41 research outputs found
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
Effect of Sodium Chloride and Varied Alkyl Chain Length on Aqueous Cationic Surfactant−Bile Salt Systems. Cryo-TEM and Fluorescence Quenching Studies
Effect of Cationic Polymers on Foam Rheological Properties
We study the effect of two cationic polymers, with trade
names
Jaguar C13s and Merquat 100, on the rheological properties of foams
stabilized with a mixture of anionic and zwitterionic surfactants
(sodium
lauryloxyethylene sulfate and cocoamidopropyl betaine). A series of
five cosurfactants are used to compare the effect of these polymers
on foaming systems with high and low surface dilatational moduli.
The experiments revealed that the addition of Jaguar to the foaming
solutions leads to (1) a significant increase of the foam yield stress
for all systems studied, (2) the presence of consecutive maximum and
minimum in the stress vs shear rate rheological curve for foams stabilized
by cosurfactants with a high surface modulus (these systems cannot
be described by the Herschel–Bulkley model anymore), and (3)
the presence of significant foam–wall yield stress for all
foaming solutions. These effects are explained with the formation
of polymer bridges between the neighboring bubbles in slowly sheared
foams (for inside foam friction) and between the bubbles and the confining
solid wall (for foam-wall friction). Upon addition of 150 mM NaCl,
the effect of Jaguar disappears. The addition of Merquat does not
noticeably affect any of the foam rheological properties studied.
Optical observations of foam films, formed from all these systems,
show a very good correlation between the polymer bridging of the foam
film surfaces and the strong polymer effect on the foam rheological
properties. The obtained results demonstrate that the bubble–bubble
attraction can be used for efficient control of the foam yield stress
and foam–wall yield stress, without significantly affecting
the viscous friction in sheared foams
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