Emulsion Droplet Coalescence Method

Assessment of the stability of an emulsion against coalescence involves droplet counting218. The most unequivocal method (but one which is rather laborious) is to introduce a suitably diluted sample of the emulsion into a haemocytometer cell and count the microscopically visible particles manually. 

To apply the phase inversion principle, the transitional inversion method should be used, as demonstrated by Shinoda and coworkers [11, 12] when using nonionic surfactants of the ethoxylate type. These surfactants are highly dependent on temperature, becoming lipophilic with increasing temperature due to dehydration of the poly(ethylene oxide) (PEO) chain. When an O/W emulsion that has been prepared using a nonionic surfactant of the ethoxylate type is heated, at a critical temperature - the PIT - the emulsion will invert to a W/O emulsion. At the PIT, the droplet size reaches a minimum and the interfacial tension also reaches a minimum, but the small droplets are unstable and coalesce very rapidly. Rapid cooling of an emulsion that has been prepared close to the PIT results in very stable and small emulsion droplets. 

This is perhaps the most sensitive method for predicting coalescence. G is a measure of the contact points of the emulsion droplets, as well as their strength. Provided that no flocculation occurs (which would cause an increase in G ), any reduction in G on storage indicates the presence of coalescence. 

Microcapsules containing polymer and pigment were prepared in [299] by dispersing a viscous suspension of pigment and oil-soluble shell monomer forming o/w emulsions. Subsequently, a water-soluble shell monomer was added to the emulsion droplets, encapsulating them via interfacial polycondensation. These microcapsules were then heated for free radical polymerisation of the core monomers. It has been shown that polyvinyl alcohol (PVOH) used as stabiliser reacts with the oil-soluble shell monomers. The decrease of PVOH concentration as result of this interaction leads to coalescence of the particles and to the increase of their equilibrium particle size, however, methods are proposed to prevent the depletion of PVOH. 

An emulsion has been defined above as a thermodynamically unstable heterogeneous system of two immiscible liquids where one is dispersed in the other. There are two principal possibilities for preparing emulsions the destruction of a larger volume into smaller sub-units (comminution method) or the construction of emulsion droplets from smaller units (condensation method). Both methods are of technical importance for the preparation of emulsions for polymerization processes and will be discussed in more detail below. To impart a certain degree of kinetic stability to emulsions, different additives are employed which have to fulfil special demands in the particular applications. The most important class of such additives, which are also called emulsifying agents, are surface-active and hence influence the interfacial properties. In particular, they have to counteract the rapid coalescence of the droplets caused by the van der Waals attraction forces. In the polymerization sense, these additives can be roughly subdivided into surfactants for emulsion polymerization, polymers for suspension and dispersion polymerization, finely dispersed insoluble particles (also for suspension polymerization), and combinations thereof (cf. below). 

The determination of droplet-size distributions in water-in-oil or oil-in-water emulsions by pulsed-gradient diffusion measurements is now a routine industrial method for monitoring the quality of food emulsions such as mayonnaise and salad creams. The method was first described in a classic paper by Packer and Rees (see Further reading section) and subsequently developed into a rapid analytical method. The theoretical model assumes spherical droplets and a log-normal size distribution, but other distributions have been considered. The MRI generalization of the method to spatially dependent droplet-size distributions in emulsions undergoing coalescence and/or phase separation has yet to be implemented, although a spatially dependent study of emulsion crystallization has been reported. 

From most of the studies described above, it can be noted that the multiple emulsions are proposed mainly for sustained release of anticancer drugs but not for drug targeting. One obvious reason is their large size, which is dependent on the size of internal aqueous phase droplets. If it was possible to reduce the size of the internal aqueous droplets to nanometers without coalescence, it may be possible to make multiple emulsions that are very small in size. A method for obtaining small-sized multiple emulsion with a solidified oil phase is reported by Morel et al. (1994) to avoid internal droplet coalescence and migration into external phase. 

The conditions for obtaining O/W nano-emulsions with a minimum droplet size and consequently low polydispersity by phase inversion emulsification methods (PIT and PIC) can be summarized as follows A bicontinuous microemulsion or a lamellar liquid crystalline phase (D or L , respectively), with all the oil dissolved, must be formed immediately before reaching the final two-phase region where the nano-emulsions form. These are composition conditions necessary but not sufficient, because the kinetics of incorporation of oil to these phases or the coalescence of droplets can make the nano-emulsion droplet size also dependent on preparation variables such as mixing rate and aqueous phase addition rate for the PIC method, or cooling rate for the PIT method. 

Studies of flow-induced coalescence are possible with the methods described here. Effects of flow conditions and emulsion properties, such as shear rate, initial droplet size, viscosity and type of surfactant can be investigated in detail. Recently developed, fast (3-10 s) [82, 83] PFG NMR methods of measuring droplet size distributions have provided nearly real-time droplet distribution curves during evolving flows such as emulsification [83], Studies of other destabilization mechanisms in emulsions such as creaming and flocculation can also be performed. 

---