Emulsion type kinetic theory

J.T. Davies, A quantitative kinetic theory of emulsion type, I, Physical chemistry of the emulsifying agent, in J.H. Schulman (Ed.), Proceedings of the 2nd International Congress of Surface Activity, Butterworths, London, 1957, pp. 426-438. 

In the derivation of the kinetic relations it was assumed that free radicals enter the particles one by one the initiation process just described satisfies this condition. This is not the case when radicals are formed by thermal decomposition of an oil-soluble initiator. Such decomposition produces pairs of radicals in the hydrocarbon phase. One would expect a pair of radicals, confined to the extremely small volume of a latex particle, to recombine rapidly. The kinetics of this type of polymerization have been described above. It is recalled here that the subdivision factor, z, and hence rate and degree of polymerization are smaller than 1 and decrease with a. These predictions from kinetic theory are in contradiction to experimental observations. Although some oil-soluble initiators, which are good catalysts in solution systems, are poor initiators in emulsion polymerizations—e.g., benzoyl peroxide—other thermally decomposing peroxides and azo compounds produce polymer in emulsion at rates comparable to those observed in polymerization initiated by water-soluble catalysts, where the radicals enter the particles one by one. Such is the case for cumene hydroperoxide, which at low concentrations yields a rate of polymerization per particle equal to that of a persulfate-initiated reaction. It must therefore be concluded that, although oil-soluble initiators may decompose into radical pairs within the particles, polymer radicals are formed one by one. The following mechanisms are consistent with formation of polymer radicals singly. 

Davies JT. A quantitative kinetic theory of emulsion type. II. Hydrodynamic factors. Proc 3rd Int Congr Surf Activity 1960 2 585-594. 

However, the kinetics of PVC emulsion does not foUow the above theory. The rate shows the same increasing behavior with conversion as mass polymerization (94,95). [N depends on [3], but the relationship varies with the emulsifier type (96,97). However, the rate is nearly independent of [N (95). The average number of radicals per particle is low, 0.0005 to 0.1 (95). The high solubiUty of vinyl chloride in water, 0.6 wt %, accounts for a strong deviation from tme emulsion behavior. Also, PVC s insolubiUty in its own monomer accounts for such behavior as a rate dependence on conversion. 

A second theory considers the relative ease with which the two types of droplets can coalesce. Upon shaking, drops of both phases are formed. Sodium stearate ionizes, and the electrical potential hinders approach and coalescence of oil droplets water droplets, on the other hand, experience no such hindrance and readily touch and coalesce. Zinc distearate, being un-ionized, does not interfere with the mutual approach of oil droplets, whereas van der Waal s forces favor subsequent coalescence. Thus the type of emulsion formed depends on the relative kinetics of oil-oil and water-water coalescence. 

We have not studied all types of colloidal systems in detail but limited ourselves to suspensions, siufac-tants, emulsions and foams. In terms of properties, the stability and associated concepts (double layer, van der Waals forces, steric effects) as well as the DLVO theory have been presented in detail, while kinetic and especially the optical properties have been discussed more briefly. 

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