Multiphase emulsions

The w/o/w or o/w/o emulsions are important, in particular when the protection of sensitive ingredients or controlled release of active substances is required [54]. Silicone-based emulsifiers are suited for these applications. They are strongly adsorbed at the oil interface, and do not migrate from one interface to the other, thus preventing the destabilization. Two emulsifiers have to be used to stabilize these multiphase emulsions. The hydrophilic-lipophilic balance (HLB) values of the emulsifiers should be above 10 for the hydrophilic emulsifier and below 6 for the hydrophobic emulsifier [54]. 

Uses Thickener and gelling agent esp. effective pH 6-8.5 improves texture and feel of formulated skin prods. primary emulsifier for multiphase emulsions of oil and water rec. for controlled-release skin or eye drug formulations Properties Off-wh. to straw-colored powd. odorless swells but does not dissolve in water sp.gr. 0.7-0.8 dec. before melting dec. before flash > 150 C 100% cone. 

Other solubilization and partitioning phenomena are important, both within the context of microemulsions and in the absence of added immiscible solvent. In regular micellar solutions, micelles promote the solubility of many compounds otherwise insoluble in water. The amount of chemical component solubilized in a micellar solution will, typically, be much smaller than can be accommodated in microemulsion fonnation, such as when only a few molecules per micelle are solubilized. Such limited solubilization is nevertheless quite useful. The incoriDoration of minor quantities of pyrene and related optical probes into micelles are a key to the use of fluorescence depolarization in quantifying micellar aggregation numbers and micellar microviscosities [48]. Micellar solubilization makes it possible to measure acid-base or electrochemical properties of compounds otherwise insoluble in aqueous solution. Micellar solubilization facilitates micellar catalysis (see section C2.3.10) and emulsion polymerization (see section C2.3.12). On the other hand, there are untoward effects of micellar solubilization in practical applications of surfactants. Wlren one has a multiphase... 

Citric acid is utilized in a large variety of food and industrial appHcations because of its unique combination of properties. It is used as an acid to adjust pH, a buffer to control or maintain pH, a chelator to form stable complexes with multivalent metal ions, and a dispersing agent to stabilize emulsions and other multiphase systems (see Dispersants). In addition, it has a pleasant, clean, tart taste making it useful in food and beverage products. 

In the late 1940s, the demand for styrene homopolymers (PS) and styrene-acrylonitrile copolymers (SAN) was drastically reduced due to their inherent brittleness. Thus, the interest was shifted to multiphase high-impact polystyrene (HIPS) and rubber-modified SAN (ABS). In principle, both HIPS and ABS can be manufactured by either bulk or emulsion techniques. However, in actual practice, HIPS is made only by the bulk process, whereas ABS is produced by both methods [132,133]. 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

The formation of nitrosamines in aprotic solvents has applicability to many practical lipophilic systems including foods (particularly bacon), cigarette smoke, cosmetics, and some drugs. The very rapid kinetics of nitrosation reactions in lipid solution indicates that the lipid phase of emulsions or analogous multiphase systems can act as "catalyst" to facilitate nitrosation reactions that may be far slower in purely aqueous media (41, 53, 54). This is apparently true in some cosmetic emulsion systems and may have important applicability to nitrosation reactions in vivo, particularly in the GI tract. In these multiphase systems, the pH of the aqueous phase may be poor for nitrosation in aqueous media (e.g., neutral or alkaline pH) because of the very small concentration of HONO or that can exist at these pH ranges. 

The number of the constituent phases of a disperse system can be higher than two. Many commercial multiphase pharmaceutical products cannot be categorized easily and should be classified as complex disperse systems. Examples include various types of multiple emulsions and suspensions in which solid particles are dispersed within an emulsion base. These complexities influence the physicochemical properties of the system, which, in turn, determine the overall characteristics of the dosage forms with which the formulators are concerned. 

Many polymers can be produced via interfacial polymerization. These multiphase reactions occur in emulsions, suspensions, slurries, or at the interface between a gas and a solid. 

The two main assumptions underlying the derivation of Eq. (5) are (1) thermodynamic equilibrium and (2) conditions of constant temperature and pressure. These assumptions, especially assumption number 1, however, are often violated in food systems. Most foods are nonequilibrium systems. The complex nature of food systems (i.e., multicomponent and multiphase) lends itself readily to conditions of nonequilibrium. Many food systems, such as baked products, are not in equilibrium because they experience various physical, chemical, and microbiological changes over time. Other food products, such as butter (a water-in-oil emulsion) and mayonnaise (an oil-in-water emulsion), are produced as nonequilibrium systems, stabilized by the use of emulsifying agents. Some food products violate the assumption of equilibrium because they exhibit hysteresis (the final c/w value is dependent on the path taken, e.g., desorption or adsorption) or delayed crystallization (i.e., lactose crystallization in ice cream and powdered milk). In the case of hysteresis, the final c/w value should be independent of the path taken and should only be dependent on temperature, pressure, and composition (i.e.,... 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

In all these reactors gravity plays an important role. To obtain good contact between phases, we need to overcome the separations that will be driven by gravity whenever the phases have different densities. This requires that the flow conditions and drop, bubble, or particle sizes are properly chosen. It is also important that the phases be separated after the reactor, and mists, emulsions, and dust in separation units can cause major problems in design of these multiphase reactors. Reactor orientation plays an obvious role in any multiphase reaction processes. 

Kobayashi, I., Nakajima, M. (2006). Generation and multiphase flow of emulsions in microchannels. In Kockmann, N. (Ed.). Advanced Micro and Nanosystems 5 Micro Process Engineering, Weinheim Wiley, pp. 149-171. 

Most food products and food preparations are colloids. They are typically multicomponent and multiphase systems consisting of colloidal species of different kinds, shapes, and sizes and different phases. Ice cream, for example, is a combination of emulsions, foams, particles, and gels since it consists of a frozen aqueous phase containing fat droplets, ice crystals, and very small air pockets (microvoids). Salad dressing, special sauce, and the like are complicated emulsions and may contain small surfactant clusters known as micelles (Chapter 8). The dimensions of the particles in these entities usually cover a rather broad spectrum, ranging from nanometers (typical micellar units) to micrometers (emulsion droplets) or millimeters (foams). Food products may also contain macromolecules (such as proteins) and gels formed from other food particles aggregated by adsorbed protein molecules. The texture (how a food feels to touch or in the mouth) depends on the structure of the food. 

Mixing processes involved in the manufacture of disperse systems, whether suspensions or emulsions, are far more problematic than those employed in the blending of low-viscosity miscible liquids, due to the multiphasic character of the... 

Foams and emulsions are other multiphase systems to which research on the application of NMR imaging is on-going. There are a great many practical uses of aqueous foams and emulsions in food, pharmaceuticals, and engineering. Understanding and particularly assigning specific functional roles to individual components in the foam has been severely... 

In fact, extremum tendencies expressing the dominant mechanisms in systems like turbulent pipe flow (Li et al, 1999), gas-liquid-solid flow (Liu et al, 2001), granular flow, emulsions, foam drainages, and multiphase micro-/nanoflows also follow similar scenarios of compromising as in gas-solid and gas-liquid systems (Ge et al., 2007), and therefore, stability conditions established on this basis also lead to reasonable descriptions of the meso-scale structures in these systems. We believe that such an EMMS-based methodology accords with the structure of the problems being solved, and hence realize the similarity of the structures between the physical model and the problems. That is the fundamental reason why the EMMS-based multi-scale CFD improves the... 

A further improvement of the multiphase reactor concept using lipase for enantioselective transformation has been recently reported, that is, an emulsion enzyme membrane reactor. Here, the organic/water interface within the pores at the enzyme level is achieved by stable oil-in-water emulsion, prepared by membrane emulsification. In this way, each pore forms a microreactor containing immobilized... 

The technology also represents a suitable strategy for the preparation of multiphase reaction systems that use phase transfer (bio)catalysts. Giorno et al. [88] reported on the use of membrane emulsification to distribute lipase from Candida rugosa at the interface of stable oil-in-water emulsions. The enzyme itself was used as a surfactant. Shirasu Porous Glassy (SPG) membranes having a nominal pore... 

Emulsion Polymerization in a CSTR. Emulsion polymerization is usually carried out isothermally in batch or continuous stirred tank reactors. Temperature control is much easier than for bulk or solution polymerization because the small (. 5 Jim) polymer particles, which are the locus of reaction, are suspended in a continuous aqueous medium as shown in Figure 5. This complex, multiphase reactor also shows multiple steady states under isothermal conditions. Gerrens and coworkers at BASF seem to be the first to report these phenomena both computationally and experimentally. Figure 6 (taken from ref. (253)) plots the autocatalytic behavior of the reaction rate for styrene polymerization vs. monomer conversion in the reactor. The intersection... 

Anthony Pearson The deviatoric stress is an important feature. I used the term stress there, but when one does these calculations on multiphase mixtures, suspension, emulsions—one is really looking not at the stresses initially, but one tends to be looking at rates of deformation. Although you get no Brownian motion, you do get very considerable structure development. My question is, is there any way in which thermodynamics can deal with structure development in a nonequilibrium state If you stop shearing the material, the structure disappears. 

Although there have been several references on the theory of emulsion polymerization kinetics, it is surprising that its commercial importance and multiphase kinetics have not generated more interest. Smith and Ewart (46) predicted a constant rate of polymerization per particle, based on initiation in the water phase and three ranges of radical concentration per particle. Van der Hoff (55) confirmed this for concentrations... 

In spatially evolving multiphase media (e.g., during dissolution of a porous medium, or phase separation in a polymer blend), the mean curvature of the interface between two phases is of interest. Curvature is a sensitive indicator of morphological transitions such as the transition from spherical to rod-like micelles in an emulsion, or the degree of sintering in a porous ceramic material. Furthermore, important physicochemical parameters such as capillary pressure (from the Young-Laplace equation) are curvature-dependent. The local value of the mean curvature K — (1 /R + 1 /Ri) of an interface of phase i with principal radii of curvature Rx and R2 can be calculated as the divergence of the interface normal vector ,... 

Obviously, one solvent, ideal from many points of view, is water. Nobody cares about trace amounts of water in polymer films that might be used to wrap food, for example. Trace amounts of benzene (a grade A carcinogen) would be unacceptable, however. The problem is, of course, that most polymers (or monomers for that matter) do not dissolve in water. This brings us to the topic of multiphase processes where polymerizations are performed in water with the monomer or polymer suspended in the form of droplets or dispersed in the form of an emulsion. 

There are various types of multiphase processes that are widely used in the mass production of polymers. The two phases can both be liquids, as in suspension and emulsion polymerization, or can be a gas/solid, gas/melt (liquid) or liquid/solid system. In the interfacial polymerization of nylon 6,6, for example, the two monomers are initially dissolved in different solvents, hexameth-... 

The influence of the molecular weight of homopolystyrene POO on the formation of polystyrene occlusions is vividly seen in fig,7, where photographs of miorostructures of HIPS specimens are represented The specimens of HIPS are obtained by the polymerization of model emulsions, prepetred by mixing solutions of rubber - 8% and polystjprene - 30% in ratio 1 1, MW PS varied at 0,7-r-3 10, As seen in fig,7 the rubber particles, formed from multiphase model emulsion as a result of redistribution of monomer, differ greatly from each other, M of polystyrene... 

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