Particle generation micellar

This is the nucleation period. Ideally, all M/P particles are generated in stage I. The particles grow in volume and adsorb surfactant molecules, resulting in diminution of micelles. If the area occupied by each surfactant molecule is the same on particle and micellar surfaces and the particles are completely covered by surfactant, the combined areas of micelles and particles in unit volume of the reaction medium will remain constant The particle surface area will grow at the expense of micellar surface area and when the total surface area of particles equals the total surface area occupied by the surfactant initially added, all micellar soap will have disappeared and further nucleation will cease. 

Formation of latex particles can proceed via the micellar nucleation, homogeneous nucleation and monomer droplet nucleation. The contribution of each particle nucleation mechanism to the whole particle formation process is a complex function of the reaction conditions and the type of reactants. There are various direct and indirect approaches to determine the particle nucleation mechanism involved. These include the variations of the kinetic, colloidal and molecular weight parameters with the concentration and type of initiator and emulsifier. There are some other approaches, such as the dye method where the latex particles generated via homogeneous nucleation do not contribute to the amount of dye detected in the latex particles since diffusion of the extremely hydrophobic dye molecules from the monomer droplets to the latex particles generated in water is prohibited. On the contrary, nucleation of the dye containing monomer droplets leads to the direct incorporation of dye into the polymer product. However, the dye also act as a hydrophobe and enhances the stability of monomer droplets as well as the monomer droplet nucleation. 

An expression for the number of particles formed during Stage I was developed, assuming micellar entry as the formation mechanism (13), where k is a constant varying from 0.37 to 0.53 depending on the relative rates of radical adsorption in micelles and polymer particles, r is the rate of radical generation, m is the rate of particle growth, is the surface area covered by one surfactant molecule, and S is the total concentration of soap molecules. 

Deriving an expression for f(t) a considerable simplification occurs if one takes all polymer particles to be nucleated at the same size dp(t,t). The generation of new polymer particles in an emulsion system is basically due to two mechanisms micellar and homogeneous particle production. Then, the rate of particle nucleation, fit), can be expressed as (12) ... 

The number of polymer particles is the prime determinant of the rate and degree of polymerization since it appears as the first power in both Eqs. 4-5 and 4-7. The formation (and stabilization) of polymer particles by both micellar nucleation and homogeneous nucleation involves the adsorption of surfactant from the micelles, solution, and monomer droplets. The number of polymer particles that can be stabilized is dependent on the total surface area of surfactant present in the system asS, where as is the interfacial surface area occupied by a surfactant molecule and S is the total concentration of surfactant in the system (micelles, solution, monomer droplets). However, N is also directly dependent on the rate of radical generation. The quantitative dependence of N on asS and R,- has been derived as... 

The abihty to modify the surface of the CdS nanoparticle within the cavity of the w/o microemulsions has been extended for their immobihzation. CdS nanoparticles (e.g., prepared in AOT/isooctane w/o microemulsions) were immobilized onto thiol-modified aluminosihcate particles [218] and thiol-modified alumina [219] by a simple addition of thiol-modified aluminosih-cates and alumina, respectively, in the micellar solution. The resulting CdS nanoparticles-aluminosihcate composites were used as photocatalysts for H2 generation from 2-propanol aqueous solution. 

The emulsion polymerization process involves the polymerization of liquid monomers that are dispersed in an aqueous surfactant micelle-containing solution. The monomers are solubilized in the surfactant micelles. A water-soluble initiator catalyst, such as sodium persulfate, is added to the aqueous phase. The free radicals generated cause the dispersed monomers to react to produce polymer molecules within the micellar environment. The surfactant plays an additional role in stabilizing dispersion of the produced polymer particles. Thus, the surfactants used both provide micelles to house the monomers and macroradicals, and also stabilize the produced polymer particles [193,790], Anionic surfactants, such as dodecylbenzene sulfonates, are commonly used to provide electrostatic stabilization [193], These tend to cause production of polymer particles having diameters of about 0.1-0.3 pm, whereas when steric stabilization is provided by, for example, graft copolymers, then diameters of about 0.1-10 pm tend to be produced [790,791]. 

All quantitative theories based on micellar nucleation can be developed from balances of the number concentrations of particles, and of the concentrations of aqueous radicals. Smith and Ewart solved these balances for two limiting cases (i) all free radials generated in the aqueous phase assumed to be absorbed by surfactant micelles, and (ii) micelles and existing particles competing for aqueous phase radicals. In both cases, the number of particles at the end of Interval I in a batch macroemulsion polymerization is predicted to be proportional to the aqueous phase radical flux to the power of 0.4, and to the initial surfactant concentration to the power of 0.6. The Smith Ewart model predicts particle numbers accurately for styrene and other water-insoluble monomers. Deviations from the SE theory occur when there are substantial amounts of radical desorption, aqueous phase termination, or when the calculation of absorbance efficiency is in error. 

In contrast to emulsion polymerization, the reaction kinetics of microemulsion polymerization is characterized by two polymerization rate intervals the interval of constant rate characteristic of emulsion polymerization is missing [42,49,53], as shown in Fig. 2. Polymer particles are generated continuously during the reaction by both micellar and homogeneous mechanisms. As the solubility of the monomer in the continuous domain increases, homogeneous... 

A common industrial practice used to avoid oscOlations is to seed the first reactor with small-diameter seed particles produced earber in batch reactors. By then keeping the emulsifier concentration below the CMC one avoids further micellar generation and simply grows the seed particles. Although in some cases this may introduce additional flexibility in that a... 

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