Influence of Surfactants on the Rheology and Stability of Crystallizing Fatty Acid Pastes

Complex fluids containing crystallizing fatty acids are important for consumer care products. The key features of these materials are their ability to support their weight under gravity due to the formation of a fatty acid crystal network, and to yield or flow beyond a critical applied strain. In model formulations comprised of two synthetic surfactants and a fatty acid in water, we have shown that the fatty acid crystal network consists of crystal aggregates linked by a non-crystallized mixed fatty acid—surfactant mesophase. We hypothesize that this mixed surfactant—fatty acid mesophase is critical for the macroscopic stability of the formulations. Rheological measurements combined with differential scanning calorimetry (DSC), X-ray scattering, and polarized light microscopy (PLM) measurements show the importance of surfactant loading on the overall stability of the formulations by linking morphology to rheology. Macroscopically homogeneous formulations are realized with 7–10 wt% of fatty acid. Increasing the fatty acid content without adding surfactant leads to inhomogeneous, phase separating formulations. Although both stable and unstable formulations show the presence of a surfactant—fatty acid mixed phase, a critical loading of surfactants is found to be necessary to create macroscopically homogenous formulations. We demonstrate how the rheology, microstructure and the macroscopic stability can be tuned by varying the relative amounts of surfactants and fatty acid.by Prachi Thareja et al.

New micellar morphologies from amphiphilic block copolymers: disks, toroids and bicontinuous micelles

Amphiphilic AB and ABA block copolymers have been demonstrated to form a variety of self-assembled aggregate structures in dilute solutions where the solvent preferentially solvates one of the blocks. The most common structures formed by these amphiphilic macromolecules are spherical micelles, cylindrical micelles and vesicles (polymersomes). Interest into the characterisation and controlled formation of block copolymer aggregates has been spurred on by their potential as surfactants, nano- to micro-sized carriers for active compounds, for the controlled release of encapsulated compounds and for inorganic materials templating, amongst numerous other proposed applications. Research in the past decade has focussed not only on manipulating the properties of aggregates through control of both the chemistry of the constituent polymer blocks but also the external and internal morphology of the aggregates. This review article will present an overview of recent approaches to controlling the self-assembly of amphiphilic block copolymers with a view to obtaining novel micellar morphologies. Whilst the article touches upon multi-compartment micelles particular focus is placed upon control of the overall shape of micelles; i.e. those systems that expand the range of accessible morphologies beyond ‘simple’ spherical and cylindrical micelles namely disk-like, toroidal and bicontinuous micelles

Structure development in octadecyl trimethylammonium templated silicate films grown at the air/water interface

The mechanism of growth of silicate films at the air/liquid interface has been investigated in situ by a series of grazing incidence diffraction experiments using a 20 x 25 cm(2) imaging plate as the detector. C(18)TAX (X = Br- or Cl-) has been used as the film templating surfactant. The formation of a layered phase, prior to growth of the hexagonal mesophase in C(18)TABr templated films. has been seen. This layered structure has a significantly shorter d spacing compared to the final hexagonal film (43 versus 48 Angstrom, respectively). The correlation lengths associated with the development of the hexagonal in-plane diffraction spots are much longer in-plane than perpendicular to the air/liquid interface (300 Angstrom versus 50 Angstrom). This implies that the film forms via the growth or aggregation of islands that are initially only a micelle or two thick. which then grow down into the solution