Colloidal dispersion, definition

Emulsions are colloidal dispersions of liquid droplets in another liquid phase, sometimes stabilized by surface active agents. Emulsions thus consist of a discontinuous phase, dispersed in a continuous phase. The most common types of emulsions are water-in-oil (W/O) in which oil is the continuous phase, and oil-in-water (OAV) in which water forms the continuous phase. However, this traditional definition of an emulsion is too narrow to include most food emulsions. For example, in foods the dispersed phase may be partially solidified, as in dairy products or the continuous phase may contain crystalline material, as in ice cream. It may also be a gel, as in several desserts. In addition to this, air bubbles may have been incorporated to produce the desired texture. 

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). 

Simple colloidal dispersions are two-phase systems, comprising a dispersed phase of small particles, droplets or bubbles, and a dispersion medium (or dispersing phase) surrounding them. Although the classical definition of colloidal species (droplets, bubbles, or particles) specifies sizes of between one nanometre and one micrometre, in dealing with practical applications the upper size limit is frequently extended to tens or even hundreds of micrometres. For example, the principles of colloid science can be usefully applied to emulsions whose droplets exceed the 1 tm size limit by several orders of magnitude. At the other extreme, the field of nano-... 

Although most colloidal dispersions are not thermodynamically stable, a consequence of the small size and large surface area in colloids, and of the presence of an interfacial film on droplets, bubbles or particles, is that dispersions of these species, having reasonable kinetic stability, can be made. That is, suspended droplets or particles may not aggregate quickly nor settle or float out rapidly and droplets in an emulsion or bubbles in a foam may not coalesce quickly. Many food and personal care product emulsions and suspensions, for example, are formulated to remain stable for months to years. It is crucial that stability be understood in terms of a clearly defined process, and one must consider the degree of change and the time-scale in the definition of stability. 

This is termed the a rent miscibility pressure because there is a considerable amount of evidence that asphaltenes, which are colloidally dispersed in most crude oils, will aggregate under appropriate dilution conditions and precipitate, so that the physicochemical definition of miscibility (single-phase for all proportions of the fluids in question) is not realized. There are also conditions, particularly found in oil reservoirs at temperatures below about 135 F, at which two liquid phases, or two liquid phases plus a gas phase, appear in addition to an asphaltene precipitate (11-13). In general, however, this does not prevent the attainment of 95 % oil recovery or more in slim-tube tests at the same pressures at which these multiple phases appear. Hence, the process is deemed "miscible," for all practical purposes. 

Definition and Classification of Emulsions. Colloidal droplets (or particles or bubbles), as they are usually defined, have at least one dimension between about 1 and 1000 nm. Emulsions are a special kind of colloidal dispersion one 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. Emulsions also form a rather special kind of colloidal system in that the droplets often exceed the size limit of 1000 nm. In petroleum emulsions one of the liquids is aqueous, and the other is hydrocarbon and referred to as oil. Two types of emulsion are now readily distinguished in principle, depending upon which kind of liquid forms the continuous phase (Figure 2) ... 

A more modern approach to colloidal dispersions is based on fractal geometry. The fractal approach, as explained later, provides a new basis for the definition and characterization of colloidal systems. 

In Section 1.1, we defined colloidal systems as systems in which particles dispersed in a medium are snbjected to both thermal motion and motion due to external forces (e.g., gravity). This definition leads directly to the notion of stability of a colloidal dispersion. A colloidal dispersion is considered to be stable if no rapid phase separation occnrs through sedimentation (if the density of the particles is higher than that of the medium) or creaming (if the density of the particles is lower than that of the medinm). Thus, colloidal stability refers to the ability of a dispersion to resist aggregation into larger entities that then would segregate from the medium. 

An aerosol is a (colloidal) dispersion of particles in a gas, which for therapeutic aerosols is air. There is no definition for the particle size disttibutimi of an aerosol, but most airborne particles are within the size range between 0.2 and 20 pm. 

Greger considered these ionic polymer systems to be colloids, and indeed they do possess behavior similar to hydrophylic colloids. " Because of the complexity of the strongly acidic, ionic-covalent poly nature of the system, no definitive experiments have been made to determine whether or not the solutions are true solutions or colloidal dispersions. Perhaps light scattering would be one of the simpler approaches, but I am unaware of this technique being applied to the system. 

In network colloids the definition of colloids in terms of dispersed phase and dispersion medium breaks down since the networks consist of interpenetrating continuous channels. Examples include porous solids, where a solid labyrinth contains a continuous gas phase. There are also examples of colloids where three or more phases coexist, two or more of which can be finely divided. These are called multiple colloids. An example is an oilbearing porous rock, since both oil and water will be present within the solid pores. 

The classical definition of an emulsion is the colloidal dispersion of one liquid in another . 

Waterborne resin systems can be divided roughly into two classes - water soluble and water insoluble. A true water soluble resin is a solution of a single polymer molecule solubilised on its own, and completely surrounded by water. The water insoluble types are emulsions, these consist of a particle (or micelle - see Chapter II for full definition) consisting of a number of polymer molecules solubilised by a surface covering of hydrophilic molecules. Consider the continuous reduction in particle size from an emulsion particle to a single molecule. There is a zone of colloidal dispersion where the particle size is so small that it is invisible to the eye and the dispersion is clear. These colloidal dispersions are sometimes referred to as a hydrosol. 

Thomas Graham used the term gel in 1861 for the first time, and nowadays it is a common household item which is easily identified by the simple inversion test if the material is able to support its own weight without falling out when a pot is turned upside down, it is considered as a gel [6, 7]. However, as Dorothy Jordan Lloyd noted in 1926, gels are. . easier to recognize than to define. The generally accepted definition is that given by Flory in 1974, which is the next A gel is a two-component, colloidal dispersion with a continuous structure with macroscopic... 

The traditional view of emulsion stability (1,2) was concerned with systems of two isotropic, Newtonian Hquids of which one is dispersed in the other in the form of spherical droplets. The stabilization of such a system was achieved by adsorbed amphiphiles, which modify interfacial properties and to some extent the colloidal forces across a thin Hquid film, after the hydrodynamic conditions of the latter had been taken into consideration. However, a large number of emulsions, in fact, contain more than two phases. The importance of the third phase was recognized early (3) and the lUPAC definition of an emulsion included a third phase (4). With this relation in mind, this article deals with two-phase emulsions as an introduction. These systems are useful in discussing the details of formation and destabilization, because of their relative simplicity. The subsequent treatment focuses on three-phase emulsions, outlining three special cases. The presence of the third phase is shown in order to monitor the properties of the emulsion in a significant manner. 

With foams, one is dealing with a gaseous state or phase of matter in a highly dispersed condition. There is a definite relationship between the practical application of foams and colloidal chemistry. Bancroft (4) states that adopting the very flexible definition that a phase is colloidal when it is sufficiently finely divided, colloid chemistry is the chemistry of bubbles, drops, grains, filaments, and films, because in each of these cases at least one dimension of the phase is very small. This is not a truly scientific classification because a bubble has a film round it, and a film may be considered as made up of coalescing drops or grains. ... 

An aqueous colloidal polymeric dispersion by definition is a two-phase system comprised of a disperse phase and a dispersion medium. The disperse phase consists of spherical polymer particles, usually with an average diameter of 200-300 nm. According to their method of preparation, aqueous colloidal polymer dispersions can be divided into two categories (true) latices and pseudolatices. True latices are prepared by controlled polymerization of emulsified monomer droplets in aqueous solutions, whereas pseudolatices are prepared starting from already polymerized macromolecules using different emulsification techniques. 

Microemulsions are a convenient medium for preparing microgels in high yields and rather uniform size distribution. The name for these special emulsions was introduced by Schulman et al. [48] for transparent systems containing oil, water and surfactants, although no precise and commonly accepted definitions exist. In general a microemulsion may be considered as a thermodynamically stable colloidal solution in which the disperse phase has diameters between about 5 to lOOnm. 

Graham s definitions were expanded, and the concept of a colloidal state of matter evolved. According to this view, a substance could occur in a colloidal state just as it could occur under various conditions as a gas, liquid, or solid. If a colloidal solution was, at that time, defined as a solution in which the dispersed particles were comprised of large molecules, the ascertion would have been more acceptable. 

The methods of disintegration rely entirely upon increasing the dispersity of a solids which process can, at least theoretically, be stopped at any instant resulting in the formation of a suspension of definite dispersity but one that is not necessarily stable. The processes of suspension formation by methods of condensation on the other hand are more complicated, owing to the fact that unless the resulting colloidal suspension possesses at least some degree of stability the process of condensation once set in operation will not cease but proceed until the transformation to the macrocrystalline structure is complete. 

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