Dispersion systems immiscible liquid droplets

Emulsions are a class of disperse systems consisting of two immiscible liquids [1-3], whereby the Hquid droplets (the disperse phase) are dispersed in a liquid medium (the continuous phase). Several classes of emulsion may be distinguished, namely Oil-in-Water (O/W), Water-in-Oil (W/O), and OU-in-Oil (0/0). The latter class may be exemplified by an emulsion consisting of a polar oil (e.g., propylene glycol) dispersed in a nonpolar oil (paraffinic oil), and vice versa. In order to disperse two immiscible liquids a third component is needed, namely the emulsifier. The choice of the emulsifier is cmcial in the formation of an emulsion and its long-term stability [1-3]. 

Emulsions are heterogeneous dispersions of immiscible liquids pharmaceutically both types, oil in water (o/w) and water in oil (w/o), are of interest. Mechanical work is required to break up the liquid to be dispersed to small droplets. Thermodynamically emulsions are unstable systems, because the interfacial tension between the two liquids causes droplets of the disperse phase to coalesce, approaching the state of complete phase separation. To counteract this tendency an emulsifying agent must be added that occupies the interfaces between dispersed droplets and bulk liquid thereby lowering the interfacial tension. 

It is known that, in a water phase, immiscible liquids such as gasoline or other petroleum products may form multicomponent droplets of various forms and sizes, under dispersive conditions. These droplets are transported by convection and diffusion, which contributes to the contamination of fresh water systems. However, during droplet transport, more volatile substances partition to the gas phase at the droplet surface, leaving less volatile material that volatilizes more slowly. More volatile material still exists in the droplet interiors, and it tends to diffuse toward the surface because of concentration gradients created by prior volatilization. Different components in a droplet have different volatilization rates, which may vary significantly during droplet transport, and as a result, the contamination of fresh water is affected accordingly. 

An emulsion can be defined as a system in which an immiscible liquid (e.g., citrus oil) is dispersed as droplets in another immiscible liquid (water) by mechanical agitation (2 ). 

Coalescence. The process of coalescence in water-treating systems is more time dependent than dispersion. In dispersion of two immiscible liquids, immediate coalescence seldom occurs when two droplets collide. If the droplet pair is exposed to turbulent pressure fluctuations, and the kinetic energy of oscillations induced in the coalescing droplet pair is larger than the energy of adhesion between them, contact will be broken before coalescence is completed. 

Unlike micelles, an emulsion is a liquid system in which one liquid is dispersed in a second, immiscible liquid, usually in droplets, with emulsiLers added to stabilize the dispersed system. Conventional emulsions possess droplet diameters of more than 200 nm, and are therefore optically opaque or milky. Conventional emulsions are thermodynamically unstable, tending to reduce their total free energy by reducing the total area of the two-phase interface. In contrast, microemulsions with droplet diameters less than 100 nm are optically clear and thermodynamically stable. Unlike conventional emulsions that require the input of a substantial amount of energy, microemulsions are easy to prepare and form spontaneously on mixing, with little or no mechanical energy applied (Lawrence and Rees, 2000). 

Example. An emulsion is a dispersion of one immiscible liquid in another. In most cases one of the liquids is aqueous and the other is in some sense, an oil. Emulsions are another kind of colloidal system in which interfacial properties are very important because emulsified droplets have a large interfacial area. Even a modest interfacial energy per unit area can become a considerable total interfacial energy to be reckoned with. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

Although the dominant mixing mechanism of an immiscible liquid polymeric system appears to be stretching the dispersed phase into filament and then form droplets by filament breakup, individual small droplet may also break up at Ca 3> Ca. A detailed review of this mechanism is given by Janssen (34). The deformation of a spherical liquid droplet in a homogeneous flow held of another liquid was studied in the classic work of G. I. Taylor (35), who showed that for simple shear flow, a case in which interfacial tension dominates, the drop would deform into a spheroid with its major axis at an angle of 45° to the how, whereas for the viscosity-dominated case, it would deform into a spheroid with its major axis approaching the direction of how (36). Taylor expressed the deformation D as follows... 

Emulsions are a class of disperse systems consisting of two immiscible liquids, one constituting the droplets (the disperse phase) and the second the dispersion medium. The most common class of emulsions is those whereby the droplets constitute the oil phase and the medium is an aqueous solution (referred to as O/W emulsions) or where the droplets constitute the disperse phase, with the oil being the continuous phase (W/O emulsions). To disperse a liquid into another immiscible liquid requires a third component, referred to as the emulsifier, which in most cases is a surfactant. Several types of emulsifiers may be used to prepare the system, ranging from anionic, cationic, zwitterionic, and nonioinic surfactants to more specialized emulsifiers of the polymeric type, referred to as polymeric... 

Upon mixing two immiscible liquids, one of the two liquids (i.e., the dispersed phase) is subdivided into smaller droplets. The surface area and the interfacial free energy increase, and the system is then thermodynamically unstable. Without continuous mixing, the droplets will be stabilized throughout the dispersion medium by dissolving the surface-active agent. There are several theories for the stabilization of emulsions but a single theory cannot account for the stabilization of all emulsions. 

By careful selection of the composition of a two immiscible liquid system, one can reduce the surface tension to near zero. In such conditions, with gentle agitation, a very fine dispersion (under micrometer size droplets) may be formed. This is a very stable system for an emulsion. In some conditions the size dispersion may be limited. 

Indirectly related to the cell models of this section is the work of Davis and Brenner (1981) on the rheological and shear stability properties of three-phase systems, which consist of an emulsion formed from two immiscible liquid phases (one, a discrete phase wholly dispersed in the other continuous phase) together with a third, solid, particulate phase dispersed within the interior of the discontinuous liquid phase. An elementary analysis of droplet breakup modes that arise during the shear of such three-phase systems reveals that the destabilizing presence of the solid particles may allow the technological production of smaller size emulsion droplets than could otherwise be produced (at the same shear rate). 

Emulsion A dosage form consisting of a two-phase system comprised of at least two immiscible liquids, one of which is dispersed as droplets (internal or dispersed phase) within the other liquid (external or continuous phase), generally stabilized with one or more emulsifying agents. (Note Emulsion is used as a dosage form term unless a more specific term is applicable, e.g. cream, lotion, ointment.). 

According to Beoher, an emulsion is a heterogeneous system oonsisting of two immiscible liquids one of whioh (the dispersed phase) is intimately dispersed in the other (the continuous phase) in the form of small droplets whose diameters generally exoeed 0.1 pm [28], Although emulsions are heterogeneous systems, — they oonsist of two immiscible phases — emulsification is intended to produce a homogeneous system in terms of chemical structure. 

EMULSION IS USUALLY DEFINED as a system consisting of a liquid dispersed with or without an emulsifier in an immiscible liquid, usually in droplets of larger than colloidal sizes. In petroleum emulsions, solids play an extremely important role in both the formation and stability of emulsions. These solids can be oil-phase components such as wax crystals or precipitated asphaltenes, or mineral components that are partially oleophilic, a property that allows them to act as stabilizers between the oil and water phases. 

An emulsion is a system consisting of a liquid dispersed as droplets in a second immiscible liquid, often stabilized by an emulsifying agent. In the oil field, the two basic types of emulsions are water-in-oil and oil-in-water oil-in-water emulsions are often termed reverse emulsions. More than 95% of the crude-oil emulsions formed in the oil field are of the water-in-oil type. Nonetheless, oil-in-water emulsions are receiving growing interest in pollution abatement as they are readily miscible with water. 

Macroemulsions have been known for thousands of years. The survey of ancient literature reveals that the emulsification of beeswax was first recorded in the second century by the Greek physician, Galen (1). Macroemulsions are mixtures of two immiscible liquids, one of them being dispersed in the form of fine droplets with diameter greater than 0.1 ym in the other liquid. Such systems are turbid, milky in color and thermodynamically unstable (i.e. the macroemulsion will ultimately separate into two original immiscible liquids with time). Since the early 1890s, extensive and careful studies have been carried out on macroemulsions and several excellent books have been written on various aspects of formation and stability of these systems (2,10). In addition, several theories and methods of macroemulsion formation have been discussed in the recent articles (13 ... 

This ambiguity in the microemulsion terminology remains today (41). The microemulsions are defined as the clear thermodynamically stable dispersions of two immiscible liquids containing appropriate amounts of surfactants or surfactants and cosurfactants. The dispersed phase consists of small droplets with diameter in the range of 100-1000A0. Because of these properties, such systems have several advantages over macroemulsions for industrial applications. 

This process involves extraction of fine particles from an aqueous phase into an oil phase. The effectiveness of this technique, as shown in Figure 2, is based on the stability of emulsion droplets with solid particles. If a particle is partially wetted by two immiscible liquids the particle will concentrate at the liquid-liquid interface. The thermodynamic criteria for distribution of solids at the interface of two immiscible liquids is the lowering in the interfacial free energy of the system when particles come in contact with two immiscible liquids. (12) If ygw, yWQ and ygp are the interfacial tensions of solid-water, water-oil and solid-oil interfaces respectively, and if ygQ > y + ygw then the solid particles are preferentially dispersed within the water phase. However, if ygw > ywq + ygQ, the solid is dispersed within the oil phase. On the other hand, if yWQ > ygQ + ysw, or if none of the three interfacial tensions is greater than the sum of the other two, the solids in such case will be distributed at the oil-water interface. 

Dispersion in liquid/liquid L/L) systems is associated with the enlargement of the interface area between two immiscible liquids, so that e.g. an extraction process or a chemical reaction (saponification, nitration, etc.) can proceed rapidly or dispersions of particular droplet size are produced (bead and suspension polymerizations, etc.). In this chapter only dispersion by stirrers is considered. If this process is assisted by the addition of surface-active substances, it is termed emulsification, for which completely different laws generally apply, see e.g. [201]. 

These are dispersions of liquid drops in an immiscible liquid medium. The most common systems are oil-in-water (O/W) and water-in-oil (W/O). It is also possible to disperse a polar liquid into an immiscible nonpolar liquid, and vice versa these are referred to as oil-in-oil (0/0) emulsions. In order to disperse a liquid into another immiscible liquid, a third component is needed that is referred to as the emulsifier. Emulsifiers are surface-active molecules (surfactants) that adsorb at the liquid/liquid interface, thus lowering the interfacial tension and hence the energy required for emulsification is reduced. The emulsifier plays several other roles (i) it prevents coalescence during emulsification (ii) it enhances the deformation and break-up of the drops into smaller units (iii) it prevents flocculation of the emulsion by providing a repulsive barrier that prevents close approach of the droplets to prevent van der Waals attraction (iv) it reduces or prevents Ostwald ripening (disproportionation) (v) it prevents coalescence of the drops and (vi) it prevents phase inversion. 

Emulsions are defined as disperse multi-phase systems of immiscible liquids. In simplest terms, they are either oil-in-water (O/W) or water-in-oil (W/O), where the dispersed phase is listed first and exists as discrete droplets within the continuous phase. In some cases, multiple emulsions can be formed, such as water-in-oil-in-water (W/O/W). Both natural and manufactured emulsions have been studied and utilized extensively, especially within the past 200 years, and Lissant has given an historical overview of the subject [106]. 

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