Solubilization locus

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. 

It is widely accepted that GBSS activity is a function of the protein coded by the waxy gene. The waxy locus gene product is a protein of molecular weight 58 KDa that is associated with starch granules and is similar to that found for the solubilized maize endosperm GBSSI.173 This protein has been extracted by heating the starch with a solution of SDS or by incubation at 37°C with 9M urea, but starch synthase activity... 

The theory of Harkins (57), which was developed during discussions with many other workers during the expansion of the synthetic rubber program, suggests that the principal locus for the initiation of the reaction was in the extremely small amount of solubilized monomer oil inside the soap micelles. This monomer is surrounded on all sides by a monomolecu-lar film of soap molecules, oriented as in Fig. 8 with their hydrocarbon groups towards the oil and their polar groups (e.g., —COO) towards the water outside. By this process small nuclei of polymer particles are formed. 

The exact location in the micelle at which solubilization occurs (i.e., the locus of solubilization) varies with the nature of the material solubilized and is of importance in that it reflects the type of interaction occurring between surfactant and solubilizate. Data on sites of solubilization are obtained from studies on the solubilizate before and after solubilization, using X-ray diffraction (Hartley, 1949 Philipoff, 1950), ultraviolet spectroscopy (Reigelman, 1958), NMR spectrometry (Eriksson, 1963, 1966), and fluorescence spectroscopy (Saito, 1993 Paterson, 1999). Diffraction studies measure changes in micellar dimensions on solubilization, whereas UV, NMR and fluorescence spectra indicate changes in the environment of the solubilizate on solubilization. Based on these studies, solubilization is believed to occur at a number of different sites in the micelle (Figure 4-2) (1) on... 

In concentrated aqueous surfactant solutions, although the shape of the micelles may be very different from that in dilute solution, the locus of solubilization for a particular type of solubilizate appears to be analogous to that in dilute solution that is, polar molecules are solubilized mainly in the outer regions of the micellar structures, whereas nonpolar solubilizates are contained in the inner portions. 

The extent to which a substance can be solubilized into a particular micelle depends upon the portion of the micelle that is the locus of the solubilization. The volume of that portion depends upon the shape of the micelle. As we have seen (Chapter 3, Section IIA), the shape of the micelle is determined by the value of the parameter Vj/lcao. As that value increases, the micelle in aqueous medium becomes increasingly asymmetrical, with the result that the volume of the inner core increases relative to that of the outer portion. We can therefore expect that the solubilization of material in the core will increase relative to that in the outer region of the micelle with increase in asymmetry (increase in the value of Vf/lcad). The amount solubilized in any location will also increase with increase in the volume of the micelle, e.g., with increase in the diameter of a spherical micelle. 

Thermal decomposition of initiator molecules produces pairs of radicals which are very likely to recombine when produced within the small volume of a latex particle or of monomer solubilized within a micelle. But if one radical escapes to the aqueous phase, a single radical is left in an isolated locus which is the prerequisite for emulsion polymerization. This still seems the most probable reason... 

As noted in the previous subsection, several types of contacts inside a micelle can also be measured directly For solubilized solutes, the average number of contacts per solute bead with solvent, tail, and head beads—and Uch, respectively—are counted. These values give information on the locus of solubilization, as discussed later in the results. 

We emphasize that a micelle may for many purposes be considered as a microscopic droplet of oil. This explains the large solubilization capacity towards a broad range of non-polar and weakly polar substances. We note, however, that the locus of solubilization will be very different for different solubilizates. While a saturated hydrocarbon will be rather uniformly distributed over the micelle core, an aromatic compound, being slightly surface-active, will be concentrated to the interfacial region. An amphiphilic solubilizate, like a long-chain alcohol, tends to orient in the same way as the surfactant itself. 

Other structural factors, such as the charge on the surfactant head group, can significantly affect the locus of solubilization. Materials containing aromatic rings, for example, may be solubilized in or near the core of anionic systems but in the palisades layer of cationic surfactants, due to electronic interactions between the ring and the cationic head group. 

Other factors that can affect the ability of a particular surfactant system to solubilize materials include pH and pressure. The effects of such factors, however, have not been as extensively reported in the literature as the factors discussed above, and they are often very specific to each surfactant system. Obviously, surfactants that show extreme sensitivity to pH such as the carbox-ylate salts can also be expected to exhibit significant changes in solubilization with changes in that factor. In addition, changes in pH can affect the nature of the additive itself, producing dramatic changes in its interactions with the micelle, including the locus of solubihzation. Such effects can be especially important in many apphcations of solubilization, such as in the pharmaceutical field. 

The role of surfactants is twofold, firstly to provide a locus for the monomer to polymerize and secondly to stabilize the polymer particles as they form. In addition, surfactants aggregate to form micelles (above the critical micelle concentration) and these can solubilize the monomers. In most cases a mixture of anionic and nonionic surfactant is used for optimum preparation of polymer latexes. Cationic surfactants are seldom used, except for some specific applications where a positive charge is required on the surface of the polymer particles. 

It is most likely that none of the preservative combinations used had exactly the same locus or are solubilized by the same mechanism, so that simple competition between the solubilizates in the micelle is unlikely alternatively the interaction of some of the preservatives with the micelle (or monomers) leads to perturbations of micelle size and shape such that binding sites are altered in their capacity to accept solubilizate molecules. 

Surfactant—polymer systems have additional technological significance since surfactants are normally used in the emulsion polymerization of many materials, often involving the solubilization of monomer in micelles prior to polymerization and particle formation. Surfactants have also been shown to increase the solubility of some polymers in aqueous solution. The combined actions of the surfactant as a locus for latex particle formation (the micelle) in some cases, particle stabilization by adsorbed surfactant, and as a solubilizer for monomer permit us to expect quite complex relationships between the nature of the surfactant and that of the resulting latex. 

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