Phase Behavior of an Extended Surfactant in Water and a Detailed Characterization of the Concentrated Phases

Abstract

The formation of microemulsions with triglycerides at ambient conditions can be improved by increasing the surfactant-H2O 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 H2O phase and enhance the soly. of oil in H2O. The phase behavior of a chosen extended surfactant (C12-14-PO16-EO2-SO4Na, X-AES) in H2O/D2O at high surfactant concns. (30-100%) and at 0-90° is studied for the 1st time. The lyotropic liq. crystals formed were detd. by optical microscopy, small-angle x-ray scattering (SAXS), and 2H and 23Na NMR, and a detailed phase diagram of the concd. area is given. The obtained mesophases are a hexagonal phase (H1), at low temps. and small concns., a lamellar phase (Lα) at high temps. or concns., a bicontinuous cubic phase (V2) as well as a reverse hexagonal phase (H2). To our knowledge, this is the 1st surfactant that forms both H1 and H2 phases without the addn. of a 3rd compd. From the 2H NMR quadrupole splittings of D2O, the authors have examd. H2O binding in the Lα and the H2 phases. There is no marked difference in the bound H2O between the 2 phases. Where sufficient H2O is present, the no. of bound H2O mols. per X-AES is ∼18 with only small changes at different temps. 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 av. length in the Lα phase compared to the all-trans length than in the case for conventional surfactants. At very high surfactant concns. an inverse isotropic soln. (L2), contg. a small fraction of solid particles, is formed. This isotropic soln. is clearly identified and the size of the reversed micelles was detd. using 1H NMR measurements. Also, the solid particles within the L2 phase and the neat surfactant were analyzed. The obsd. results were compared to common conventional surfactants (e.g., Na dodecyl sulfate, Na lauryl ether sulfate, and Na dodecyl-p-benzene sulfonate), and the influence of the hydrophilic/lipophilic linkers on the phase behavior was discussed.