Emulsion Formation by Nucleation and Growth Mechanisms

A common example to illustrate this sequence of events would be the formation of a foam (bubble in liquid dispersion) on opening a can of lager. Inside the can the system is one-phase, CO2 having been dissolved under applied pressure. When the pressure is released, small gas bubbles form, which will aggregate and coalesce, until sufficient protein, also present in the beer, adsorbs at the growing bubble/ aqueous solution interface to give a stable foam. 

Examples of the two primary routes, physical and chemical, for preparing emulsions by the nucleation and growth route are described respectively in the following two sections. 

One suspects that the reason why it was difficult to form reasonable emulsions at higher volume fractions, say in the 50/50 PEO/PDMS region, by this method is that nucleation and initial growth are too fast for the macrosurfactant to adsorb quickly enough (even at the higher concentrations employed) to prevent substantial coalescence of the growing droplets. 

Clearly this whole area of emulsification by physically crossing phase boundary lines is one which warrants much more research effort. 

---

After the emulsion of the monomer phase in the water phase and the presence of the emulsifier micelles established, the pol3mierization is initiated by the addition of initiator. According to the theories proposed by Harkins and Smith and Ewart, conventional emulsion pol)nnerization mechanism occurs into three intervals including the initial (particle formation or nucleation) stage, the particle growth stage and the completion stage. 

The prediction of the evolution of the PSD in Interval II is simpler than that in the ether intervals and it was for this reason that it was discussed first. Even the qualitative features of particle formation in Interval I are in doubt and the relative importance of homogeneous (ije., oligomeric precipitation) versus heterogeneous (i.e., micellar) nucleation mechanisms are not fully understood. For tbis reason, detailed solutions to Eq. (S) in this Interval, when c is nonzero, appear to be premature. Moreover, in many emulsion polymerizations, the precise details of events occurring in Interval I are masked by the subsequent particle growth in Intervals II and III. 

---