc.200700289