Thermodynamically stable vesicle formation and vesicle-to-micelle transition of single-tailed anionic surfactant in water

J Phys Chem B. 2013 May 2;117(17):5081-9. doi: 10.1021/jp402314f. Epub 2013 Apr 18.

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.

MeSH terms

  • Calorimetry, Differential Scanning
  • Coloring Agents
  • Fatty Acids / chemistry*
  • Micelles*
  • Oils / chemistry
  • Polyethylene Glycols / chemistry*
  • Solutions / chemistry
  • Surface Tension
  • Surface-Active Agents / chemistry*
  • Thermodynamics
  • Water / chemistry*

Substances

  • Coloring Agents
  • Fatty Acids
  • Micelles
  • Oils
  • Solutions
  • Surface-Active Agents
  • Water
  • Polyethylene Glycols
  • 3,6,9,12,15-pentaoxaheptacosanoic acid