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 surfactant water 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/D2O at high surfactant concentrations (30-100 wt %) and at temperatures between 0 and 90 degrees C is studied for the first time; The lyotropic liquid crystals formed were determined by optical microscopy. small-angle X-ray scattering (SAXS). and H-2 and Na-23 NMR, and a detailed phase diagram of the concentrated area is given. The obtained mesophases are a hexagonal phase (H-1), at low temperatures and small concentrations, a lamellar phase (L-alpha) at high temperatures or concentrations, a bicontinuous cubic phase (V-2) as well as a reverse hexagonal phase (H-2). To our knowledge. this is the first surfactant that forms both H-1 and H-2 phases without the addition of a third compound. From the H-2 NMR quadrupole splitting of D2O. we have examined water binding in the L-a and the H-2 phases. There is no marked difference in the hound 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 Na-23 NMR data, which again showed little difference in the ion binding between the L-alpha and the H-2 phases. The X-ray diffraction data show that X-AES has a much smaller average length in the L-alpha phase compared to the all-trans length than in the case for conventional surfactants. At very high surfactant concentrations an inverse isotropic solution (L-2), 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 H-1 NMR measurements. Furthermore, the solid particles within the L-2 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.