Bimolecular reactions, droplet phase

As discussed in Section 1.2.7, the formation of the final charged droplets that lead to gas-phase ions is associated with a very large loss of solvent by evaporation. This leads to a large increase in the concentration of the solutes present in the electrosprayed solution. Sucdi a large increase in concentration will promote bimolecular reactions, and these will have an effect on the observed mass spectrum. We will consider two examples. In the first example bimolecular reactions are shown to take place on native proteins, indicating various levels of protein surface gas-phase chemistry. In the second example, a comparison of biochemical and physical organic applications of ESMS to the determination of equilibrium constants indicates the limitations on such measurements imposed by the electrospray mechanism. 

Interval III Particle Growth in the Absence of Monomer Droplets.—James and Sundberg have published the results of an experimental study of ideal and non-ideal behaviour in the seeded emulsion polymerization of styrene. Unlike the experiments on seeded emulsion polymerization reported in papers referred to above, the amounts of monomer added to the seed latices were less than those required to saturate the particles and form a separate monomer droplet phase. The reaction systems were therefore the seed analogues of Interval III of a conventional emulsion polymerization reaction. The results are found to be in good agreement with the predictions of the Stockmayer-O Toole theory, provided that allowance is made for the effect of monomer/polymer ratio at the reaction locus upon the rate coefficient for bimolecular mutual termination. A paper by Hamielec and Marten is concerned with the effects of chain entanglements and the rubber-glass transition... 

This problem was first treated in detail by Haward (1949). He considered the case of a bulk polymerization that has been compartmentalized by subdividing the reaction system into a large number of separate droplets, each of volume v. Radicals are generated exclusively within the droplets and always in pairs. An example would be the polymerizatiim of styrene in emulsified droplets dispersed in water initiated the thermal decomposition of an oil-soluble initiator which partitions almost exclusively within the monomer droplets. In the model considered by Haward, radicals are unable to exit from the droplets into the external phase. The only radical-loss process is in fact bimolecular mutual termination. It therefore follows that all the droplets must always contain an even number (including zero) of propagating radicals, and that the state of radical occupancy will change in increments of 2. The conclusion reached by Haward is that in this case the effect of compartmentalization is to reduce the overall rate of polymerization per unit volume of disperse phase. The f ysical reason for this is that, as the volume of the droplets is reduced, so are the opportunities for a radical to escape from the others—and hence to avoid mutual... 

Strictly speaking, any model based on the time-independent thermodynamics cannot be used to adequately predict the concentration of monomer in latex particles during Smith-Ewart Interval II. This is because the free radical polymerization of monomer in the discrete latex particles is governed by the simultaneous kinetic events such as the generation of free radicals in the continuous aqueous phase, the absorption of free radicals by the particles, the propagation of free radicals with monomer molecules in the particles, the bimolecular termination of free radicals in the particles, and the desorption of free radicals out of the particles. The equilibrium (or saturation) concentration of monomer in the growing latex particles may not be achieved if the rate of consumption of monomer in the major reaction loci is much faster than that of diffusion of monomer molecules from the monomer droplets to the reaction loci. Therefore, the equilibrium concentration of monomer in the latex particles represents an upper limit that is ultimately attainable in the course of polymerization. Nevertheless, the general... 

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