Dispersed systems continuous medium

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. 

The nucleation mechanism of dispersion polymerization of low molecular weight monomers in the presence of classical stabilizers was investigated in detail by several groups [2,6,7]. It was, for example, reported that the particle size increased with increasing amount of water in the continuous phase (water/eth-anol), the final latex radius in their dispersion system being inversely proportional to the solubility parameter of the medium [8]. In contrast, Paine et al.[7] reported that the final particle diameter showed a maximum when Hansen polarity and the hydrogen-bonding term in the solubility parameter were close to those of steric stabilizer. 

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... 

Let us consider that the microemulsion contains spherical globules of uniform size. Their dispersion in the continuous medium is accompanied by an increase in the entropy of the system. As noted in Sec. I, the Helmholtz free energyfper unit volume of microemulsion (j=F/V, where V is the volume of the microemulsion) will be written as the sum between a frozen noninteracting free energy fa and a free energy A f due to the entropy of dispersion of the globules in the continuous phase and to their interactions ... 

Microemulsions are thermodynamically stable dispersions of oil and water stabilized by a surfactant and, in many cases, also a cosurfactant.1-4 The microemulsions can be of the droplet type, either with spherical oil droplets dispersed in a continuous medium of water (oil-in-water microemulsions, O/W) or with spherical water droplets dispersed in a continuous medium of oil (water-in-oil microemulsions, W/O). The droplet-type microemulsions can be either a single-phase system or part of a two-phase system wherein the microemulsion phase coexists with an excess dispersed phase (an upper phase of excess oil in the case of O/W and a lower phase of excess water in the case of W/O microemulsions). There are also nondroplet-type microemulsions, referred to as middle-phase microemulsions. In this case, the microemulsion phase is part of a three-phase system with the microemulsion phase in the middle coexisting with an upper phase of excess oil and a lower phase of excess water. One possible structure of this middle-phase microemulsion, characterized by randomly distributed oil and water microdomains and bicontinuity in both oil and water domains, is known as thebiccntinuous microemulsion. Numerous experimental studies have shown1 2 4 that one can achieve a transition... 

As seen clearly from Equation (4.81), the settling rate will be zero when the density difference between the particles and the dispersing medium is zero (i.e., po - p = 0). This method can only be applied to systems with smaller density differences because of the limitation to increase the density of the continuous medium by dissolving some inert simple molecules (e.g., sugar and water-miscible solvents) in Newtonian fluids. In addition, even if the matched density is obtained at one temperature, it cannot be maintained at other temperatures. 

Figure 8-41 Effect of Increasing the Concentration of the Disperse Phase on the Flow Behavior of a Disperse System. 1—continuous phase, 2— low solids content, 3—medium solids content, 4—high solids content.

Dispersions of gas in solids are also called foams but the foam cells (bubbles) formed are isolated from one another. An example of such foams are the natural porous materials, cellular concrete, cellular glass and polymer foams. However, if in such disperse systems both phases are continuous (such as in many foamed polymers), they are called sponges. Many porous materials are partially sponge and partially solid foam. The properties of solid foams differ drastically from those of foams with liquid dispersion medium. At the same time the strength and other physical and mechanical characteristics of solid foams depend significantly... 

Although the production of highly deflocculated suspensions is a primary objective for formulation of suspension concentrates, these systems tend to settle under gravity forming dilatant sediments (clays). The latter must be prevented either by controlled flocculation or by the addition of a second disperse phase to the continuous medium (1). One method which may be applied to sterlcally stabilised dispersions, is to add a free (ie. non-adsorbing) polymer to the continuous medium. 

The stability of mlcroemulslons has been treated In the form of a model with dispersed droplets In a continuous medium (14,15). The free energy of such a system may schematically be described as the sum of a series of terms according to Table I. 

A porous system resulting from dispersion of a (macroscopically) continuous medium, condensation, a chemical reaction, or from any other specific process (e.g. physical or biological) may be called a growth system. Such a system usually possesses an inimitable morphology. Growth systems include the following natural or man-made porous materials pumice, cokes, activated carbon, carbon, ceolites, cellulose fibers, and finally most foamed polymers. 

Diesel oil and BCO is a two phases system since Diesel is insoluble in BCO and vice versa. They are not miscible. If the Diesel oil/BCO system must be used as fuel, a stable emulsion is necessary. In the simplest emulsion a phase (oil or water) is dispersed in the continuous medium (water or oil) in the form of droplets. In this case Diesel oil has been considered as the oil phase and BCO as the water phase because of its consistent water percentage. Three kinds of emulsions can be prepared according to the value of the BCO/Diesel oil ratio ... 

Disperse systems consist normally of two or more phases in which the continuous phases are intermixed. If, in a continuous phase (the dispersion medium), the elements of the disperse matter are embedded such that they can be individually distinguished, the system is called discretely disperse, A coherent disperse phase, which may also consist of well-defined elements adhering to or intermixed with each other, is called compact disperse. 

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]. 

A colloidal system is composed of one or more dispersed phases and a continuous medium. Colloids are present as dispersed systems, characterized by slow diffusion and slow (often negligible) sedimentation under normal gravity, which sets the size of the colloidal particles to the range of 1 nm to 1 pm. Systems in which a significant fraction of particles is in a size range that is larger than the colloidal range are termed suspensions. 

A dilute polymer solution is a system where polymer molecules are dispersed among solvent molecules. An assumption common to any existing theory for flow properties of polymer solutions is that the structure of solvent molecules is neglected and the solvent is assumed to be replaced by a continuous medium of a Newtonian nature. Thus, macroscopic hydrodynamics may be used to describe the motion of the solvent. Recently, some ordering or local structure of solvent molecules around a polymer chain has been postulated as an explanation of the stress-optical coefficient of swollen polymer networks (31,32) so that the assumption of a solvent continuum may not apply. The high frequency behavior shown in Chapter 4 could possibly due to such a microscopic structure of the solvent molecules. Anyway, the assumption of the continuum is employed in every current theory capable of explicit predictions of viscoelastic properties. In the theories of Kirkwood or... 

Many disperse systems with solid continuous phase are the common subjects for studies in such areas of science as material science, physics of materials, physics of metals and others. This is related to the existing great variety of such systems. Obviously, their properties (among which mechanical ones are of primary importance) are significantly different from those of systems with liquid dispersion medium. At the same time, the investigation of processes leading to the formation of such systems and their interactions with ambient media constitute direct subjects of colloid science. 

The formation of systems with solid backbones is often the result of aggregation processes taking place in suspensions and sols which lead to the development of spacial networks and final conversion of disperse systems into materials with valuable properties (Chapter IX, 2). In some cases, e.g. during solidification of metal alloys, the processes of structuring accompany formation of new phases. The systems with solid dispersion medium also form upon the solidification of a continuous phase in foams, emulsions, suspensions and sols. 

This description corresponds to the case of disperse structures of globular type in which the strength originates from a continuous skeleton that forms due to adhesion of individual particles upon the conversion of free disperse system into structured disperse system. There are, however, other types of structures, such as, e.g., cellular structures (in solidified foams and emulsions), in which the skeleton consists of continuous films of solid-like dispersion medium. Such structures, typical for some polymeric systems, may... 

The range of disperse systems of interest in colloid science is very broad. These include coarse disperse systems consisting of particles with sizes of 1 pm or larger (surface area S < 1 m2/g), and fine disperse systems, including ultramicroheterogeneous colloidal systems with fine particles, down to 1 nm in diameter, and with surface areas reaching 1000 m2/g ( nanosystems ). The fine disperse systems may be both structured (i.e. systems in which particles form a continuous three-dimensional network, referred to as the disperse structure), and free disperse, or unstructured (systems in which particles are separated from each other by the dispersion medium and take part in Brownian motion and diffusion). 

The state of thermodynamic equilibrium (implying the formation of a single-phase blend of components) can rarely be reached due to the high viscosity of polymer composite systems. Polymer CM are most often dispersed systems whose composition varies with time and individual components may form phase areas of different sizes. One of their components (the polymer phase) is a continuous dispersed medium, i.e. a matrix, in which all other components are spread as a dispersion of spatially separated particles called a dispersed phase [31],... 

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