Micelle particle formation

Aqueous micelles 3D 20-40 A diameter Dissolution of surfactant in water at concentrations above the critical micelle concentration (CMC) Dynamic equilibrium between monomers and micelles Particle formation was only possible at the micellar surface or close to it 55, 101... 

In the case of the more water-soluble monomers which were studied later, their solubility though low (<3%) is much in excess of the amount of monomer which may be solubilized by the emulsifier. The latex particles are certainly formed by homogeneous nucleation [54] which is also the only mechanism which can apply to styrene-like monomers in the absence of surfactant micelles. Particle formation involves polymerizaticm in the aqueous phase until the surface-active oligomer radical, which is formed when the initiator used introduces an ionic end-group, either precipitates [55] or micellizes [56] to form a latex particle into... 

Smith and Ewart [6] assume that the particles are formed by entry of primary radicals into the monomer-swollen emulsifier micelles or polymer particles where they initiate or terminate the growth events. Roe [23] assumes that the particles are formed outside the micelles. Particle formation stops when the surface of the particles has grown to such a size that the emulsifier concentration in the aqueous phase is below a critical point, somewhat lower than CMC. Fitch et al. [22,25, 67] have proposed a mechanism which implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. Such primary particles are colloidally unstable, undergoing coagulation with other primary or with large polymer particles and polymerize very slowly. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

No version of micellar entry theory has been proposed, which is able to explain the experimentally observed leveling off of the particle number at high and low surfactant concentrations where micelles do not even exist. There is a number of additional experimental data that refute micellar entry such as the positively skewed early time particle size distribution (22.), and the formation of Liesegang rings (30). Therefore it is inappropriate to include micellar entry as a particle formation mechanism in EPM until there is sufficient evidence to do so. 

Unlike TEOS hydrolysis, Si02 particles have been also prepared by hydrolysis of Na2Si02 and Na4Si02 in nonionic reversed micelle systems. Spherical and poly-disperse particles of 31.8 nm mean diameter were produced in polyoxyethylene(9.5) octylphenyl ether-hexanol-cyclohexane systems (25), but more uniform and dense particles were precipitated by hydrochloric acid-catalyzed hydrolysis in a mixture of polyoxyethylene(5) nonylphenyl ether and polyoxyethylene(9) nonylphenyl ether in cyclohexane systems at pH 11 (26). The uniform particle formation at higher pH is attributed to the charge repulsion by OH- adsorbed on particle surface. The particles of specific surface area of 347 m2 g-1 can be obtained by calcination of particles produced at pH 2. 

In particle formation in ionic and nonionic reversed micelles as mentioned earlier, simple electrolytes such as AgN03 and NaCl were used as reactants. If the... 

AOT-isooctane-HjO reversed micelles 150 A Cu particles Mixing Cu-AOT [Cu(AOT)2] = 10" 3 M in [AOT] = 0.25 M with NaBHt-AOT [NaBHJ = 2 x 10-3 M in [AOT] = 0.25 M in isooctane led to Cu-particle formation (monitored by absorption spectra and TEM). Particle diameters did not change greatly with the water content in the reversed micelles but at low [H20] to [AOT] ratios isolated clusters formed, while at higher ratios particles became more aggregated 55... 

Nomura and Harada already reported an experimental and theoretical study on the effect of lowering the amount of monomer initially charged on the number of polymer particles formed in a batch reactor(14). Under usual conditions in batch operation, micelles disappear and the formation of particles terminates before the disappearance of monomer droplets in the water phase. However, if the initial monomer concentration is extremely low, micelles would exist even after the disappearance of monomer droplets and hence, particle formation will continue until all emulsifier molecules are adsorbed on the surfaces of polymer particles. This condition is quantitatively expressed by the following emulsifier balance equation., ... 

Solubilization of insoluble oxidation products and soot particles. Reverse micelles (RMs) formations manage the prevention of agglomeration and the contamination process of insoluble oxidation particles and soot particles by both steric stabilization (Fig.2.1) and electrostatic stabilization mechanisms (Fig.2.2). The steric stabilization mechanism provides a physical barrier to agglomeration of particles by adsorption on particle surfaces. Adsorbed dispersant acts as a physical barrier to attraction between particles. 

On the other hand, they derived an expression that predicts the number of polymer particles produced, ATp, assuming that (i) a monomer-swollen emulsifier micelle is transformed into a polymer particle by capturing a free radical from the aqueous phase, (ii) the volumetric growth rate per particle p is constant, at least during particle formation, and (iii) free radical activity does not transfer out of a growing particle... 

Particle formation stops at the time t, when the emulsifier micelles have just disappeared because all of the emulsifier molecules comprising the emulsifier micelles have been transferred to the surfaces of growing polymer particles for adsorption. The volume Vp at time t of a particle formed at time r is Vp c=p(tc )> and so the surface area Up of this particle at time t is given by... 

Case B Radicals enter both micelles and polymer particles at rates that are proportional to their surface areas (collision theory), so that the rate of new particle formation is given by... 

Particle formation by radical entry into a micelle... 

As we discussed in Sect. 3.1.1, Hansen et al. [15] made significant improvements to the concept of the radical capture efficiency proposed by Nomura et al. [ 14]. Taking this concept into consideration, they examined the effect of radical desorption on micellar particle formation in emulsion polymerization [ 65 ]. Assuming that radical entry is proportional to the x power of the micelle radius and the polymer particle radius, they proposed the following general expression for the rate of particle formation ... 

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