Inner phase

Although it is hard to draw a sharp distinction, emulsions and foams are somewhat different from systems normally referred to as colloidal. Thus, whereas ordinary cream is an oil-in-water emulsion, the very fine aqueous suspension of oil droplets that results from the condensation of oily steam is essentially colloidal and is called an oil hydrosol. In this case the oil occupies only a small fraction of the volume of the system, and the particles of oil are small enough that their natural sedimentation rate is so slow that even small thermal convection currents suffice to keep them suspended for a cream, on the other hand, as also is the case for foams, the inner phase constitutes a sizable fraction of the total volume, and the system consists of a network of interfaces that are prevented from collapsing or coalescing by virtue of adsorbed films or electrical repulsions. 

An emulsion may be defined as a mixture of particles of one liquid with some second liquid. The two common types of emulsions are oil-in-water (O/W) and water-in-oil (W/0), where the term oil is used to denote the water-insoluble fiuid. These two types are illustrated in Fig. XIV-1, where it is clear that the majority or outer phase is continuous, whereas the minority or inner phase is not. These two emulsion types are distinguished by their ability to disperse oil or water-soluble dyes, their dilution with oil or water, and their conductivity (O/W emulsions have much higher conductivity than do W/0 ones see Ref. 1 for reviews). 

An important aspect of the stabilization of emulsions by adsorbed films is that of the role played by the film in resisting the coalescence of two droplets of inner phase. Such coalescence involves a local mechanical compression at the point of encounter that would be resisted (much as in the approach of two boundary lubricated surfaces discussed in Section XII-7B) and then, if coalescence is to occur, the discharge from the surface region of some of the surfactant material. 

A foam can be considered as a type of emulsion in which the inner phase is a gas, and as with emulsions, it seems necessary to have some surfactant component present to give stability. The resemblance is particularly close in the case of foams consisting of nearly spherical bubbles separated by rather thick liquid films such foams have been given the name kugelschaum by Manegold [175]. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Warmuth R The Inner Phase of Molecular Container Compounds As a Novel Reaction Environment J. Inclusion Phenom. Macrocyclic Chem. 2000 37 1-38 Keywords inciusion reaction, photochemistry, photoinduced eiectron transfer, fuiierenes... 

Sustained release from disperse systems such as emulsions and suspensions can be achieved by the adsorption of appropriate mesogenic molecules at the interface. The drug substance, which forms the inner phase or is included in the dispersed phase, cannot pass the liquid ciystals at the interface easily and thus diffuses slowly into the continuous phase and from there into the organism via the site of application. This sustained drug release is especially pronounced in the case of multilamellar liquid crystals at the interface. 

Porous materials used for chromatography result from a chemically induced phase separation using chain-wise polymerization of vinyl-containing monomers crosslinked with a portion of divinyl functional monomers. Frechet has improved this technique for the preparation of porous PS beads [48]. In this approach the inner phase consists of a mixture containing the reactive styrene and divinylbenzene monomers as well as an unreactive polymeric porogen. After polymerization, the soluble polymeric porogen is removed, leaving behind ma-croporous beads with pore sizes of around 100-500 nm. 

A very recent development is encapsulation of actives in colloidosomes [16, 41]. The method is analogous to liposome entrapment. Selectively permeable capsules are formed by surface-tension-driven deposition of solid colloidal particles onto the surface of an inner phase or active ingredient in a water-in-oil or an oil-in-water emulsion composed of colloidal particles. Initially synthetic polymer microparticles were used but more recently a natural alternative has been described based on small starch particles. After spray-drying, redispersible emulsions can be formed. 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destruct in the bulk phase or transform and react under atmospheric conditions. 

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. 

How can we experimentally determine which is the outer and which the inner phase One possibility is to use electron microscopy which provides detailed images of the emulsion structure. Electron microscopes are relatively expensive and sample preparation requires time and skill. Therefore alternative techniques are often used ... 

An important quantity, which characterizes a macroemulsion, is the volume fraction of the disperse phase 4>a (inner phase volume fraction). Intuitively one would assume that the volume fraction should be significantly below 50%. In reality much higher volume fractions are reached. If the inner phase consists of spherical drops all of the same size, then the maximal volume fraction is that of closed packed spheres (fa = 0.74). It is possible to prepare macroemulsions with even higher volume fractions volume fractions of more than 99% have been achieved. Such emulsions are also called high internal phase emulsions (HIPE). Two effects can occur. First, the droplet size distribution is usually inhomogeneous, so that small drops fill the free volume between large drops (see Fig. 12.9). Second, the drops can deform, so that in the end only a thin film of the continuous phase remains between neighboring droplets. 

The inner phase volume fraction determines many properties of an emulsion. One example is the viscosity r/em. For small volume fractions one can often regard the disperse phase as consisting of rigid, spherical particles instead of liquid, flexible drops. Then we can apply Einstein s3 equation [541], with rj being the viscosity of the pure dispersing agent ... 

Practically, no emulsion is monodisperse and drops of the inner phase have different sizes. To characterize their size distribution, the following log-normal distribution function has proved to be useful ... 

The rationale for introducing emulsions as a DDS for topical ocular application is their ability to incorporate within their oil inner phase lipophilic active molecules, which exhibit low water solubility and cannot be normally administered in an aqueous eyedrop formulation. Thus, the preferred emulsion for topical ocular application should be of the oil-in-water type formulation. 

The advantages derived from the use of microscopic liquid-liquid interfaces have been highlighted in Sect. 5.5.3, and different approaches to support such small liquidlliquid interfaces in pores, pipettes, and capillaries have been addressed. The theoretical treatment of ion transfer through these interfaces needs to consider the asymmetry of the diffusion fields inside and outside the pore or pipette (i.e., diffusion can be approximated as linear in the inner phase, whereas radial diffusion is significant in the outer phase, especially for small sizes) [36, 40, 42-44]. 

Dalgarno, S. J., Tucker, S. A., Bassil, D. B., Atwood, J. L., Fluorescent guest molecules report ordered inner phase of host capsules in solution. Science 2005, 309, 2037-2039. 

Singlet phenylnitrene thermally ring expands in the inner phase of a hemicarcerand to the cyclic ketenimine (54), whose polymerization is prevented by the surrounding host.104 This allowed the activation parameters for the ring contraction of (54) to be measured and for the NMR spectroscopic characterization of (53). 

A last note about the continuous phase is the fact that it must completely immerse the packing section where the mixing of the two phases takes place. The inner phase between the two liquid phases is therefore to be near the extractor column s dispersed-phase outlet. The extract stream, having gained the transferred solute, exits the column at the opposite end from where the raffinate stream exits. The raffinate stream is the inlet feed stream containing the extracted solute. 

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