Solubilized emulsifier molecules

The function of emulsifier in the emulsion polymerization process may be summarized as follows [45] (1) the insolubilized part of the monomer is dispersed and stabilized within the water phase in the form of fine droplets, (2) a part of monomer is taken into the micel structure by solubilization, (3) the forming latex particles are protected from the coagulation by the adsorption of monomer onto the surface of the particles, (4) the emulsifier makes it easier the solubilize the oligomeric chains within the micelles, (5) the emulsifier catalyzes the initiation reaction, and (6) it may act as a transfer agent or retarder leading to chemical binding of emulsifier molecules to the polymer. 

Based on the Smith-Ewart theory, the number of latex particles formed and the rate of polymerization in Interval II is proportional with the 0,6 power of the emulsifier concentration. This relation was also observed experimentally for the emulsion polymerization of styrene by Bartholomeet al. [51], Dunn and Al-Shahib [52] demonstrated that when the concentrations of the different emulsifiers were selected so that the micellar concentrations were equal, the same number of particles having the same size could be obtained by the same polymerization rates in Interval II in the existence of different emulsifiers [52], The number of micelles formed initially in the polymerization medium increases with the increasing emulsifier concentration. This leads to an increase in the total amount of monomer solubilized by micelles. However, the number of emulsifier molecules in one micelle is constant for a certain type of emulsifier and does not change with the emulsifier concentration. The monomer is distributed into more micelles and thus, the... 

Micelles are approximately spherical aggregates of surfactant molecules with their nonpolar tails in the interior and their hydrophilic ends oriented towards the aqueous medium. They are some 50-100 A in diameter. The bulk concentration of surfactant is usually around 0.1 M and this corresponds to approximately I o micelles per milliliter of aqueous phase, since there are typically about 50-100 emulsifier molecules per micelle. The apparent water solubility of organic molecules is enhanced by micellar surfactants, because the organic molecules are absorbed into the micelle interiors. The extent of this solubilization of organic molecules depends on the surfactant type and concentration, the nature of the solubilized organic substance, and the concentration of electrolytes in the aqueous phase. As an example, there will be about an equal number of styrene molecules and potassium hexadecanoate (palmitate) molecules in a micelle of the latter material. In this case about half the volume of the micelle interior is occupied by solubilized monomer, and the concentration of styrene is approximately 4.5 M at this site. Thus radical polymerization starts very rapidly in the interior of a micelle once it is initiated there. 

Like a detergent, bile salts contain hydrophobic and hydrophilic components. The hydrophobic portions of the molecule associate with the fat, and the hydrophilic parts associate with water, serving to solubilize (emulsify) the otherwise insoluble fat. See Figure 18.4. 

The system initially consists of water, a practically water-insoluble monomer, an emulsifier, and a water-soluble initiator (see Figure 20-10). The emulsifier forms a great number of micelles above the critical micelle concentration. The micelles solubilize monomer, which causes them to swell. Another fraction of the monomer forms monomer droplets of about 1000 nm diameter. The initiator dissociates into free radicals, which can either travel into the micelle and start a polymerization directly (Smith-Ewart-Harkins theory) or react first with an emulsifier molecule, under... 

The adsorption of surfactant molecules at oil-water interfaces has attracted much interest, in relation to the oil recovery techniques (1). The systems containing oil, water and emulsifier molecules form generally two phases an aqueous phase containing sometimes solubilized oil in the form of small droplets surrounded by emulsifier molecules and an oil phase which also can contain solubilized water. When the amount of emulsifier is large enough, the system can form only one phase, i.e. all the water (or oil) can be solubilized in the oil (or water). The system is again a dispersion of very small droplets of water (or oil), surrounded by emulsifier molecules, in a continuous medium containing the oil (or water). Such dispersions are currently called microemulsions. The droplet size is usually of the order of lOOA (2). 

Emulsion polymerization is applicable only to monomers that are relatively insoluble in water, such as styrene. A coarse emulsion of monomer in aqueous surfactant is prepared with a water-soluble initiator, say, H202 in the solution. The surfactant concentration is above the CMC, so surfactant molecules are present as monomers, micelles, and emulsifiers at the oil-water interface. Even an insoluble liquid like styrene dissolves in water to some extent. Therefore the monomer is present in coarse emulsion drops, solubilized in micelles, and as dissolved molecules in water. A schematic illustration of the distribution of surfactant, monomer, and polymer in an emulsion polymerization process is shown in Figure 8.14. 

It is believed that polymerization of hydrophobic monomers is initiated by free radicals in the aqueous phase and that the surface-active oligomers produced migrate to the interior of the emulsifier micelles where propagation continues. Monomer molecules dispersed in the water phase also solubilize by diffusing —to the expanding lamellar micelles. These micelles disappear as the polymerization continues and the rate may be measured by noting the increase in surface tension of llie system. 

Answer Lecithin (see Fig. 10-14 for structure), an amphipathic molecule, is an emulsifying agent, solubilizing the fat (triacylglycerols) in butter. Lecithin is such a good emulsifying agent that it... 

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