Dispersion medium, 477 table

Another classification scheme is based on the size of the dispersed particles within the dispersion medium (Table 2). The particles of the dispersed phase may vary considerably in size, from large particles visible to the naked eye, down to particles in the colloidal size range, and particles of atomic... 

A suspension of tiny particles in some medium is called a colloidal dispersion, or a colloid. The suspended particles can be single, large molecules or aggregates of molecules or ions ranging in size from 1 to 1000 nanometers. Colloids are classified according to the states of the dispersed phase and the dispersing medium. Table 17.7 summarizes various types of colloids. 

This theory is supported by comparison of the contents of the dispersion medium or the maltenes with the conversion during the DSC experiments. The ratio of the conversion in 1 bar argon to the eoncentration of dispersion medium (Table 4-44) has a value weU above one for the vacuum residue and bitumens, showing a mean 3c=1.14( V = 7.24 %). The... 

Nomenclature. Colloidal systems necessarily consist of at least two phases, the coUoid and the continuous medium or environment in which it resides, and their properties gready depend on the composition and stmcture of each phase. Therefore, it is useful to classify coUoids according to their states of subdivision and agglomeration, and with respect to the dispersing medium. The possible classifications of colloidal systems are given in Table 2. The variety of systems represented in this table underscores the idea that the problems associated with coUoids are usuaUy interdisciplinary in nature and that a broad scientific base is required to understand them completely. 

Particles whose dimensions are between 1 nanometer and 1 micrometer, called colloids, are larger than the t3/pical molecule but smaller than can be seen under an optical microscope. When a colloid is mixed with a second substance, the colloid can become uniformly spread out, or dispersed, throughout the dispersing medium. Such a dispersion is a colloidal suspension that has properties intermediate between those of a true solution and those of a heterogeneous mixture. As Table 12-3 demonstrates, colloidal suspensions can involve nearly any combination of the three phases of matter. Gas-gas mixtures are the exception, because any gas mixes uniformly with any other gas to form a true solution. 

Disperse systems can be classified in various ways. Classification based on the physical state of the two constituent phases is presented in Table 1. The dispersed phase and the dispersion medium can be either solids, liquids, or gases. Pharmaceutically most important are suspensions, emulsions, and aerosols. (Suspensions and emulsions are described in detail in Secs. IV and V pharmaceutical aerosols are treated in Chapter 14.) A suspension is a solid/liquid dispersion, e.g., a solid drug that is dispersed within a liquid that is a poor solvent for the drug. An emulsion is a li-quid/liquid dispersion in which the two phases are either completely immiscible or saturated with each other. In the case of aerosols, either a liquid (e.g., drug solution) or a solid (e.g., fine drug particles) is dispersed within a gaseous phase. There is no disperse system in which both phases are gases. 

Table 1 Classification Scheme of Disperse Systems on the Basis of the Physical State of the Dispersed Phase and the Dispersion Medium... 

Despite this, the expected heat transfer coefficients obtainable in a fluidized bed are greater than those for forced convection in a gas (Ditchev and Richardson, 1999) although not as high as in the dynamic dispersion medium (DDM) method described by these authors. Comparative data are presented in Table 3.3. 

The physical picture of emulsion polymerization is based on the original qualitative picture of Harkins [1947] and the quantitative treatment of Smith and Ewart [1948] with subsequent contributions by other workers [Blackley, 1975 Casey et al., 1990 Gao and Penlidis, 2002 Gardon, 1977 Gilbert, 1995, 2003 Hawkett et al., 1977 Piirma, 1982 Poehlein, 1986 Ugelstad and Hansen, 1976]. Table 4-1 shows a typical recipe for an emulsion polymerization [Vandenberg and Hulse, 1948]. This formulation, one of the early ones employed for the production of styrene-1,3-butadiene rubber (trade name GR-S), is typical of all emulsion polymerization systems. The main components are the monomer(s), dispersing medium, emulsifier, and water-soluble initiator. The dispersing medium is the liquid, usually water,... 

A colloid is also called a colloidal dispersion. A colloid dispersion consists of two components similar to a solution. The particles themselves are the dispersed phase and are analogous to the solute in a solution. The dispersing medium is similar to the solvent. Some examples of different types of colloids are summarized in Table 11.5. 

In a colloidal system the particles are dispersed in a medium the dispersed particles form the dispersed phase and they are dispersed in a dispersion medium. Both the dispersed phase and the dispersion medium can be solid, liquid or gaseous, with the exception of the gas - gas combination. For a number of these colloidal system specific names are used (table 5.1). This table illustrates what was mentioned earlier, namely that the literal meaning of the word colloidal only applies to few colloidal systems. 

The particles in a colloidal dispersion are sufficiently large for definite surfaces of separation to exist between the particles and the medium in which they are dispersed. Simple colloidal dispersions are, therefore, two-phase systems. The phases are distinguished by the terms dispersed phase (for the phase forming the particles) and dispersion medium (for the medium in which the particles are distributed) - see Table 1.1. The physical nature of a dispersion depends, of course, on the respective roles of the constituent phases for example, an oil-in-water (O/W) emulsion and a water-in-oil (W/O) emulsion could have almost the same overall composition, but their physical properties would be notably different (see Chapter 10). 

Colloidal suspensions can be classified in a number of ways. Most obvious is by the phase of the homogeneous dispersing medium and that of the dispersed particles, as shown in Table 2. 

Colloidal solutions may be divided roughly into two main groups, designated as lyophobic (Greek solvent hating) and lyophilic (Greek solvent loving) when water is the dispersion medium, the terms hydrophobic and hydrophilic are employed. The chief properties of each class are summarized in Table 1.14, but it must be emphasized that the distinction is not an absolute one since some, particularly sols of metallic hydroxides, exhibit intermediate properties. 

The time that is required to produce a homogeneous suspension of particles in the dispersion medium using an oscillatory shaking machine equipped with several containers is measured. Not only can small quantities of the millbase with the same composition be tested (as with other types of apparatus), but various millbases can also be tested under the same conditions. A low-viscosity alkyd resin system of the stoving or oxidatively drying type can be used as a test medium. For standards, see Table 1.1 ( Ease of dispersion Oscillatory shaking machine ). 

Until recently, only three chlorofluorocarbon (CFC) propellants, namely CFCs 11, 12 and 114 (Table 1), had been approved worldwide for use in medical MDIs. Their widespread acceptance was due to their ability to substantially meet the ideal propellant properties. All the CFC MDIs that are currently marketed employ CFC 12 as the major constituent mixed with either CFC 11 or with a mixture of CFC 11 and CFC 114. These mixtures of propellants closely obey Raoult s law and therefore the blend selected can be used to give a defined vapor pressure (Table 1). The inclusion of CFC 11 in the formulation also offered advantages in that it increased the solvency of most propellant systems, thereby facilitating the dissolution of surfactants in suspension formulations. By virtue of it being a liquid below 24° C, it was used as the primary dispersion medium for either suspending or dissolving the drug. 

Colloids are categorized according to the phases of their dispersed particles and dispersing mediums. Milk is a colloidal emulsion because liquid particles are dispersed in a liquid medium. Other types of colloids are described in Table 15-6 and shown in Figure 15-20. How many of them are familiar to you Can you name others ... 

What are colloids Referring to Table 12.4, why is there no colloid in which both the dispersed phase and the dispersing medium are gases ... 

In a thermoplastic acrylic automotive coating, a CAB is used as the pigment dispersion medium and at 15% concentration based on polymer weight provides better sprayability, increased cold crack resistance, better pigment control, better solvent release, and improved exterior durability. (See Table IX.)... 

Based on the aggregate states of the dispersed phase and the dispersion medium one can recognize different kinds of disperse systems, which can be described by the abbreviation of two letters, the first of which characterizes the aggregate state of the dispersed phase, and the second one that of the dispersion medium. In these notations gaseous, liquid and solid states are labeled as G, L, and S, respectively. In the case of two phase systems, one can outline eight different types of disperse systems, as shown in the table below. 

Virtually any polymer that is appropriately soluble in the dispersion medium is effective as a stabilizing moiety and any polymer that is insoluble in the dispersion medium is effective as an anchor polymer. Some typical stabilizing moieties and anchor polymers for aqueous and nonaqueous dispersion media are shown in Table 2.3. 

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