Emulsions hydrophilic component

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

A stable emulsion requires balancing of hydrophobic and hydrophilic components into a uniform suspension. The suspension may be a multicomponent system of emulsifiers, surfactants, stabilizers, and thickeners that are included only to make an aesthetically pleasing product — not for their clinical significance. 

A recurring feature in many process industries is the rag layer, mentioned earlier. A rag layer is a gel-like emulsion that forms and accumulates at the oil/water interface in a separation vessel (Figure 8.1). Rag layers tend to concentrate a range of emulsion-stabilizing components that may be hydrophilic, oleophilic, inorganic,... 

Emulsion Capacity and Stability. The emulsion capacities of the three protein preparations are shown in Table IV. and again both the unheated and microwave treated samples were significantly superior to that of the hot water process. Although it is difficult to relate these data to those of other workers, since a wide variety of conditions are employed for measuring this property, it has generally been found that emulsifying properties are related to the aqueous solubility of proteins (7.) which further bears on the capacity of proteins to lower interfacial tension between hydrophobic and hydrophilic components. 

Within the last 30 years, micro emulsions have also become increasingly significant in industry. Besides their application in the enhanced oil recovery (see Section 10.2 in Chapter 10), they are used in cosmetics and pharmaceuticals (see Chapter 8), washing processes (see Section 10.3 in Chapter 10), chemical reactions (nano-particle synthesis (see Chapter 6)), polymerisations (see Chapter 7) and catalytic reactions (see Chapter 5). In practical applications, micro emulsions are usually multicomponent mixtures for which formulation rules had to be found (see Chapter 3). Salt solutions and other polar solvents or monomers can be used as hydrophilic component. The hydrophobic component, usually referred to as oil, may be an alkane, a triglyceride, a supercritical fluid, a monomer or a mixture thereof. Industrially used amphiphiles include soaps as well as medium-chained alcohols and amphiphilic polymers, respectively, which serve as co-surfactant. 

Proteins adsorbed at an oil-water interface may stabilize the oil droplets by the Derjaguin, Landau, Verwey, and Overbeek (DLVO) interactions and/or the steric stabilization mechanism. The proteins may possess or be capable of adopting extended structures, which protrude into a solution for a considerable distance from the interface. This extended hydrated layer may form the basis for steric stabilization of the emulsion. Interactions between the adsorbed protein layers can involve a reduction in conhgurational entropy as molecular chains overlap (Darling and Birkett, 1987). In addition, hydration of adsorbed hydrophilic components can lead to an enthalpic repulsion when two particles are in close proximity. This tends to force the oil droplets apart (Darling and Birkett, 1987). 

This should be an amphiphilic compound which by itself promotes water-in-oil emulsions and is capable of complexing with the hydrophilic component at the oil-water interface. 

Its concentration should at least be sufficient to form a close-packed mixed monolayer with the hydrophilic component. To promote semi-solid emulsions at room temperature it should be near or above its saturation concentration in the oil. 

At low temperature, nonionic surfactants are water-soluble but at high temperatures the surfactant s solubUity in water is extremely smaU. At some intermediate temperature, the hydrophile—Hpophile balance (HLB) temperature (24) or the phase inversion temperature (PIT) (22), a third isotropic Hquid phase (25), appears between the oil and the water (Fig. 11). The emulsification is done at this temperature and the emulsifier is selected in the foUowing manner. Equal amounts of the oil and the aqueous phases with aU the components of the formulation pre-added are mixed with 4% of the emulsifiers to be tested in a series of samples. For the case of an o/w emulsion, the samples are left thermostated at 55°C to separate. The emulsifiers giving separation into three layers are then used for emulsification in order to find which one gives the most stable emulsion. 

Internal surfactants, i.e., surfactants that are incorporated into the backbone of the polymer, are commonly used in PUD s. These surfactants can be augmented by external surfactants, especially anionic and nonionic surfactants, which are commonly used in emulsion polymerization. Great attention should be paid to the amount and type of surfactant used to stabilize urethane dispersions. Internal or external surfactants for one-component PUD s are usually added at the minimum levels needed to get good stability of the dispersion. Additional amounts beyond this minimum can cause problems with the end use of the PUD adhesive. At best, additional surfactant can cause moisture sensitivity problems with the PUD adhesive, due to the hydrophilic nature of the surfactant. Problems can be caused by excess (or the wrong type of) surfactants in the interphase region of the adhesive, affecting the ability to bond. 

GA is well recognized as emulsifier used in essential oil and flavor industries. Randall et al., 1998, reported that the AGP complex is the main component responsible for GA ability to stabilize emulsions, by the association of the AGP amphiphilic protein component with the surface of oil droplets, while the hydrophilic carbohydrate fraction is oriented toward the aqueous phase, preventing aggregation of the droplets by electrostatic repulsion. However, only 1-2% of the gum is absorbed into the oil-water interface and participates in the emulsification thus, over 12% of GA content is required to stabilize emulsions with 20%... 

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