Emulsion external phase

Maybe the first and most important emulsion property to be determined is type, i.e.. either oil-in-watcr (0/W) or waler-in-oil (W/0), and evcmually the occurrence of a multiple emulsion. This is essentially equivalent to detennine which is the emulsion external phase. 

Emulsion viscosity is set to be proportional to its external phase viscosity, and this assumption is obviously correct at low internal phase content, say up to 20-30% and often at higher content as long as Newtonian behavior is exhibited. The second most important factor related to emulsion viscosity is the internal phase content, i.e., the volumetric proportion of the drops. As increasing numbers of drops crowd the emulsion external phase, the interdrop interactions produce increased friction that results in esca-... 

An emulsifying agent generally produces such an emulsion that the liquid in which it is most soluble forms the external phase. Thus the alkali metal soaps and hydrophilic colloids produce O/W emulsions, oil-soluble resins the W/O type (see emulsion). 

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

Fig. 2. Aerosol emulsion droplets containing propellant (a) in the internal phase with subsequent formation of aerosol foam and (b) in the external phase...

Sprays. Aerosol spray emulsions are of the water-in-oil type. The preferred propellant is a hydrocarbon or mixed hydrocarbon—hydrofluorocarbon. About 25 to 30% propellent, miscible with the oil, remains in the external phase of the emulsion. When this system is dispensed, the propellant vaporizes, leaving behind droplets of the w/o emulsion (Fig. 2b). A vapor tap valve, which tends to produce finely dispersed particles, is employed. Because the propellant and the product concentrate tend to separate on standing, products formulated using this system, such as pesticides and room deodorants, must be shaken before use. 

Emulsion—Suspension Polymerized Pigment Ink. Polymerization of a polar prepolymer as the internal phase in an oil-based external phase (24) gives a fluorescent ink base in which spherical fluorescent particles are dispersed. This base is suitable for Htho and letterpress inks (qv). An... 

Conventional cosmetic emulsions (macroemulsions) normally contain about 70% or more of the external phase, which may be a mixture of components. The internal phase is routiaely iatroduced iato the external phase at an elevated temperature with vigorous agitation. The emulsifiers are distributed according to their solubility between the two phases. The level of emulsifiers (rarely more than about 10%) is kept low siace excessive amounts may destabilize emulsions or form a clear solubilizate. Auxiliary emulsifiers and other components are iacluded ia the phases ia which they are soluble. 

Conductivity test Immerse a pair of electrodes connected to an external electric source in the emulsion. If the external phase is water, a current passes through the emulsion. If the oil is the continuous phase, the emulsion fails to carry the current. 

The viscosity of an emulsion can be of crucial importance for its stability, especially the viscosity of the external phase. A high viscosity reduces creaming and also lessens the tendency of particles to coalescence and produce phase separation. Examples of the widely used viscosity-imparting agents are alginates, bentonite, carboxymethylcellulose, polyvinyl pyrrolidone, hydroxypropylcellulose, and carbomer. 

Phase inversion technique the external phase is added to the internal phase. For example, if an O/W emulsion is to be prepared, the aqueous phase is added to the oil phase. First a W/O emulsion is formed. At the inversion point, the... 

Removal of NAPL can present problems, particularly if emulsion is involved. Emulsion is an intimate mixture of two liquids not miscible with each other, as oil and water. Water-in-oil emulsions have water as an internal phase and oil as the external phase, whereas oil-in-water emulsions reverse the order. Oil-water separation is required prior to downstream treatment processes. Several specific oil removal technologies are presented below. 

Under osmotic pressure gradients between the two aqueous phases of W/OAV emulsions, water may migrate either from the internal to the external phase or vice versa, depending on the direction of the osmotic pressure gradient. This process is entropically driven and is another manifestation of compositional ripening. Such... 

In order to solve the mathematical model for the emulsion hquid membrane, the model parameters, i. e., external mass transfer coefficient (Km), effective diffu-sivity (D ff), and rate constant of the forward reaction (kj) can be estimated by well known procedures reported in the Hterature [72 - 74]. The external phase mass transfer coefficient can be calculated by the correlation of Calderback and Moo-Young [72] with reasonable accuracy. The value of the solute diffusivity (Da) required in the correlation can be calculated by the well-known Wilke-Chang correlation [73]. The value of the diffusivity of the complex involved in the procedure can also be estimated by Wilke-Chang correlation [73] and the internal phase mass transfer co-efficient (surfactant resistance) by the method developed by Gu et al. [75]. 

In a dispersed system, it is possible to have both phases in existence at the same time. However, whenever fuels emulsify with water, water-in-oil emulsion typically forms. Agents which comprise the external phase of an emulsion are usually the most soluble in the bulk liquid in which the emulsion exists. 

A third approach is emulsification. Most emulsified commercial products are the oil-in-water type, in which the oil is suspended in the form of small spheres in the water. The oil is the discontinuous or internal phase, and the water is the continuous or external phase. Stabilization of these systems is effected by surface-active compounds that prevent the oil drops from coalescing and by proportioning the two phases so that the lighter phase cannot separate to the top. In applying the emulsion ap-... 

With a given emulsion, the most practical way to determine which is the external phase is to place a few drops of the emulsion on a clean glass plate and then carefully drop a single drop of one of the pure liquids (usually water) upon it. If the emulsion is merely diluted, the external phase and the added drop are the same. If the added drop merely remains an isolated drop upon the emulsion, the external phase and the drop are different liquids. 

Obtain a sample of mixed lead paint and also one of raw linseed oil. To 20 cc. of the mixed paint in a bottle, add 70 cc. of water in 5-cc. lots, shaking vigorously after each addition. This emulsion will contain about 72 per cent of water, and oil is the external phase, as will be shown by a drop test. 

Basically, there are three components in a water-ln-oil emulsion (1) Water, the dispersed or internal phase, (2) oil, the continuous or external phase, (3) the emulsifying agent, which stabilizes the dispersion. 

Different kinds of dispersions can be formed. Most of them have important applications and have special names (Table 1.1). While there are only five types of interface, we can distinguish ten types of disperse system because we have to discriminate between the continuous, dispersing (external) phase and the dispersed (inner) phase. In some cases this distinction is obvious. Nobody will, for instance, mix up fog with a foam although in both cases a liquid and a gas are involved. In other cases the distinction between continuous and inner phase cannot be made because both phases might form connected networks. Some emulsions for instance tend to form a bicontinuous phase, in which both phases form an interwoven network. 

Emulsions are two-phase systems formed from oil and water by the dispersion of one liquid (the internal phase) into the other (the external phase) and stabilized by at least one surfactant. Microemulsion, contrary to submicron emulsion (SME) or nanoemulsion, is a term used for a thermodynamically stable system characterized by a droplet size in the low nanorange (generally less than 30 nm). Microemulsions are also two-phase systems prepared from water, oil, and surfactant, but a cosurfactant is usually needed. These systems are prepared by a spontaneous process of self-emulsification with no input of external energy. Microemulsions are better described by the bicontinuous model consisting of a system in which water and oil are separated by an interfacial layer with significantly increased interface area. Consequently, more surfactant is needed for the preparation of microemulsion (around 10% compared with 0.1% for emulsions). Therefore, the nonionic-surfactants are preferred over the more toxic ionic surfactants. Cosurfactants in microemulsions are required to achieve very low interfacial tensions that allow self-emulsification and thermodynamic stability. Moreover, cosurfactants are essential for lowering the rigidity and the viscosity of the interfacial film and are responsible for the optical transparency of microemulsions [136]. 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

Emulsions are colloidal dispersions in which a liquid is dispersed in a continuous liquid phase of different composition. The dispersed phase is sometimes referred to as the internal (disperse) phase and the continuous phase as the external phase. Practical emulsions may well contain droplets that exceed the classical size range limits given above, sometimes ranging upwards to tens or hundreds of micrometres. In most emulsions, one of the liquids is aqueous while the other is hydrocarbon and referred to as oil. Two types of emulsion are readily distinguished in principle, depending upon which kind of liquid forms the continuous phase (Figure 1.1) ... 

The texture of an emulsion frequently reflects that of the external phase. Thus O/W emulsions usually feel watery or creamy while W/O emulsions feel oily or greasy . This distinction becomes less evident as the emulsion viscosity increases, so that a very viscous O/W emulsion may feel oily. 

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