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the cohesion of particles in a colloidal system upon collision during thermal (Brownian) movement, mixing, or directed motion in a force field. Coagulation produces aggregates —larger (secondary) particles consisting of masses of small (primary) particles. The primary particles in such masses are held together by the force of intermolecular interaction directly or through an interlayer of the surrounding (dispersion) medium. Coagulation results in progressive enlargement of the particles (increase in the size and mass of the aggregates) and a decrease in their number in the dispersion medium (liquid or gas).
A distinction is made between rapid and slow coagulation. In rapid coagulation, almost every collision of the particles is effective—that is, causes them to combine. In slow coagulation, only some of the colliding particles combine. In a liquid medium— for example, in the coagulation of sols—enlargement of the particles to a certain point (approximately 10-4 cm in diameter) does not result in their precipitation or emersion. This is latent coagulation, in which the system retains its resistance to sedimentation. Further growth of the particles results in the formation of lumps or flakes (floccules) that are precipitated (coagulum or coagel) or accumulate in the form of a film on the surface; this is visible coagulation. In some cases a loose three-dimensional network (a coagulation structure) forms throughout the dispersion medium, and the system does not divide into layers. If colloidal particles (droplets of liquid or gas bubbles) are present, coagulation may end in their adhesion or coalescence.
Coagulatior is a spontaneous process, which according to the laws of thermodynamics is a consequence of the system’s tendency toward a state with lower free energy. However, such a conversion is difficult and sometimes virtually impossible to achieve if the system has aggregate stability—that is, the ability to resist enlargement (aggregation) of the particles. The electric charge and/or adsorptive-solvate layer on the surface of the particles, which prevents them from coming together, may provide protection against coagulation in such cases. Aggregate stability can be disrupted, for example, by an increase in temperature (thermocoagulation), mixing or shaking, the introduction of coagulants, or other external influences on the system. The lowest concentration of a substance, electrolyte, or nonelectrolyte that brings about coagulation in a system with a liquid dispersion medium is called the coagulation threshold. Polydisperse systems, in which the particles have different sizes, may exhibit orthokinetic coagulation, which is the adhesion of small particles to larger particles upon their precipitation or emersion. The adhesion of similar particles is called homocoagulation, adhesion of dissimilar particles, heterocoagulation or adagulation. Heterocoagulation often occurs when disperse systems of different compositions are mixed. Coagulation may take place in the absence of any external action on the colloid system (autocoagulation), as a result of physical or chemical changes that occur during aging. Coagulation is sometimes reversible. Under favorable conditions, particularly after the introduction of surface-active agents that reduce interphase surface energy and promote dispersion, the aggregates may break down into the primary particles (peptization), and the coagel may become a sol.
Coagulation plays an important role in many technological, biological, atmospheric, and geological processes. When biopolymers (proteins or nucleic acids) are heated or subjected to some other influence—for example, a change in pH—they coagulate. Coagulation phenomena are important in many biological disperse systems (for example, blood and lymph) because of some aspects of their aggregate stability. The purification of natural water and sewage from highly disperse mechanical impurities, control of air pollution by aerosols, separation of rubber from latex, and the production of butter and other foods are typical examples of practical uses of coagulation. Coagulation is undesirable during the preparation and storage of suspensions, emulsions, powders, and other disperse systems used in industry or at home.
REFERENCESNauka o kolloidakh, vol. 1. Edited by H. Kruyt. Moscow, 1955. (Translated from English.)
Voiutskii, S. S. Kurs kolloidnoi khimii. Moscow, 1964.
L. A. SHITS