Micellar emulsion, definition

An emulsion polymerization requires the mechanism of polymer particle nudeation to reside outside the monomer droplets. This physical-chemical process involves a series of radical reactions in the continuous phase followed by homogeneous or micellar particle formation. Either of these mechanisms require the initiator to be insoluble in the monomer phase, such as a water soluble initiator and an organically soluble monomer, or vice versa. If, in contrast, the initiator is soluble in the monomer phase, all the components of the reaction are contained in the dispersed phase and the continuous phase serves only to decrease the viscosity and dissipate heat Such polymerizations are categorized as suspensions. The second definition, however, makes no statement as to the magnitude of n and therefore the two criteria are mutually exclusive. 

The term microemulsion was initially introduced in 1959 by Schulman who suggested the following definition A microemulsion is formed on addition of an aliphatic alcohol (co-surfactant) to an ordinary emulsion. Generally, micro-emulsions are defined as thermodynamically stable homogeneous mixtures of oil and water stabilised by surfactants and, in some cases, by co-surfactants. Thus, a water in oil microemulsion, i.e. a reversed micellar solution, is a transparent, isotropic and thermodynamically stable fluid in which nanometer-sized water droplets are dispersed in a continuous oil phase. 

Scattering techniques provide the most definite proof of micellar aggregation. Zielinski et aL (34) employed SANS to study the droplet structures in these systems. Conductivity measurements (35) and SANS (36) were also used to study droplet interactions at high volume fraction in w/c microemulsions formed with a PFPE-COO NH4 surfactant (MW = 672). Scattering data were successfully fitted by Schultz distribution of polydisperse spheres (see footnote 37). A range of PFPE-COO NH/ surfactants were also shown to form w/c emulsions consisting of equal amount of CO2 and brine (38-40). 

In the case of small Brownian deformable droplets the interaction energy, when the continuous phase is a micellar surfactant solution of sodium nonylphenol polyoxy-ethylene-25 (SNP-25S), is illustrated in Fig. 8 as a function of the thickness, A, and film radius, R (see Fig. 3 for the definition of the geometry). The parameters of the micro-emulsion system are R = 2 im, d = 9.8 nm, (j> = 0.38, Ah = 5x10" J, <7= 7.5 mN/m, % = -135 mV, r = 1.91 nm, the electrolyte concentration 25 mM. The points on the contour plot (Fig. 8) correspond to tree local minima of -406 AbT, -140 k T and -37 k T corresponding to film containing 0, 1 and 2 micellar layers, respectively (Ivanov et al. 1999). These three possible films are thermodynamically stable and they act like barriers against the closer approach and flocculation (or coalescence) of the droplets in emulsions. 

The expression single phase means that all observable properties are independent from the macroscopic locus of the sampling point. By using this definition, micellar solutions, microemulsions, lyotropic liquid crystals or colloids covered by surfactants are single-phase fluids, while emulsions, i.e. microphase... 

A distinction has to be made between kinetically and thermodynamically stable emulsions. Mixtures of water, an oil, a surfactant, and a cosurfactant in proper proportions can spontaneously form transparent, themiodynamically stable emulsions, called microemulsions. The definition is not quite unequivocal. Thermodynamically unstable but kinetically stable systems which form spontaneously have also been called microemulsions [117]. Micellar solutions have been included in the definition of microemulsions and the debate on microemulsion structure is ongoing. 

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