text: At the start of the 20th century, scientists had become interested in the stability of colloids, both the dispersions of solid particles and the solutions of polymeric molecules. It was known that salts and temperature could often be used to cause flocculation of a colloid. The German chemist F.W. Tiebackx reported in 1911  that flocculation could also be induced in certain polymer solutions by mixing them together. In particular, he reported the observation of opalescence (a turbid mixture) when equal volumes of acidified 0.5% “washed” gelatine solution, and 2% gum arabic solution were mixed. Tiebackx did not further analyse the nature of the flocs, but it is likely that this was an example of complex coacervation.
  Dutch chemist H. G. Bungenberg-de Jong reported in his PhD thesis (Utrecht, 1921) two types of flocculation in agar solutions: one that leads to a suspensoid state, and one that leads to an emulsoid state. He observed the emulsoid state under the microscope and described small particles that merged into larger particles (Thesis, p. 82), most likely a description of coalescing coacervate droplets. Several years later, in 1929, Bungenberg-de Jong published a seminal paper with his PhD advisor, H. R. Kruyt, entitled “Coacervation. Partial miscibility in colloid systems”. In their paper, they give many more examples of colloid systems that flocculate into an emulsoid state, either by varying the temperature, by adding salts, co-solvents or by mixing together two oppositely charged polymer colloids, and illustrate their observations with the first microscope pictures of coacervate droplets. They term this phenomenon coacervation, derived from the prefix co and the Latin word acervus (heap), which relates to the dense liquid droplets. Coacervation is thus loosely translated as ‘to come together in a heap’. Since then, Bungenberg-de Jong and his research group in Leiden published a range of papers on coacervates, including results on self-coacervation, salt effects, interfacial tension, multiphase coacervates and surfactant-based coacervates.
  In the meantime, Russian chemist Alexander Oparin, published a pioneering work in which he laid out his protocell theory on the origin of life. In his initial protocell model, Oparin took inspiration from Graham's description of colloids from 1861 as substances that usually give cloudy solutions and cannot pass through membranes. Oparin linked these properties to the protoplasm, and reasoned that precipitates of colloids form as clots or lumps of mucus or jelly, some of which have structural features that resemble the protoplasm. According to Oparin, protocells could therefore have formed by precipitation of colloids. In his later work, Oparin became more specific about his protocell model. He described the work of Bungenberg-de Jong on coacervates in his book from 1938, and postulated that the first protocells were coacervates.
  Other researchers followed, and in the 1930s and 1940s various examples of coacervation were reported, by Bungenberg-de Jong, Oparin, Koets, Bank, Langmuir and others. In the 1950s and 1960s, focus shifted to a theoretical description of the phenomenon of (complex) coacervation. Voorn and Overbeek developed the first mean-field theory to describe coacervation. They estimated the total free energy of mixing as a sum of mixing entropy terms and mean-field electrostatic interactions in a Debye-Hückel approximation. Veis and Aranyi suggested to extend this model with an electrostatic aggregation step in which charge-paired symmetrical soluble aggregates are formed, followed by phase separation into liquid droplets.
  In the decades after that, until about 2000, the scientific interest in coacervates had faded. Oparin's theory on the role of coacervates in the origin of life had been replaced by interest in the RNA world hypothesis. Renewed interest in coacervates originated as scientists recognized the relevance and versatility of the interactions that underlie complex coacervation in the natural fabrication of biological materials and in their self-assembly.
  Since 2009, coacervates have become linked to membraneless organelles and there has been a renewed interest in coacervates as protocells.