Macroemulsion system

An investigation of the copolymer composition demonstrated the important effect of monomer transport on the copolymerization. The droplets in the macroemulsion act as monomer reservoirs. In this system, the effect of monomer transport will be predominant when an extremely water-insoluble comonomer, such as DOM, is used. In contrast with the macroemulsion system, the miniemulsion system tends to follow the integrated Mayo Lewis equation more closely, indicating less influence from mass transfer. 

The small droplet size in microemulsions also leads to a large surface-to-volume ratio in an oil-water system. This is important for chemical reactions in which the rate of reaction depends on the interfacial area. The microemulsion can also be classified as W/O or 0/W similar to macroemulsion systems. 

Table II. Effect of Chain Length Compatibility on Flow Through Porous Media Behavior of Foams and Macroemulsion Systems...

Demonstrations are lacking of the benefits that can be obtained from using nanoemulsions when compared to classic macroemulsion systems. 

The preparation of sulphated zirconia designed for catalyst supports was studied by Boutonnet et al. . Zirconia prepared in microemulsion showed a pure tetragonal structure compared with zirconia prepared by an impregnation -precipitation procedure which also contained monoclinic phase. Platinum-promoted sulphated zirconia catalysts were prepared both in anionic and non-ionic microemulsions. Furthermore, the catalytic activity and selectivity for the isomerization of hexanes were tested. The catalysts produced by the microemulsion method showed a higher selectivity towards isomers but a lower activity when compared to catalysts prepared by impregnation technique. More recently, a study of zirconia synthesis from micro and macroemulsion systems has been conducted . Spherical ZrOa particles ranging from tens of nanometers to a few micrometers were produced. 

Lobo and co-workers (11) investigated the influence of oil (in the micellar environment) on the stability of foam. Two different types of emulsified oil systems were studied, i.e. (a) a microemulsion (solubilized within the micelle), and (b) a macroemulsion system. It was found that in each case, the foam stability was affected by a completely different mechanism. In the case of (a), where the foam films containing oil is solubilized within the micelle to form a microemulsion, the normal micellar interactions are changed. It had been earlier demonstrated that micellar structuring causes a step-wise thinning due to layer-by-layer expulsion of the micelles and such as effect was found to inhibit drainage and thus increase the foam stability. 

Summary of Significant Shortcomings. The emulsion liquid membranes studied each possess advantages when compared against one another. In specific, the microemulsion systems displayed separation kinetics which were typically an order of magnitude faster than the macroemulsion counterparts. However, tlm increased rate of separation was at the expense of product recovery. The concentrated mercury was easily recovered by electrostatic demulsification for the macroemulsion system, but required the addition of butanol to the microemulsion before mercury recovery was achieved. The requirement of chemical demulsification is a major disadvantage of the microemulsion-based liquid membrane system. 

Microemulsions in selected compositional areas of water-oil-surfactant systems are thermodynamically stable, and isotropic dispersions of nanosized droplets of one liquid in another immiscible liquid (W/O or O/W) are obtained. The water phase, similarly to the case of macroemulsions, could be water in which an alkoxide is to be added, as well as a sol or an alkoxide solution. The difference between microemulsion and macroemulsion systems consists of the amount of surfactants used (10-40% in the former case and 1-2% in the latter case). 

In 1959, J. H. Schulman introduced the term microemulsion for transparent-solutions of a model four-component system [126]. Basically, microemulsions consist of water, an oily component, surfactant, and co-surfactant. A three phase diagram illustrating the area of existence of microemulsions is presented in Fig. 6 [24]. The phase equilibria, structures, applications, and chemical reactions of microemulsion have been reviewed by Sjoblom et al. [127]. In contrast to macroemulsions, microemulsions are optically transparent, isotropic, and thermodynamically stable [128, 129]. Microemulsions have been subject of various... 

In contrast to the conventional emulsions or macroemulsions described earlier are the disperse systems currently termeraiicroemulsions. The term was Lrst introduced by Schulman in 1959 to describe a visually transparent or translucent thermodynamically stable system, with much smaller droplet diameter (6-80 nm) than conventional emulsions. In addition to the aqueous phase, oily phase, and surfactant, they have a high proportion of a cosurfactant, such as an alkanol of 4-8 carbons or a nonionic surfactant. Whereas microemulsions have found applications in oral use (as described in the next chapter), parenteral use of microemulsions has been less common owing to toxicity concerns (e.g., hemolysis) arising from the high surfactant and cosolvent levels. In one example, microemulsions composed of PEG/ethanol/water/medium-chain triglycerides/Solutol HS15/soy phosphatidylcholine have been safely infused into rats at up to 0.5 mL/kg. On dilution into water, the microemulsion forms a o/w emulsion of 60-190 nm droplet size (Man Corswant et al., 1998). 

An emulsion is a dispersed system in which the phases are immiscible or partially miscible liquids. The globules of the dispersed liquid in the usual type of emulsion (sometimes now called a macroemulsion) are usually between 0.1 fim and 10 fim in diameter, and so tend to be larger than the particles found in sols. 

Several generic kinds of results are pertinent to the properties of dispersions. The surfactant solutions formulated to stabilize microemulsions, and some kinds of macroemulsions, can exhibit marked dynamic interfacial tension behaviour. Figure 3.14 shows an example in which a series of commerical surfactant additions are made to a system containing crude oil and a base. Under alkaline conditions the interfacial tension is already dynamic due to the saponification of natural surfac-... 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

To synthesize water-soluble or swellable copolymers, inverse heterophase polymerization processes are of special interest. The inverse macroemulsion polymerization is only reported for the copolymerization of two hydrophilic monomers. Hernandez-Barajas and Hunkeler [62] investigated the copolymerization of AAm with quaternary ammonium cationic monomers in the presence of block copoly-meric surfactants by batch and semi-batch inverse emulsion copolymerization. Glukhikh et al. [63] reported the copolymerization of AAm and methacrylic acid using an inverse emulsion system. Amphiphilic copolymers from inverse systems are also successfully obtained in microemulsion polymerization. For example, Vaskova et al. [64-66] copolymerized the hydrophilic AAm with more hydrophobic methyl methacrylate (MMA) or styrene in a water-in-oil microemulsion initiated by radical initiators with different solubilities in water. However, not only copolymer, but also homopolymer was formed. The total conversion of MMA was rather limited (<10%) and the composition of the copolymer was almost independent of the comonomer ratio. This was probably due to a constant molar ratio of the monomers in the water phase or at the interface as the possible locus of polymerization. Also, in the case of styrene copolymerizing with AAm, the molar fraction of AAm in homopolymer compared to copolymer is about 45-55 wt% [67], which is still too high for a meaningful technical application. 

If the surfactant concentration in a macroemulsion is greatly increased, or if the monomer concentration is greatly reduced, a microemulsion results. Microemulsions are thermodynamically stable systems in which all of the monomer resides within the micelles. At high surfactant concentration, the micelles may form a bicontinuous network, rather than discrete micelles. Polymerization (with water- or oil-soluble initiator) of the monomer within a microemulsion is referred to as microemulsion polymerization. The particles produced in this way are extremely small, ranging from 10 to 100 nm. 

Interval III begins when aU monomer droplets have vanished and/or the aqueous phase becomes unsaturated. Since each droplet in a macroemulsion actually absorbs radicals, they cannot disappear but rather shrink to a point where they have no excess monomer. The monomer in the aqueous phase decreases corresponding to the decrease in the particles. The conversion at which Interval III begins varies for different monomers and systems,but is typically around 40 to 50%. However, it may not be as distinguishable in miniemulsions due to early initiation of the gel effect. 

The problem with applying correlations derived from other systems to emulsion polymerization is twofold. First, normal macroemulsion particles are said to be created with 30 to 40% monomer in them and so the unbiased (zero conversion) termination rate is unknown. Secondly, the diffusional limitations in particles might be quite different from those observed in bulk or suspension polymerizations. It is for these reasons that an empirical approach is suggested. 

Even though two droplets are always able to find an equilibrium when put together, because of the presence of the costabilizer, it is useful to check whether this equilibrium is stable or not. Going back to the case of the macroemulsion depicted in Fig. 6, point A is an unstable point because, if the system is perturbed and a new droplet is formed, it will diverge from the equilibrium point. Clearly, the necessarily condition to have a stable equilibrium is that the slope of the chemical potential versus radius is positive at the point of equilibrium [ 122,124]. For a macroemulsion, this condition leads to the following expression ... 

As the water solubility of the comonomer decreases, the difference in incorporation of the hydrophobic monomer between the mini- and macroemulsion polymerization becomes more pronounced. This was seen in the copolymerization of VH/MMA. The fraction of the hexanoate in the copolymer formed in the miniemulsion polymerization was substantially higher than that found with the macroemulsion. This incorporation closely follows the copolymer equation. The VEH/MMA miniemulsion copolymerization also followed the copolymer equation. Differences between the mini- and macroemulsion polymerization are not as pronounced in this system. For the VD/MMA and VS/MMA systems there were large differences between the two copolymerizations. In addition, none of the mini- or macroemulsion copolymerizations of vinyl decanoate or vinyl stearate are predicted by the copolymer equation. The miniemulsion copolymerizations fall above the prediction curve (more hydrophobic monomer incorporation than predicted), and the macroemulsions fall below. In these cases, both micellar and droplet nucleation took place in the miniemulsion polymerizations, and the presence of micelles tended to enrich the concentration of the hydrophobic monomer in the droplets, since the micelles would likely be richer in the more water-soluble MMA. 

Wu and Schork [152] compared batch and semibatch and mini- and macroemulsion polymerization for three monomer systems, VAc/BA, VAc/dioctyl maleate (DOM) and VAc/n-methylol acrylamide (NMA), with large differences in reactivity ratios and water solubilities. HD was used as the costabilizer. (It should be noted that DOM could function as a costabilizer itself, but for the sake of consistency, HD was added to the DOM polymerizations.) KPS and the... 

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