Hydrophobic loci

The porphyrin skeleton is essentially hydrophobic this feature may be an important factor affecting the preferential acciunulation in cellular hydrophobic loci since such molecules must be able to get into cells by crossing Upid membranes. That brings insolubility in physiological fluids to overcome this fact adequate formulations have to be used, such as incorporation into hpo-somes, biopolymers and cyclodextrins [43-45]. 

As depicted in Figure 4.5, the surfactant bilayer created at the solid/aqueous solution interface provides hydrophobic loci for the solubilization of monomer or radicals these can polymerize further on the modified inorganic surface and effectively coat the sohd particles according to an emulsion-like polymerization reaction. Three steps are involved in the coating reaction. In the first step, the emulsifier adsorbs onto the mineral surface, forming micelle-like aggregates. In a second step, the monomer is solubihzed in the adsorbed micelles. Finally, the polymerization takes place as a conventional emulsion polymerization in the monomer-sweUed admiceUes (Fig. 4.6). 

The reaction described in this example is carried out in miniemulsion.Miniemulsions are dispersions of critically stabilized oil droplets with a size between 50 and 500 nm prepared by shearing a system containing oil, water,a surfactant and a hydrophobe. In contrast to the classical emulsion polymerization (see 5ect. 2.2.4.2), here the polymerization starts and proceeds directly within the preformed micellar "nanoreactors" (= monomer droplets).This means that the droplets have to become the primary locus of the nucleation of the polymer reaction. With the concept of "nanoreactors" one can take advantage of a potential thermodynamic control for the design of nanoparticles. Polymerizations in such miniemulsions, when carefully prepared, result in latex particles which have about the same size as the initial droplets.The polymerization of miniemulsions extends the possibilities of the widely applied emulsion polymerization and provides advantages with respect to copolymerization reactions of monomers with different polarity, incorporation of hydrophobic materials, or with respect to the stability of the formed latexes. 

One should consider, however, that siloxane (like any polymeric matrix) will not create a hermetic interface. Even though it is hydrophobic in nature, it may still allow water penetration. The second possible explanation involves the change in the relative number of Si—O bonds that would need to be broken to create weakness in the system to the point of changing the peel locus of failure. It may be considered that the APS-siloxane network on the plasma-treated F-contaminated Si02 surface effectively brings another layer of Si—O bonds, the number of which may be too high to be effectively broken during the peel test [21]. 

The effect of micelles on organic reactions can be attributed to both electrostatic and hydrophobic interactions (Rosen, 1979). Electrostatic interaction is important because it may affect the transition state of a reaction orthe concentration of reactant in the vicinity ofthe reaction site. The hydrophobic interactions are important because they determine the extent and the locus of solubilization in the micelle. 

The balance of electrostatic and hydrophobic interactions can be such as to cause the locus of solubilisation to be anywhere in the micelle from close to the surface to the inner core. 

The First Stage. The preferential polymerization of AA at the initial stage of copolymerization means that the main reaction locus is the aqueous phase just as Juang and Krieger pointed it out for the aqueous copolymerization of St with sodium styrenesul-fonate ( SSS ) (9). In the St-SSS system, SSS polymerized preferentially up to a few percent conversion under the condition of SSS/St (w/w) - 0.014. Copolymerization of hydrophobic monomer with a large amount of hydrophilic comonomer was considered to yield a greater amount of information with respect to the reaction mode. By use of a relatively large amount of AA or its derivatives the characteristic reaction mode of the copolymerization of St with acrylamides could be clarified. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

All the results discussed in this chapter helped to delineate a unique physicochemical model for micelle structure and solute-micelle interactions. The SDS micelle is viewed as an entity composed of numerous compartments of distinct hydrophobicities at least three of them can be readily defined the inner core, the interface, and the surface. The interface can be further divided into a number of levels. The MEKC solute set used in the LSER studies predominantly occupies the interface distributing themselves into these levels according to their hydrophobicity. Therefore, the characterization of phase selectivity depends strongly on the composition of the solute set. By knowing the preferential locus of the solute series into the micelle, it is possible to promote changes in that specific locus and thus that must be the only way to alter and to contrast phase selectivity. Moreover, meaningful studies on solute-micelle interactions by QSRR can only derive from an RSS assessing a particular locus in the micelle and, of course, reliable chemometry. 

Upon initiation of the polymer reaction, these monomer droplets become the main locus of particle generation. At this stage of polymerization, the function of hydrophobe is to retain the monomer originally present in the nucleated droplets. 

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