Phase behavior of an extended surfactant
in water and a detailed characterization of the dilute and semidilute phases ,
Klaus Angelika, Tiddy Gordon J T, Touraud Didier, Schramm Anette, Stuhler
Georgine, Drechsler Markus, Kunz Werner,
Langmuir : the ACS journal of surfaces and colloids
26(8) (2010) 5435-43.
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. |
