Disperse Structure

Disperse Structure

 

a disordered three-dimensional network in a disperse system (a framework), formed by the particles of the dispersed phase that are bonded by molecular forces of various types. The formation of disperse structures is accompanied by condensation (an increase in the structural viscosity) or by solidification of an initially liquid system. In this case, the structural lattice may occupy several hundredths of a percent to several dozen percent of the volume of the system; in some cases, it may occupy virtually the entire volume. A distinction is made between disperse structures of the coagulation and condensation types according to the nature of the bonds between particles.

Disperse structures of the coagulation type take shape during the coagulation of particles in the dispersed phase or upon an increase in the volume of the disperse system occupied by the particles. In disperse structures of the coagulation type, the particles are bound through a thin layer of the liquid dispersion medium as a result of the action of weak (van der Waals’) forces of attraction. Such disperse structures have low stability and plasticity, some elasticity, and thixotropy (the capability for spontaneous and reversible regeneration after mechanical destruction as a consequence of collisions during Brownian movement). Lyogels and pastes of various types have disperse structures of a typically coagulation type.

Condensation disperse structures are formed during the process of separation of a new phase from supersaturated vapors, melts, and solutions. The smallest particles of the new (dispersed) phase, having been generated in the interior of the homogeneous phase, grow together to form a three-dimensional network with strong phase, or cohesion, contacts. Such a disperse structure may be resilient-brittle or elastic (depending on the mechanical properties of the phase that makes up the system), but it is devoid of plasticity and thixotropy (its destruction is irreversible). If the new phase separates in the form of crystals that grow together or intertwine during the growth stage, the resulting disperse structure is called a crystallization or condensation-crystallization structure. Disperse structures of this type are formed, for example, during the interaction of inorganic binders (cements) with water. Disperse structures of the condensation-crystallization type also include ceramics and metal ceramics, as well as high-density disperse structures of microgranular solids formed by crystalization from melts, such as the pyroceramics, which are crystallized glasses. The formation of disperse structures of the condensation type from supersaturated polymer solutions may proceed in two ways: through the intermediate stage of coacervate droplets with a high polymer content and through the formation in the elastic polymer gel of droplets of a dilute solution, similar to vacuoles. In the first case, depletion of the solvent and partial coalescence of the droplets, which have passed into a high-elastic state, lead to the formation of a three-dimensional network accretion structure. Analogous structures are formed from spherical polymeric particles during the gelation of a latex—for example, in the production of sponge rubber—or from fat particles in the preparation of butter. In the second case, the growth and coalescence of the “vacuoles” generates a system of interconnecting channels; at the same time, as a result of syneresis, depletion of the gel phase in the solvent occurs and a structural network of the cellular type is generated. The removal of the solvent from polymeric disperse structures of the condensation type (so-called pseudogel) yields polymeric xerogels, which are of practical interest in the production of microporous materials such as membrane filters and man-made leather and of macroporous ion-exchange resins.

Natural and man-made materials, such as some rocks, filled plastics, and rubbers may have a complex coagulation-condensation structure. The study and directed synthesis of disperse structures with predetermined properties are the objectives of a separate scientific discipline, physicochemical mechanics.

REFERENCES

Rebinder, P. A., and I. N. Vlodavets. “Fiziko-khimicheskaia mekhanika poristykh i voloknistykh dispersnykh struktur.” In Problemy fizio-khimicheskoi mekhaniki voloknistykh i poristykh dispersnykh struktur i materialov. Riga, 1967. Pages 5-40.
Voiutskii, S. S. Kurs kolloidnoi khimii. Moscow, 1964. Pages 334-40 and 533-44.

L. A. SHITS

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It is shown that structural changes, which occur at the stage of solidification of the R6M5 high speed steel melt in EBCHR, exerts favorable influence on phase transformations and formation of homogeneous disperse structure of cast metal.