Miniemulsions, definition

When chloro-octadecane was found to give the same result as a so-called cosurfactant, an argument arose in terms of the real role of this highly hydrophobic compound because it is not surface active and has no cooperation with surfactant. Taking account of these systems, the definition of miniemulsion polymerization will be revised to the polymerization in which a water-insoluble compound in the dispersed phase retards or inhibits diffusion degradation of the emulsion. ... 

In Fig. 8 the calorimetric curve of a typical miniemulsion polymerization for 100-nm droplets consisting of styrene as monomer and hexadecane as hydrophobe with initiation from the water phase is shown. Three distinguished intervals can be identified throughout the course of miniemulsion polymerization. According to Harkins definition for emulsion polymerization [59-61], only intervals I and III are found in the miniemulsion process. Additionally, interval IV describes a pronounced gel effect, the occurrence of which depends on the particle size. Similarly to microemulsions and some emulsion polymerization recipes [62], there is no interval II of constant reaction rate. This points to the fact that diffusion of monomer is in no phase of the reaction the rate-determining step. 

Blythe et al. argue (with justification) that polymer should not be termed a costabilizer since it does not cause super-swelhng however, this review will take a functional, rather than thermodynamic definition of a costabilizer, and treat polymer-stabilized miniemulsions under the heading of costabihzers. 

Although it may seem obvious, the definition of miniemulsion polymerizatim can vary depending on the objectives of the specific research programme. In its narrowest sense, miniemulsion polymerization could be defined as the polymerization of all the monomer droplets present in the initial emulsion, where the final particle size distribution is reflected in the initial droplet size distribution (i.e. there is a one-to-one correspondence between the droplets and particles). It quickly became evident that this definition was too narrow to accommodate most (if not all) of the reactions termed miniemulsion polymerization because there was no such correspondence. Typically, fewer particles were found than the original number of monomer droplets. The definition, therefore, could be taken as the polymerization in miniemulsion droplets where not all the droplets succeed in becoming polymer particles. This definition only allows for nucleation in monomer droplets. 

Unfortunately, no such simple solution is available for the ESA effect at the high frequencies at which it is ciu-rently used. Goetz and El-Aasser (24) attempted to compare the electroacoustic and electrophoretic behavior of concentrated miniemulsion systems of toluene in water, stabilized by cetyl alcohol and sodium laiuyl sulfate. They concluded that the simple correction which works well for CVP does not produce a similar reconciliation in the case of the ESA effect. Their conclusions are, however, suspect because of uncertainties arising from the dilution of the emulsion system this can so easily lead to changes in siuface properties, no matter how carefully it is done. Texter s results (34) referred to above are perhaps more definitive in this case. He showed that in the range from 2 to 5% by volume where his particles showed a nonlinear dependence of the ESA signal on volume fraction, the Levine and Neale model (3 8) was unable to account for the nonlinearity. His particles were, however, nonspherical and that may at least partially explain the discrepancy. 

Scientifically described for the first time in 1943 by Hoar and Schulman (2), the latter author coined the term microemulsion in 1959 to describe these optically isotropic transparent oil and water dispersions (3). Since this early work, many experimental and theoretical efforts have shown that these dispersions are actually solutions, namely thermodynamically stable equilibrium phases (4). Consequently, the most widely, but still not universally accepted definition of a microemulsion is that of a thermodynamically stable mixture of oil and water. Occasionally, the term microemulsion (5) or miniemulsion (6) is used to describe long-lived emulsions with ultra-small droplet sizes (30-100 nm). Sometimes, stable emulsions may be created by agitation of systems while passing through regions of the phase diagram where microemulsion phases form however, the final state is in the emulsion region (7, 8). In this present chapter, we use the most widely accepted definition of microemulsions, namely equilibrium phases of oil and water (9). 

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