Emulsions continuous phase

Figure 3.6 Effect of Ca2+ content on predicted values of osmotic pressure (H, , left axis) of caseinate nanoparticles in emulsion continuous phase and the free energy of the depletion interaction (AGdep, , right axis) between a pair of emulsion droplets (

Bubbles/voids move violently, rendering it difficult to distinguish the emulsion (continuous) phase from the bubble/void (discrete) phase in the bed. 

Antioxidants in emulsion (20 ppm) Added emulsifier (%) Olive oil emulsion Continous phase Salmon oil emulsion Continuous phase... 

Differences in emulsion viscosities are principally due to the different viscosities of the emulsion continuous phase (oil phases). Although for some emulsions clear phase separation was visible to the naked eye, viscosity values revealed no significant changes during storage, even at higher temperature, due to the dominance of the continuous phase viscosity on the bulk viscosity. 

Fig. 8. Emulsion morphology diagram, illustrating where the microemulsion in various macroemulsion morphologies is a continuous phase or dispersed phase. Morphology boundaries (—), aqueous, continuous (--------------), oleic, continuous (--), microemulsion, continuous.

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

If a linear mbber is used as a feedstock for the mass process (85), the mbber becomes insoluble in the mixture of monomers and SAN polymer which is formed in the reactors, and discrete mbber particles are formed. This is referred to as phase inversion since the continuous phase shifts from mbber to SAN. Grafting of some of the SAN onto the mbber particles occurs as in the emulsion process. Typically, the mass-produced mbber particles are larger (0.5 to 5 llm) than those of emulsion-based ABS (0.1 to 1 llm) and contain much larger internal occlusions of SAN polymer. The reaction recipe can include polymerization initiators, chain-transfer agents, and other additives. Diluents are sometimes used to reduce the viscosity of the monomer and polymer mixture to faciUtate processing at high conversion. The product from the reactor system is devolatilized to remove the unreacted monomers and is then pelletized. Equipment used for devolatilization includes single- and twin-screw extmders, and flash and thin film evaporators. Unreacted monomers are recovered for recycle to the reactors to improve the process yield. 

An emulsion system in which the propellant is in the external or continuous phase is shown in Figure 2b. As the Hquefied propellant vaporizes, it escapes direcdy into the atmosphere, leaving behind droplets of the formulation which are emitted as a wet spray. This system is typical of many water-based aerosols or w/o emulsions. 

Emulsifiers are incorporated in oil and synthetic mud formulations to maintain a stable emulsion of the internal brine phase. These materials include calcium and magnesium soaps of fatty acids and polyamines and amides and their mixtures (123,127). The specific chemistry of these additives depends on the nature of the continuous phase of the mud, ie, whether diesel oil, mineral oil, or a synthetic Hquid. Lime is added along with the fatty acid to form the... 

Emulsion Polymerization. When the U.S. supply of natural mbber from the Far East was cut off in World War II, the emulsion polymerization process was developed to produce synthetic mbber. In this complex process, the organic monomer is emulsified with soap in an aqueous continuous phase. Because of the much smaller (<0.1 jira) dispersed particles than in suspension polymerization and the stabilizing action of the soap, a proper emulsion is stable, so agitation is not as critical. In classical emulsion polymerization, a water-soluble initiator is used. This, together with the small particle size, gives rise to very different kinetics (6,21—23). 

Viscosity Increase. The flocculation rate of an emulsion is iaversely proportional to the viscosity of the continuous phase and an iacrease of the viscosity from 1 mPa-s (=cP) (water at room temperature) to a value of 10 Pa-s (100 P) (waxy Hquid) reduces the flocculation rate by a factor of 10,000. Such a change would give a half-life of an unprotected emulsion of a few hours, which is of Httle practical use. 

To prepare stable emulsions ia this way gelation of the continuous medium is necessary. The appearance of a Hquid emulsion may be retained by choosing a polymer for the continuous phase, giving a thixotropic solution with short breakdown and buildup times. The polymers used for this purpose are natural gums (qv) or synthetic polymers. Clay particles also act as viscosity enhancers. The members of the bentonite family derived from... 

Two kinds of barriers are important for two-phase emulsions the electric double layer and steric repulsion from adsorbed polymers. An ionic surfactant adsorbed at the interface of an oil droplet in water orients the polar group toward the water. The counterions of the surfactant form a diffuse cloud reaching out into the continuous phase, the electric double layer. When the counterions start overlapping at the approach of two droplets, a repulsion force is experienced. The repulsion from the electric double layer is famous because it played a decisive role in the theory for colloidal stabiUty that is called DLVO, after its originators Derjaguin, Landau, Vervey, and Overbeek (14,15). The theory provided substantial progress in the understanding of colloidal stabihty, and its treatment dominated the colloid science Hterature for several decades. 

The theory has certain practical limitations. It is useful for o/w (od-in-water) emulsions but for w/o (water-in-oil) systems DLVO theory must be appHed with extreme caution (16). The essential use of the DLVO theory for emulsion technology Hes in its abdity to relate the stabdity of an o/w emulsion to the salt content of the continuous phase. In brief, the theory says that electric double-layer repulsion will stabdize an emulsion, when the electrolyte concentration in the continuous phase is less than a certain value. 

The relative value of the two potentials reveals the destabdization action of salts added to the emulsion. Addition of an electrolyte to the continuous phase causes a reduction of the electric double-layer repulsion potential, whereas the van der Waals potential remains essentially unchanged. Hence, the reduced electric double-layer potential causes a corresponding reduction of the maximum in the total potential, and at a certain concentration of electrolyte the maximum barrier height is reduced to a level at which the stabdity is lost. 

The use of the nomograph is as follows Find the intersecting point of the curves of continuous phase and dispersed phase viscosities on the binary field (left side of nomograph). A line is drawn from this point to the common scale volume fraction of dispersed phase and continuous phase liquids. The intersection of this line with the Viscosity of Emulsion scale gives the result. 

Example. The viscosity of the continuous phase liquid is 20. The viscosity of the dispersed phase liquid is 30. The volume fraction of the dispersed phase liquid is 0.3. The nomograph shows the emulsion viscosity to be 36.2. 

Following is a typical procedure for the emulsion polymerization of hydrophobic monomers in batch fashion. The first step in this process is the preparation of the continuous phase solution. As mentioned before, water is usually selected as the base material of the continuous phase for the emulsion polymerization of hydrophobic... 

Soapless seeded emulsion copolymerization has been proposed as an alternative method for the preparation of uniform copolymer microspheres in the submicron-size range [115-117]. In this process, a small part of the total monomer-comonomer mixture is added into the water phase to start the copolymerization with a lower monomer phase-water ratio relative to the conventional direct process to prevent the coagulation and monodispersity defects. The functional comonomer concentration in the monomer-comonomer mixture is also kept below 10% (by mole). The water phase including the initiator is kept at the polymerization temperature during and after the addition of initial monomer mixture. The nucleation takes place by the precipitation of copolymer macromolecules, and initially formed copolymer nuclei collide and form larger particles. After particle formation with the initial lower organic phase-water ratio, an oligomer initiated in the continuous phase is... 

Emulsions. Emulsions are formed when one liquid is dispersed as small droplets in another liquid with which the dispersed liquid is immiscible. Mutually immiscible fluids, such as water and oil, can be emulsified by stirring. The suspending liquid is called the continuous phase, and the droplets are called the dispersed (or discontinuous) phase. There are two types of emulsions used in drilling fluids oil-in-water emulsions that have water as the continuous phase and oil as the dispersed phase, and water-in-oil emulsions that have oil as the continuous phase and water as the dispersed phase (invert emulsions). 

Oil-Base Muds. Oil-base muds contain oil as the continuous phase and water as the dispersed phase. Oil-base muds contain less than 5% (by volume) water, while oil-base emulsion muds (invert emulsions) have more than 5% water in mud. Oil-base muds are usually a mixture of diesel fuel and asphalt the filtrate is oil. 

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