Disperse Systems

Disperse Systems

 

formations consisting of two or more phases (bodies) with a highly developed interface between them. In disperse systems, at least one of the phases—the disperse phase—is distributed in the form of small particles (crystals, threads, films or platelets, droplets, or bubbles) in the other, continuous phase, the dispersion medium. According to their main characteristic—the particle size, or dispersity (which is determined by the ratio of the total interphase surface area to the volume of the dispersed phase)—disperse systems are divided into coarsely (poorly) dispersed and finely (highly) dispersed, or colloidal systems (colloids).

In coarsely dispersed systems, the particles have sizes of 10−4 cm; in colloidal systems, the particle size varies from 10−4-10−5to 10−7cm. According to the state of aggregation of the dispersion medium, a distinction is made among gaseous systems, such as aerosols (fog and smoke) and dust; liquid systems, such as sols, suspensions, emulsions, and foams; and solid systems, such as glassy or crystalline bodies with inclusions of very fine solid particles, liquid droplets, or gas bubbles (see Table 1). Dust, suspensions, and lyophobic emulsions are coarsely dispersed systems. As a rule (when density differences exist), these systems are unstable with respect to sedimentation—that is, their particles precipitate out under the influence of gravity or float to the surface. Sols are typical highly dispersed colloidal systems whose particles (micelles) take part in Brownian movement and are, therefore, sedimentationally stable. Liquid and solid foams, consisting of gas cells or bubbles separated by thin layers of the continuous phase, are a separate group of structured cellular systems.

Table 1. Classification of disperse systems according to the state of aggregation of phases
Dispersion mediumGaseousDispersed phase LiquidSolid
1 Extremely highly dispersed systems (sols) are sometimes difficult to classify according to the state of aggregation of the dispersed phase
Gaseous...............Disperse systems do not formFogSmoke and dust
Liquid...............FoamsEmulsionsSuspensions
  Sols (colloidal “solutions”)1
Solid...............AerogelsLiquid inclusions in solidsSolid sols (ruby glass)

Lyophilic and lyophobic disperse systems are distinguished according to the intensity of the molecular phase interaction. In lyophilic systems, the molecular interaction between the phases is rather strong, and the specific free surface energy (surface tension) at the interphase boundary is very low. Lyophilic systems are formed spontaneously and have maximum dispersity. In lyophobic systems, the interaction between molecules of various phases is considerably weaker than in lyophilic systems; the interphase surface tension is high, as a result of which the system exhibits a tendency toward spontaneous enlargement of the particles of the dispersed phase. A necessary condition for the existence of lyophobic disperse systems is the presence of stabilizers, which are materials that are adsorbed at the interface between the phases and that form protective layers, which prevent the approach of particles of the dispersed phase.

Disperse systems may be structureless (freely dispersed) and structured (bonded dispersed). Structured disperse systems are penetrated by a three-dimensional lattice consisting of interconnected particles (droplets or bubbles) of the dispersed phase, as a result of which they have some of the mechanical properties of solids. A characteristic feature of disperse systems is a high free energy caused by the highly developed interphase surface. For this reason, disperse systems are usually thermodynamically unstable (with the exception of lyophilic systems). Disperse systems have high adsorption capacity and chemical—and in some cases biological—activity. Disperse systems are the primary object of studies in the area of colloid chemistry.

Disperse systems are widespread in nature, industry, and daily life. Examples of disperse systems include geologic formations, soils, smoke, clouds, atmospheric precipitation, plant and animal tissues, building materials, paints, detergents, fibrous products, and important food products.

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