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

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• 作者：Angelika Klaus ; Gordon J. T. Tiddy ; Didier Touraud ; Anette Schramm ; Georgine Sthler ; Werner Kunz
• 刊名：Langmuir
• 出版年：2010
• 出版时间：November 16, 2010
• 年：2010
• 卷：26
• 期：22
• 页码：16871-16883
• 全文大小：1124K
• 年卷期：v.26,no.22(November 16, 2010)
• ISSN：1520-5827

文摘

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 °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 (L_α) 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 D₂O, we have examined water binding in the L_α 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 L_α and the H₂ phases. The X-ray diffraction data show that X-AES has a much smaller average length in the L_α 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.