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 was 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 soly. of triglycerides in water. Currently, only limited information about the properties of these surfactants and its behavior in water is available. Therefore, mixts. of a chosen extended surfactant (C12-14-PO16-EO2-SO4Na, X-AES) with H2O/D2O over the whole concn. range were studied by optical microscopy. A schematic phase diagram was obtained, which shows two isotropic liq. phases at the lowest and highest surfactant concns. Also, between the isotropic solns., four liq.-cryst. phases occur: a hexagonal phase (H1), a lamellar phase (Lα) with a change in birefringence, a bicontinuous cubic phase (V2), and a reverse hexagonal phase (H2). The structure of the micellar soln. (L1) was detd. by cryo-TEM, dynamic light scattering, and 1H NMR, which gave information about the size, the aggregation no., and the area per mol. of the micelles. Liq.-crystal formation occurs from the micellar soln. in two different ways. The 1st route appeared by increasing the temp., going from an L1 to an Lα phase. By increasing the surfactant concn. (at low temps.), a 2nd route showed a transition from L1 to H1. The effect of sodium chloride on the cloud point of the extended surfactant was examd., indicating that small amts. of NaCl have no influence on the phase behavior. The monolayer behavior of the extended surfactant at the air-water interface was also detd. Despite its water soly., an isotherm on the water subphase was found, showing slow kinetics of the mols. to go into the bulk. Thus, the detn. of the cmc of the extended surfactant using conventional methods is impossible.