Micelles core size

Surfactants are well known as stabilizers in the preparation of metal nanoparticles for catalysis in water. Micelles constitute interesting nanoreactors for the synthesis of controlled-size nanoparticles from metal salts due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained which can be easily used as catalysts. 

FIGURE 1.2. Formation of nanoparticles of metal oxide by reverse micelle method. A solution of inverse micelles is first formed by adding a long-chain alkylamine to a toluene solution. A small amount of water is trapped in the reverse micelle core. Mixing the reverse micelle solution with an aluminum alkoxy amine adduct results in hydrolysis of the aluminum alkoxide adduct and formation of nano-sized particles of aluminum oxyhydroxide after drying. These particles are shown in the SEM picture above. 

In search of easy dispersible nucleating agents with a high number of nucleants per volume, Spitael et al. [19] investigated the use of nanoscale diblock copolymer micelles on the batch-foaming behavior of PS. The diblock copolymers were composed of a PS block, and either PDMS, PEP, or PMMA as a second block. Several factors were identified as essential for nucleation, e.g., the size of the micelle and the surface tension of the micelle core material. 

The size and shape of micelles have been a subject of several debates. It is now generally accepted that three main shapes of micelles are present, depending on the surfactant structure and the environment in which they are dissolved, e.g., electrolyte concentration and type, pH, and presence of nonelectrolytes. The most common shape of micelles is a sphere with the following properties (i) an association unit with a radius approximately equal to the length of the hydrocarbon chain (for ionic micelles) (ii) an aggregation number of 50-100 surfactant monomers (iii) bound counterions for ionic surfactants (iv) a narrow range of concentrations at which micellization occurs and (v) a liquid interior of the micelle core. 

The recovery of product or catalyst from the micelle core may be simplified because the micelle size, and even its existence, are dependent on fluid pressure, in contrast to liquid systems where pressure has little or no effect. 

If the optimal micelle core radius i c,o from Eq. (12-10) exceeds the maximum effective tail length Ic (i.e., if Rc,o/ic = vjaoic > 1), then optimally sized spherical micelles are not possible, and either the surface area per head group must deviate from its optimal value Up, or there will be a transition to a shape with a higher surface-to-volume ratio. 

Finally, contrary to expectations, the apparent micelle radius decreases (by approximately 10 to 15%) as the polymerization progresses (see Figure 10). This trend was observed for all of the runs shown in Table 1. Not until almost total conversion, when coagulation and phase separation were observed in the view cell, was any increase in the apparent micelle size observed during polymerization. However, the growing polymer molecules could be in a highly collapsed state such that they fit inside a micelle core. (A one-million molecular weight molecule in its bulk state, density of approximately Ig/cc, would require a sphere of approximately 7 to 8 nm in radius). 

This approach was first described in [64, 65]. When metal nanoparticles are located in the block copolymer micelle cores [64] or in microgels [65] and these polymeric systems are used as templates for silica casting, both pore size and... 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

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