title: Thermodynamically stable vesicle formation and vesicle-to-micelle transition of single-tailed anionic surfactant in water.
abstract: The aggregation behavior of sodium 3,6,9,12,15-pentaoxa-heptacosanoate (AEC4-Na) in aqueous solution with increase of the concentration at 25 °C was investigated using differential scanning calorimetry, equilibrium surface tension, solubilization of an oil-soluble dye, steady-state fluorescence, dynamic light scattering, and freeze-fractured transmission electron microscopy. Vesicle formation of AEC4-Na preceded micelle formation below the critical micelle concentration (cmc). The vesicle-to-micelle transition was observed through a narrow concentration region above the cmc. The mean diameters of the vesicles and micelles were not affected by the concentration. All solutions over a wide range of concentrations were homogeneously transparent with a low Krafft point below 0 °C. These results indicate that the AEC4-Na vesicles have a thermodynamically stable structure. Vesicle formation may be caused by a pseudobinary mixed surfactant system composed of monomeric AEC4-Na and an acid soap that consists of a dimer complex formed between AEC4-Na and unneutralized AEC4-Na. The thermodynamic stability would then result from the inhibition of close intermolecular aggregation and flexibility of the molecular shape in the vesicles due to the oxyethylene units in AEC4-Na.
authors.affiliation: Research and Development, Eco-Innovation Research, Kao Corporation, 1334, Minato, Wakayama-shi, Wakayama 640-8580, Japan. sakai.takaya@kao.co.jp
authors.forename: Takaya
authors.initials: T
authors.lastname: Sakai
authors.forename: Risa
authors.initials: R
authors.lastname: Ikoshi
authors.forename: Natsuko
authors.initials: N
authors.lastname: Toshida
authors.forename: Mariko
authors.initials: M
authors.lastname: Kagaya
mesh_terms: D002152:Calorimetry, Differential Scanning; D004396:Coloring Agents; D005227:Fatty Acids / Q000737:chemistry*; D008823:Micelles; D009821:Oils / Q000737:chemistry; D011092:Polyethylene Glycols / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013500:Surface Tension; D013501:Surface-Active Agents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D004396:Coloring Agents; D005227:Fatty Acids; D008823:Micelles; D009821:Oils; D012996:Solutions; D013501:Surface-Active Agents; D014867:Water; D011092:Polyethylene Glycols; C584420:3,6,9,12,15-pentaoxaheptacosanoic acid