Macroemulsions preparation

An important quantity, which characterizes a macroemulsion, is the volume fraction of the disperse phase 4>a (inner phase volume fraction). Intuitively one would assume that the volume fraction should be significantly below 50%. In reality much higher volume fractions are reached. If the inner phase consists of spherical drops all of the same size, then the maximal volume fraction is that of closed packed spheres (fa = 0.74). It is possible to prepare macroemulsions with even higher volume fractions volume fractions of more than 99% have been achieved. Such emulsions are also called high internal phase emulsions (HIPE). Two effects can occur. First, the droplet size distribution is usually inhomogeneous, so that small drops fill the free volume between large drops (see Fig. 12.9). Second, the drops can deform, so that in the end only a thin film of the continuous phase remains between neighboring droplets. 

Macroemulsions are in a nonequilibrium state and their properties depend not only on parameters such as temperature and composition but also on the method of preparation. This leads to a high level of complexity when it comes to scientific experimental studies or practical applications. Recipes which work at one place often do not lead to the same result in another laboratory, because seemingly insignificant details have a big effect. How oil and water are mixed if, for instance, they are shaken or swirled or if air is bubbled through, and the wetting behavior of the vessel, can change the outcome dramatically. 

Freshly prepared macroemulsions change their properties with time. The time scale can vary from seconds (then it might not even be appropriate to talk about an emulsion) to many years. To understand the evolution of emulsions we have to take different effects into account. First, any reduction of the surface tension reduces the driving force of coalescence and stabilizes emulsions. Second, repulsive interfacial film and interdroplet forces can prevent droplet coalescence and delay demulsification. Here, all those forces discussed in Section 6.5.3 are relevant. Third, dynamic effects such as the diffusion of surfactants into and out of the interface can have a drastic effect. 

Reimers and Schork [94, 95] report the use of PMMA to stabihze MM A miniemulsions enough to effect predominant droplet nucleation. Emulsions stabilized against diffusional degradation by incorporating a polymeric costabilizer were produced and polymerized. The presence of large numbers of small droplets shifted the nucleation mechanism from micellar or homogeneous nucleation, to droplet nucleation. Droplet diameters were in the miniemulsion range and reasonably narrowly distributed. On-hne conductance measurements were used to confirm predominant droplet nucleation. The observed reaction rates were dependent on the amount of polymeric costabilizer present. The latexes prepared with polymeric costabilizer had lower polydispersities (1.006) than either latexes prepared from macroemulsions (1.049) or from alkane-stabilized miniemulsions (1.037). 

The monomer miniemulsions with PMMA as costabihzer were prepared with different amounts of alkyd resin. The PMMA costabilizer was effective in the preparation of stable miniemulsions, especially in conjunction with the alkyd. The size of monomer droplets was below 300 nm. After five days, the unpolymerized macroemulsions with alkyd separated into three phases, monomer on the top, clear water in the middle, and alkyd resin on the bottom. The miniemulsion without alkyd showed two phases, monomer and water. All miniemulsions with alkyd resin appear to remain uniform. Very stable miniemulsions were obtained when the alkyd content was higher than 30%. The shelf life of macroemulsions was only 2-8 minutes. 

Microemulsions are fluid, transparent, thermodynamically stable oil and water systems, stabilized by a surfactant usually in conjunction with a cosurfactant that may be a short-chain alcohol, amine, or other weakly amphiphilic molecule. An interesting characteristic of microemulsions is that the diameter of the droplets is in the range of 100-1000 A, whereas the diameter of droplets in a kinetically stable macroemulsion is 5000 A. The small droplet size allows the microemulsion to act as carriers for drugs that are poorly soluble in water. The suggested method of preparation of microemulsions is as follows the surfactant, oil, and water are mixed to form a milky emulsion and titrated with a fourth component, the cosurfactant,... 

It should be remembered that for microemulsions the ratio of emulsifier to oil is much higher than that used for macroemulsions. This emulsifier used is at least 10% based on the oil, and in most cases it can be as high as 20-30%. The W/O systems are prepared by blending the oil and emulsifier, with some heating if necessary. Water is then added to the oil-emulsifier blend to produce the microemulsion droplets, at which point the resulting system should appear transparent or translucent. If the maximum amount of water that can be microemulsified is not high enough for the particular application, other emulsifiers should be tried in order to reach the required composition. 

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. 

In addition to the aforementioned differences, their methods of preparation also differ distinctly. Dining preparation of macroemulsions, a large input of energy is required, whereas preparation of microemulsions does not require energy. In contrast to ordinary emulsions, microemulsions form upon simple mixing of the components and do not require the high shear conditions generally used in the formation of ordinary emulsions. ... 

Emulsified systems can be classified aceording to their thermodynamie stability and their droplets size. Macroemulsions (or simply emulsions) are metastable systems, i.e., the system is not in thermodynamic equilibrium, and it will breakdown into two distinct phases if suffieient time is allowed. However, emulsions that keep their kinetic stability for periods of months or years ean be prepared by using appropriate components and amounts (McClements et al., 2007). This is the most common type of emulsion, and it is found in many food systems such as milk and salad dressing. Macroemulsions are usually polydisperse, with droplet sizes in the range of 1-100 pm. The main destabilization mechanisms in macroemulsions are droplets creaming, flocculation, and coalescence. 

Two macroemulsion.s. one 0/W and another W/O. were tested, together with other classical fomiulaiions, as vehicles for 4-bipheny[acetic acid, a nonsteroidal anti inflammatory agent (13). According to the authors, the 0/W emulsion, prepared with Polawax, was better tolerated than the W/O emulsion (containing Dehymuls E. a mixture of high molecular weight esters) and displayed slgniticani antiinflammatory activity in a rabbit model of ocular inflammation. 

There is some disagreement within the surfactant literature as to the exact definition of solubilization, particularly as the ratio of surfactant to additive decreases, and one approaches the nebulous frontier between swollen micellar systems and the micro- and macroemulsion regions. For present purposes, solubilization will be defined as the preparation of a thermodynamically stable, isotropic solution of a substance (the additive ) normally insoluble or only slightly soluble in a given solvent by the addition of one or more amphiphilic compounds at or above their critical micelle concentration. By the use of such a definition, a broad area can be covered that includes both dilute and concentrated surfactant solutions, aqueous and nonaqueous solvents, all classes of surfactants and additives, and the effects of complex interactions such as mixed micelle formation and hydrotropes. It does not, however, limit the phenomenon to any single mechanism of action. 

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