Coacervation


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coacervation

[kō‚as·ər′vā·shən]
(chemistry)
The separation, by addition of a third component, of an aqueous solution of a macromolecule colloid (polymer) into two liquid phases, one of which is colloid-rich (the coacervate) and the other an aqueous solution of the coacervating agent (the equilibrium liquid).

Coacervation

 

the appearance in a solution of a macromolecular compound of drops enriched in the dissolved material.

Under favorable conditions, the coalescence of the coacervate drops (which may be preceded by flocculation, or their combination to give loose, flaky aggregates) leads to a separation of the system into two liquid layers separated by a clearly defined interface: a layer of the equilibrium liquid, with a low concentration of the macromolecular compound, and a layer of increased concentration, called the coacervate layer. The phase enriched in the polymer (in the form of either droplets or a layer) is called the coacérvate. This term is sometimes applied to the coacervate system as a whole, that is, to the aggregate of the coacervate drops and the equilibrium liquid in contact with the drops.

Coacervation takes place with a change in either the temperature or the composition of the system when the components forming the system lose the capacity to dissolve completely in one another and become only partially intersoluble. This kind of transition is considered the stratification of a single-phase (homogeneous) system into two new phases—the solution of the polymer in the solvent and the solution of the solvent in the polymer. Unlike the stratification of homogeneous mixtures of low-molecular substances (for example, phenol-water or aniline-water systems) near the critical mixing temperature, coacervation is not always reversible.

Coacervate drops and layers exhibit complex structural transformations, arising from the interaction of the macromolecules concentrated therein. The process may occur in two-component and multicomponent solutions of organic and inorganic compounds. The most typical and most thoroughly studied coacervation processes are those in aqueous solutions of proteins and polysaccharides.

The process of coacervation may be either simple or complex. Simple coacervation is the result of the interaction of a dissolved polymer with a low-molecular substance (for example, gelatin with alcohol or sodium sulfate). Complex coacervation occurs through the interaction of two polymers whose macromolecules bear opposite charges (for example, in mixing aqueous solutions of gelatin and gum arabic).

Coacervation may occur in polymer solutions containing a few tenths, or even a few hundredths, of a percent of the polymer, in which case the polymer concentration in the coacervate drops may be as high as several dozen percent. For this reason, coacervation is used as a means of concentrating and fractionating native and denatured biopolymers (in particular, water-soluble proteins) and synthetic polymers. According to A. I. Oparin’s hypothesis of the generation of life on earth, coacervation played an important role in concentrating proteins in isolated areas of the surrounding medium. According to the hypothesis, the combination of the individual hydrated macromolecules into molecular clusters and the subsequent concentration of these clusters into coacervate drops led to the appearance of prebiological systems in the primordial oceans that covered the earth’s surface during remote geological epochs.

REFERENCES

Serebrovskaia, K. B. Koatservaty i protoplazma. Moscow, 1971.
Evreinova, T. N. Kontsentrirovanie veshchestv i deistvie fermentov v koatservatakh. Moscow, 1966.
Pasynskii, A. G. Kolloidnaia khimiia. Moscow, 1968. Page 166.
Colloid Science, vol. 2. [Edited by H. R. Kruyt] New York (and elsewhere), 1949. (Article by H. G. Bunoenberg de Jong.)

L. A. SHITS

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