Salt-induced sphere-to-disk transition of octadecyltrimethylammonium bromide micelles

We have used surface tension measurements, differential scanning calorimetry (DSC), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM) to investigate the dynamic and structural behavior of octadecyltrimethylammonium bromide (C(18)TAB) micelles in water and NaBr solution. The surface tension data for fixed C(18)TAB concentrations of 25 mM and varied NaBr additions (0-50 mM) shows that the critical micelle concentration (cmc) increases after an initial decrease at 0.5 mM NaBr. This unusual effect has been explained using results from DSC and DLS. At low salt concentrations (below ca. 25 mM) the relaxation time distribution is bimodal with a dominant fast mode due to spherical micelles. Above ca. 35 mM NaBr disklike structures are favored and the relaxation time distribution is more closely unimodal. The postulated sphere-to-disk transition is supported by cryo-TEM micrographs. A pronounced increase in the micellar effective hydrodynamic radius (R-H) is observed as the NaBr concentration is increased above about 35 mM; below 35 mM the R-H of the spherical micelles changes Little with ionic strength

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