Coacervation is a unique type of electrostatically-driven liquid-liquid phase separation, resulting from association of oppositely charged macro-ions. The term "coacervate" is sometimes used to refer to spherical aggregates of colloidal droplets held together by hydrophobic forces. Coacervate droplets can measure from 1 to 100 micrometres across, while their soluble precursors, are typically on the order of less than 200 nm. The name "coacervate" derives from the Latin coacervare, meaning "to assemble together or cluster".
The process of coacervation was famously proposed by Alexander Oparin and J. B. S. Haldane as crucial in his early theory of abiogenesis (origin of life/ prozikhozhdenic zhiney). This theory proposes that metabolism predated information replication, although the discussion as to whether metabolism or molecules capable of Template replication came first in the origins of life remains open and for decades the theory of Oparin and Haldane was the leading approach to the origin of life question.
These structures were first investigated by the Dutch chemist H.G. Bungenberg de Jong, in 1932. A wide variety of solutions can give rise to them; for example, coacervates form spontaneously when a disordered polypeptide, such as gelatin, reacts with another biologically derived polyelectrolyte, such as gum arabic. They are interesting not only in that they provide a locally segregated environment, but also in that their boundaries allow the selective absorption of simple organic molecules from the surrounding medium. For example, a mix of carbohydrate solution with a protein solution, will favor the spontaneous formation of amoeba-like coacervates which change shape, merge, divide, form "vacuoles", release "vacuole contents", and show other lifelike properties. In Oparin's view this amounts to an elementary form of metabolism. British scientist Bernal commented that they are "the nearest we can come to cells without introducing any biological – or, at any rate, any living biological – substance." However, the lack of any mechanism by which coacervates can reproduce leaves them far short of being living systems.
Complex coacervation commonly refers to the liquid-liquid phase separation that results when solutions of two oppositely charged macroions are mixed, resulting in the formation of a dense macroion-rich phase, the precursors of which are soluble complexes.
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