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disperse systems with a liquid or gaseous dispersion medium, having some properties of solids—the ability to preserve their shape, strength, elasticity, and plasticity. These properties of gels are due to the existence in them of a structural network (framework) formed by the disperse-phase particles, which are interconnected by molecular forces of various types.
Typical gels in the form of gelatinous precipitates (coagels) are formed from sols by coagulation or from supersaturated solutions of both low-molecular-weight and macromolecular substances by phase-separation processes. Gels with an aqueous dispersion medium are known as hydrogels; those with a hydrocarbon medium are known as organogels. The hardening of sols throughout their entire volume without the separation of a precipitate or the disturbance of their homogeneity gives so-called lyogels. The entire dispersion medium in these gels is immobilized because of mechanical occlusion within the cells of the structural network. The greater the particle asymmetry, the lower the disperse-phase content at which a gel is formed. For a hydrosol of vanadium pentoxide, 0.05 percent of the disperse phase is sufficient for the production of hardening. In other cases, a few volume percent of the disperse phase produces the same effect.
Lyogels exhibit low strength and plasticity and some elasticity and thixotropy (the ability to restore their structure after disturbance by mechanical action). Gels of soap and soaplike surface-active materials, as well as of hydroxides of many polyvalent metals, are of this type. Aerogels, or xerogels, which are microporous systems devoid of plasticity and having a brittle, irreversibly destructible structure, are produced by drying lyogels. Common sorbents are produced in this way: alumogel from aluminum hydroxide gel and silica gel from silicic-acid coagels.
Gels are frequently equated with gelatins. The latter, however, are single-phase (homogeneous) systems—true solutions of organic or inorganic polymers in low-molecular-weight liquids. In the area of chemistry and technology of synthetic resins, the infusible and insoluble solid (brittle) or semisolid (elastic-viscous plastic) products of condensation polymerization or polymerization are traditionally called gels. The three-dimensional network in these systems consists of a continuous structure of chemically bonded macromolecules.