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

Abstract

The formation of microemulsions with triglycerides under ambient conditions has been a challenge for scientists for many decades. For this reason, so-called extended surfactants were developed that contained hydrophilic/lipophilic linkers to stretch further into the oil and water phase, and enhance the solubility of triglycerides in water. Currently, only limited information about the properties of these surfactants and its behavior in water is available. Therefore, in this work, mixtures of a chosen extended surfactant (C(12-14)-PO(16)-EO(2)-SO(4)Na, X-AES) with H(2)O/D(2)O over the whole concentration range were studied by optical microscopy. A schematic phase diagram has been obtained, which shows two isotropic liquid phases at the lowest and highest surfactant concentrations. Furthermore, between the isotropic solutions, four liquid-crystalline phases occur: a hexagonal phase (H(1)), a lamellar phase (L(alpha)) with a change in birefringence, a bicontinuous cubic phase (V(2)), and a reverse hexagonal phase (H(2)). The structure of the micellar solution (L(1)) was determined by cryo-TEM, dynamic light scattering, and (1)H NMR, which gave information about the size, the aggregation number, and the area per molecule of the micelles. Liquid-crystal formation occurs from the micellar solution in two different ways. The first route appeared by increasing the temperature, going from an L(1) to an L(alpha) phase. By increasing the surfactant concentration (at low temperatures), a second route showed a transition from L(1) to H(1). In addition, the effect of sodium chloride on the cloud point of the extended surfactant was examined, indicating that small amounts of NaCl have no influence on the phase behavior. The monolayer behavior of the extended surfactant at the air-water interface was also determined. Despite its water solubility, an isotherm on the water subphase was found, showing slow kinetics of the molecules to go into the bulk. Thus, the determination of the cmc of the extended surfactant using conventional methods was found to be impossible.