Phase behavior of an extended surfactant in water and a detailed characterization of the concentrated phases

The formation of microemulsions with triglycerides at ambient conditions can be improved by increasing the surfactantwater and surfactant-oil interactions. Therefore, extended surfactants were developed, which contain hydrophilic/lipophilic linkers. They have the ability to stretch further into the oil and water phase and enhance the solubility of oil in water. In this work, the phase behavior of a chosen extended surfactant (C12-14- PO16-EO2-SO4Na, X-AES) in H2O/D 2O at high surfactant concentrations (30-100 wt %) and at temperatures between 0 and 90 °C is studied for the first time. The lyotropic liquid crystals formed were determined by optical microscopy, small-angle X-ray scattering (SAXS), and 2H and 23Na NMR, and a detailed phase diagram of the concentrated area is given. The obtained mesophases are a hexagonal phase (H1), at low temperatures and small concentrations, a lamellar phase (Lα) at high temperatures or concentrations, a bicontinuous cubic phase (V2) as well as a reverse hexagonal phase (H2). To our knowledge, this is the first surfactant that forms both H1 and H2 phases without the addition of a third compound. From the 2H NMR quadrupole splittings of D 2O, we have examined water binding in the Lα and the H2 phases. There is no marked difference in the bound water between the two phases. Where sufficient water is present, the number of bound water molecules per X-AES is estimated to be ca. 18 with only small changes at different temperatures. Similar results were obtained from the 23Na NMR data, which again showed little difference in the ion binding between the Lα and the H2 phases. The X-ray diffraction data show that X-AES has a much smaller average length in the Lα phase compared to the all-trans length than in the case for conventional surfactants. At very high surfactant concentrations an inverse isotropic solution (L2), containing a small fraction of solid particles, is formed. This isotropic solution is clearly identified and the size of the reversed micelles was determined using 1H NMR measurements. Furthermore, the solid particles within the L2 phase and the neat surfactant were analyzed. The observed results were compared to common conventional surfactants (e.g., sodium dodecyl sulfate, sodium lauryl ether sulfate, and sodium dodecyl-p-benzene sulfonate), and the influence of the hydrophilic/lipophilic linkers on the phase behavior was discussed. © 2010 American Chemical Society.