Coacervate droplets

This method used the physicochemical properties of polymers like chitosan, which is insoluble in alkaline pH medium and therefore precipitates/coa-cervates when it comes in contact with alkaline solution. Particles are produced by blowing chitosan solution into an alkali solution like NaOH using a compressed air nozzle to form coacervate droplets (Agnihotri et al. 2004). 

In coacervation by Polymer 2-Polymer 3 repulsion, the addition of Polymer 3 causes phase separation between the two polymer species dissolved in a common solvent 1. This phase separation produces a viscous, liquid phase of Polymer 2, i.e., the coacervate, and a low-viscous phase of Polymer 3, often called continuous or polymer-poor phase. Under stirring, coacervate droplets are formed and dispersed in the continuous phase. The solubility of Polymer 3 in solvent 1 should be superior to that of Polymer 2 in this common solvent. For particle production, the Polymer 3 should also function as stabilizer for the coacervate droplets to prevent their aggregation. Further, for the entrapment of a biologically active material, the coacervate must have a certain degree of fluidity and a high affinity to the core material, whereas the affinity between core material and continuous phase should be low... 

Naturally, the aforementioned polymers may also be coacervated by desolvation upon adding a nonsolvent, such as hexane, heptane, liquid paraffin, or a vegetable oil. The advantage of using the coacervation by Polymer 2-Polymer 3 repulsion is that the viscosity and volume fraction of the coacervate phase and the stability of coacervated droplets can be controlled by... 

The particular feature of coacervation by Polymer 2-Polymer 3 repulsion is that phase separation occurs already after the addition of a minute volume fraction of Polymer 3, which is in contrast to the coacervation by polymer desolvation. In the very first step, a dispersion of Polymer 3-in-Polymer 2 phase is formed (Fig. 3). Further Polymer 3 addition produces a phase inversion, whereupon the Polymer 2 phase (coacervate droplets) is dispersed in the Polymer 3 phase. Upon further Polymer 3 addition, the solvent is partially extracted from the coacervate droplets thereby increasing their viscosity and physical stability against coalescence. Optimal coacervate stability is generally achieved within a certain range of Polymer 3 volume fraction. This stability window has been determined by various authors for different PLA and PLGA types.f ... 

The main advantage of coacervation by Polymer 2-Polymer 3 repulsion over polymer desolvation resides in the good control of the composition and viscosity of both the coacervate and dispersing phases. This, in turn, provides a means to control particle size and prevent undesired coalescence of the coacervate droplets. 

Fig. 3 Different stages of PLA coacervation by PLA-silicone oil (PDMS) repulsion in a ternary system using dichloromethane (DCM) as solvent (A) Dispersion of PDMS in PLA/DCM (B) Phase inversion yielding unstable droplets of PLA/DCM in PDMS (C) Stable dispersion of well defined coacervate droplets (D) Aggregation and precipitation of polymer particles upon exceeding PDMS-addition.

While the catalytic effects of enzymes or of proteinoids are intrinsically of interest, the effects on metabolic sequences of such catalysts in juxtaposition within a membrane is conceptually of far-reaching biochemical significance. Similarly, the enzyme or the proteinoid is largely retained by the membrane or boundary which serves to localize the agent within a reactive package. In the same mode, Oparin (56) has concentrated contemporary enzymes within the boundaries of micro-structural coacervate droplets. He has learned that enzyme-promoted reactions proceed more rapidly within the particles than without. 

The positive charge of the droplets is in the same way caused by adsorption of hexol nitrate at the surface of the coacervate droplets as indicated in... 

Whereas most of the work discussed so far in this essay has dealt with the synthesis of well-defined biochemical species supporting the theory of chemical evolution as first proposed by A. I. Oparin, one of Oparin s major concerns has been to develop a hypothesis of precellular evolution and to experimentally demonstrate that specific biochemical reactions can occur within simulated precellular entities (coacervates). In an elegant experiment, using polynucleotide phosphorylase in coacervate droplets and the appropriate substrate in the external medium, he showed a continuous uptake of the substrate, a rapid internal synthesis of polynucleotides and a continuous release of phosphate to the external environment. His more recent concepts on evolution of probionts and the origin of cells were presented at the 4th International Conference on the Origin of Life held in Barcelona, Spain, in 1973. Experimental models involving microspheres made of polymers of amino acids have been developed by S. W. Fox and coworkers > and other investigators. 

Coalescence of coacervate droplets was not observed. For comparative purposes an identical study was conducted using gelatin/acacia coacervates which are known to coalesce readily. Accurate determination of the mean coacervate diameter of this system could not be made as coalescence occurred very rapidly. Over a period of 10 minutes all the gelatin/acacia coacervate droplets in the field of view had coalesced into one. 

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