Cylindrical Reverse Micelles Research Articles

Growth, Branching, and Local Ordering of Lecithin Polymer-Like Micelles

The viscoelastic properties of lecithin organogels formed by the addition of trace amounts of water in a n-decane solution were studied by means of oscillatory rheology. The viscoelasticity of jelly-like phases of this sort is caused by a transient three-dimensional network consisting of entangled cylindrical reverse (polymer-like) micelles. It is shown that although the organogel properties depend on the lecithin concentration, the phase and rheological behavior is mainly regulated by the polar additive. The homogeneous jelly-like phase exists for molar ratios (nw) of water to lecithin from 1.6−1.7 to 3.2−3.4. At nw ratios below 2.7−2.8, the scaling exponents of the main rheological parametersthe zero shear viscosity, plateau modulus, and terminal relaxation timeare rather close to the theoretical predictions that follow from a model by Cates. This means that the lecithin polymer-like micelles are linear and flexible. At larger molar ratios the scaling behavior with the lecithin concentration is changed....

Biomimetic strategies for the control of size, shape and self-organization of nanoparticles

An unusual phase diagram is presented. It is composed of copper(II) bis(2-ethylhexyl) sulfosuccinate Cu(AOT) 2-isooctane-water. Keeping the concentration of the Cu(AOT) 2-isooctane solution constant, increasing in the amount of water induces various phase transitions. At low water content, spherical and cylindrical reverse micelles are formed. By increasing the water content, a bicontinuous system appears. Further addition of water leads to the formation of planar and spherulite type lamellae. As more water is added only spherulites remain in the phase. Still further addition of water leads to a reappearance of an interconnected network and then reverse micelles. Syntheses performed in the various parts of the phase diagram show that the use of colloidal assemblies as templates favors the control of the shape of nanoparticles. Cylindrical metallic copper particles having the same size can be obtained in various parts of the phase diagram when the template is made of interconnected cylinders. A very low amount of cylinders (13%) is formed when the synthesis is performed in cylindrical reverse micelles. When the colloidal self-assembly is a mixture of several phases, various types of shapes can be obtained. In some cases, the polydispersity in size is so low that metallic particles are able to self-assemble in a hexagonal network. Multilayers can be observed and are arranged in a face centered cubic structure.

Control of the Shape of Copper Metallic Particles by Using a Colloidal System as Template

In this paper we show that the use of colloidal assemblies as templates favors the control of the shape of nanoparticles. Cylindrical copper metallic particles having same size can be obtained in various parts of the phase diagram when the template is made of interconnected cylinders. A very low amount of cylinders (13%) is formed when the synthesis is performed in cylindrical reverse micelles. When the colloidal self-assembly is a mixture of several phases, various types of shapes can be obtained. In some cases, the polydispersity in size is so low that metallic particles are able to self-assemble in a hexagonal network. Multilayers can be observed and are arranged in a face centered cubic structure.

Phase Diagram of Copper(II) Bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2−Isooctane−Water

A phase diagram of copper(II) bis(2-ethylhexyl)sulfosuccinate, Cu(AOT)2−isooctane−water, is presented. Keeping the concentration of the Cu(AOT)2−isooctane constant, an increase in the amount of water induces transitions. At low water content, spherical and cylindrical reverse micelles are formed. By increasing the water content, a phase transition takes place with formation of a bicontinuous system. Further addition of water leads to the formation of planar and bended (spherulite type) lamellae. As more water is added, only spherulites remain in the phase. Still further addition of water leads to a reappearance of an interconnected network and then reverse micelles.

About entangled networks of worm-like micelles: a rejected hypothesis

We report new results from small-angle neutron scattering ond12-cyclohexane/lecithin/water micellar solutions performed as a function of the water content (w0), temperature (T) and dispersed phase volume fraction (ϕ). The data from dilute samples are interpretable in terms of the existence of giant cylindrical reverse micelles and are well fit with a core-shell model (that provides the micelle structure and dimensions) with values of 28 and 45 A for the inner core and the outer shell radii, almost independent on temperature and concentration. Such a result could appear consistent with the current idea that worm-like micelles are living polymers. On the contrary, the appearance of a sharp interference maximum at high concentrations (ϕ>0.15) suggests arguments against the current hypothesis of an entangled network of giant flexible cylinders. Further arguments against the current hypothesis are given by the close similarity between the above described results and those from free of water micelles (for sure not cylinders). All the data are well fitted in terms of a unique model taking into account the micellar form factor plus a hard sphere structure factor. The data analysis suggests a micellar size distribution determined by the competition between concentration and interaction effects on which temperature plays not a minor role. Following our results, the current hypothesis of a gel structure in terms of an entangled network can be assumed as wrong and some caution has to be taken in assuming wormlike micelles as living polymers.

Small-angle neutron scattering from lecithin reverse micelles

We report new results of small-angle neutron scattering from cyclohexane- d 12/lecithin/water micellar solutions, performed as a function of the water content, w 0, the temperature, T and the dispersed-phase volume fraction, ф. The data from dilute samples can be interpreted in terms of the existence of giant cylindrical reverse micelles, in good agreement with the current idea of an unlimited micellar growth with ф. In contrast, the appearance of a sharp interference maximum at high concentrations ( ф > 0.15) suggests that the current hypothesis of a gel structure interpreted as a random network of entangled micelles can be assumed to be incorrect, and that there must be some caution in assuming analogies with neutral polymer-like systems.

Change in the shape of copper nanoparticles in ordered phases

The shape of copper metal particles (see Figure) is demonstrated to depend on the structure of the mesophase (cylindrical reverse micelles, interconnected networks of surface aggregates or lamellar phases) i the surfactant system used as a microreactor for their synthesis. An understanding of this correlation is of great importance for magnetic recording technology magnified image.

Sol-gel transition in worm-like micelles

The statical and dynamical properties of lecithin/H2O/cyclohexane cylindrical reverse micelles are investigated as a function of lecithin volume fraction, ϕ, and temperature at fixed water/lecithin ratio,w 0. The viscosity data are well fitted by the Cates model when the breathing mode of micelles is taken into consideration, overlapping with the breaking and reforming mechanisms. We present some results from Brillouin-scattering experiment, performed across the sol-gel transition. In order to explain the experimentally observed ϕ-dependence of the hyperacoustic parameters, a mechanical model was developed from which the ϕ-dependence of the micelle size distribution was obtained. From a comparison with the viscosity data the entanglement length was estimated. Furthermore some new results from an incoherent quasi-elastic neutron scattering experiment are presented. The whole body of the experimental results suggests for a sol-gel transition triggered by topologic phenomena. When the lecithin volume fraction increases, the kinetic equilibrium between the breaking and reforming mechanisms of the micelles shifts the mean micelle length towards higher values and the entanglement of the micelles becomes highly favourable. The obtained results are discussed and compared with other findings in literature.

Shear aligned lecithin reverse micelles: a small-angle neutron scattering study of the anomalous water-induced micellar growth

We present results from a small-angle neutron scattering study of shear-aligned viscoelastic reverse micellar solutions of lecithin in isooctane. We obtain direct evidence for the presence of cylindrical reverse micelles and a water-induced anisotropic growth in these solutions. A substantial increase of the micellar contour length L with increasing water to lecithin molar ratio w 0 can be observed

Viscoelastic properties of polymerlike reverse micelles

A dramatic increase in the viscosity of reverse micellar solutions of lecithin in a variety of organic solvents of up to a factor of 106 upon the addition of a small amount of water can be observed. The formation of viscoelastic solutions can be explained by a water-induced aggregation of lecithin molecules into flexible cylindrical reverse micelles and the subsequent formation of a transient network of entangled micelles. The viscoelastic properties of these solutions are characterized as a function of water content and temperature for different organic solvents by means of dynamic shear viscosity measurements. The results are interpreted by making analogies to the behavior of semidilute polymer solutions and living polymers.