Unstable immiscibility

A wide variety of capsules loaded with water-immiscible or water-iasoluble materials have been prepared by complex coacervation. Capsule size typically ranges from 20—1000 p.m, but capsules outside this range can be prepared. Core contents usually are 80—95 wt %. Complex coacervation processes are adversely affected by active agents that have finite water solubiUty, are surface-active, or are unstable at pH values of 4.0—5.0. The shell of dry complex coacervate capsules is sensitive to variations ia atmospheric moisture content and becomes plasticized at elevated humidities. 

The factors determining the appearance of ordered cell-like motions were first investigated by Sternling and Scriven (S33) who considered the two-dimensional stability of a plane interface separating two immiscible semi-infinite fluid phases with mass transfer occurring between the phases. This system was shown to be unstable for mass transfer in one direction, but stable for transfer in the opposite direction. For an interfacial tension-lowering solute, instability... 

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). 

An emulsion can be deLned as a mixture of two immiscible phases (namely, water and oil) with an emulsiLer added to stabilize the dispersed droplets (Davis et al., 1987). As conventionally deLned, emulsions will have droplet diameters of more than 100 nm (up tprfijft and thus are opaque or milky in appearance. In addition, they are thermodynamically unstable by nature, that is, on standing they will eventually separate into two phases. However, proper choice of emulsiLer (generally 1-5%) and preparation conditions can delay this separation and thus lead to nominal shelf lives of more than 2 years, as typically required for pharmaceutical products. An emulsion can be characterized as oil-in-water (o/w) (containing up to 40% oil) or water-in-oil (w/o), depending on the identity of the dispersed and continuous phases. Multiple (e.g., w/o/w) emulsions can also be prepared, but these are less widely used in pharmaceutical applications. 

In practice, emulsification requires even more energy than that calculated in the above example, since deformation of large drops is needed before smaller drops can pinch off (Figure 3.4). In laminar flow this deformation is produced by viscous forces exerted by the surrounding liquid. If a stream of liquid is injected with low turbulence into another, immiscible liquid the cylindrical column of liquid will eventually become unstable and break up into droplets (Figure 3.4a). Under high turbulence the breakdown occurs sooner and produces smaller droplets (Figure... 

Incompatible commingling molecules separate soon after the commingling stimulus is withdrawn such systems have short lifetimes and are therefore said to be unstable. For longer life, surface active compounds (surfactants), efficacious in small quantities, are added to decrease the contact angle between the immiscible surfaces, lower a, and permit the interfusion of the immiscible surfaces. 

Time-temperature superposition works for homopolymers and miscible blends but not for immiscible blends, filled systems (e. g., glass fiber reinforced plastics) or reactive or unstable polymers. 

Emulsions made by agitation of pure immiscible liquids are usually very unstable and break within a short time. Therefore, a surfactant, mostly termed emulsifier, is necessary for stabilisation. Emulsifiers reduce the interfacial tension and, hence, the total free energy of the interface between two immiscible phases. Furthermore, they initiate a steric or an electrostatic repulsion between the droplets and, thus, prevent coalescence. So-called macroemulsions are in general opaque and have a drop size > 400 nm. In specific cases, two immiscible liquids form transparent systems with submicroscopic droplets, and these are termed microemulsions. Generally speaking a microemulsion is formed when a micellar solution is in contact with hydrocarbon or another oil which is spontaneously solubilised. Then the micelles transform into microemulsion droplets which are thermodynamically stable and their typical size lies in the range of 5-50 nm. Furthermore bicontinuous microemulsions are also known and, sometimes, blue-white emulsions with an intermediate drop size are named miniemulsions. In certain cases they can have a quite uniform drop size distribution and only a small content of surfactant. An interesting application of this emulsion type is the encapsulation of active substances after a polymerisation step [25, 26]. 

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. 

The phenomenon of droplet breakup is of great importance in the preparation of emulsions. If a stream of liquid is injected with little turbulence into another liquid with which it is immiscible, the cylinder that may form is unstable, breaks down in several spots, and breaks up into droplets (Figure 2a). If the injection rate is such as to produce turbulence, the disruption is faster, and many smaller droplets are produced (Figure 2b). If in addition the liquid impinges against a surface, many smaller droplets will be formed. 

An innovative technique involving the preparation of a lower-viscosity unstable slurry-emulsion system by mixing water with the oil as a means of conveying crude oil has yet to be proven on a commercial scale. The technical problems and issues limiting the application of this technique (analogous to crude-oil emulsion pipelining) are to sustain the two immiscible liquids in a stable emulsion during transport and to destabilize the emulsion... 

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 ... 

Pure water/oil-emulsions are unstable. For this reason, surface active agents (surfactants) are added, which adsorb at the interface between the two immiscible liquids and decreasing the interfacial tension. In this way, the stability of a water droplet in oil will be increased. In addition, there is a second stabilizing effect by steric hindrance which will be explained later (4). 

Properties Colorless, heavy liquid unstable hygroscopic. The commercial grade is amixture of the two isomers, a and (3, of which a is in a greater proportion. D 1.326 (18C), bp 213C (decomposes), wt/gal 11.1021b, fp -40C, viscosity 2.388 cP (20C). Soluble in water, alcohol, and ether immiscible with oils. Nonflammable. 

Properties Colorless liquid. Fp-26.3C, bp 120C (1 mm Hg), bp 155C (5 mm Hg), d 1.0405 (30C), viscosity 8.00 cP (30C), refr index 1.4392 (25C), flash p 180F (82.2C) (TOC) soluble in water. It is thermally unstable, yielding acrylonitrile and water at above 175C. Hydrolyzed by strong acids and bases, quite immiscible with paraffin hydrocarbons, but dissolves aromatics. Combustible. Derivation From acrylonitrile. 

An emulsion is defined as a dispersion of two immiscible liquids, one of which is finely subdivided and uniformly distributed as droplets (the dispersed phase) throughout the other (the continuous phase). A third component (or multiple additional components), the emulsifying agent(s), is necessary to help stabilize the emulsion. The emulsifying agent(s) coats the droplets and prevents droplet coalescence by either reducing the interfacial tension or by creating a physical repulsion between the droplets. The dispersed phase is occasionally also defined as the internal phase the continuous phase is occasionally also defined as the external phase or dispersion medium. Virtually all emulsions are inherently physically unstable. 

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