Solubilized monomer molecules

Emulsification is the process by which a hydrophobic monomer, such as styrene, is dispersed into micelles and monomer droplets. A measure of a surfactant s abiUty to solubilize a monomer is its critical micelle concentration (CMC). Below the CMC the surfactant is dissolved ia the aqueous phase and does not serve to solubilize monomer. At and above the CMC the surfactant forms spherical micelles, usually 50 to 200 soap molecules per micelle. Many... 

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. 

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. 

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. 

Water-miscible surfactant molecules contain both a hydrophobic and hydrophilic portion, and can solubilize many poorly water-soluble drugs. Surfactants can also self-assemble to form micelles once the surfactant monomer concentration reaches the critical micelle concentration. Thus surfactants can solubilize drug molecules by either a direct cosolvent elfect or by uptake into micelles. The non-ionic surfactants in commercially available solubilized oral formulations include polyoxyl 35 castor oil (cremophor EL), polyoxyl 40 hydrogenated... 

Harkins calculated from the solubility of styrene in water (0.00368 mol dm at 50 °C [50]) that there are 4 x 10 molecules dm . In a 3% solution of potassium dodecanoate there are about 1 x 10 micelles dm , but with 61 molecules per micelle with an unswollen radius of 2.1 nm the cross-sectional area of the monomer-swollen micelles exceeds that of the styrene molecules by a factor of at least 12. Hence the micelles are more likely to capture initiator radicals produced in the aqueous phase. Polymerization within the micelles must be much faster than in the water because the concentration of styrene will be much the same as in bulk (8.5 mol dm ). The molar mass of the polystyrene produced is much larger than the molar mass of all the styrene molecules solubilized in a micelle thus, the monomer must be able to diffuse through the aqueous phase from other micelles and monomer droplets to allow the polymer radical to continue to grow until it is finally terminated by the entry of another initiator radical from the aqueous phase. Under the standard conditions of the mutual recipe (Table 4.1) there is 180 g water to 100 g styrene taking the emulsion droplets to have a radius of 1 pm, the ratio of the total cross-sectional areas of droplets to micelles to monomer molecules is about 1 30 2.5. The ratio of total surface areas would be even more heavily biased in favour of micelles. Hence it is probable that many more radicals will be captured from the aqueous phase by the micelles than by the emulsion droplets or than react with the monomer molecules in aqueous solution. 

R = 9, the solubilized water molecules are mostly bound to the polar groups on the AOT molecules. Thus, hydrolysis is incomplete and condensation (leading eventually to particle formation) is inhibited. In the case of R = 30, free water molecules are available, and therefore extensive hydrolysis, followed by condensation, can occur. The peak in the absorbance and the subsequent decline in the absorbance mark the removal of the hydrolyzed monomers for the condensation reaction. 

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 surfactant is initially distributed through three different locations dissolved as individual molecules or ions in the aqueous phase, at the surface of the monomer drops, and as micelles. The latter category holds most of the surfactant. Likewise, the monomer is located in three places. Some monomer is present as individual molecules dissolved in the water. Some monomer diffuses into the oily interior of the micelle, where its concentration is much greater than in the aqueous phase. This process is called solubilization. The third site of monomer is in the dispersed droplets themselves. Most of the monomer is located in the latter, since these drops are much larger, although far less abundant, than the micelles. Figure 6.10 is a schematic illustration of this state of affairs during emulsion polymerization. 

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... 

At infinite dilution, 1-pentanol monomers distribute between AOT-reversed micelles and the continuous organic phase, whereas at finite alcohol concentration, given the ability of alcohol to self-assemble in the apolar organic solvent, a coexistence between reversed micelles (solubilizing 1-pentanol) and alcoholic aggregates (incorporating AOT molecules) is realized [25],... 

The cyclodextrins are stable bodies in aqueous solution, unlike the micelles, which are transitory and are in a state of dynamic equilibrium with the monomer surfactants. However, in many aspects the inclusion of analytes in the cyclodextrin cavity is reminiscent of the solubilization of hydrophobic molecules in micelles in aqueous solution. 

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