Polymer coacervation steps

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 ... 

FIGURE 47.6 Steps involved in coacervation process (a) core material dispersion in solution of shell polymer, (b) separation of coacervate from solution, (c) coating of core material by microdroplets of coacervate, and (d) coalescence of coacervate to form continuous shell around core particles. 

Interfacial coacervation has been successfully used for many types of microcapsules. Stabilization of the emulsion phase with surfactant is an important step in this process. The surfactants used have included lignin sulfonates, other sulfonated polymeric surfactants, and vinylpyrrolidone co-and terpolymers. Increased photo stabilization of the AI, apparently due to light-absorbing properties of the polymers used, has been an added benefit observed for some encapsulated formulations. 

The three basic steps in complex coacervation are (i) preparation of the dispersion or emulsion (ii) encapsulation of the core and (iii) stabilization of the encapsulated particle. First the core material (usually an oil) is dispersed into a polymer solution (e.g., a cationic aqueous polymer). The second polymer (anionic, water-soluble) solution is then added to the prepared dispersion. Deposition of the shell material onto the core particles occurs when the two polymers form a complex. This process is triggered by the addition of salt or by changing the pH, temperature or by dilution of the medium. The shell thickness can be obtained as desired by... 

Fig. 11 Schematic representation of the formation of xanthan and chitosan polyelectrolyte hydrogels through mutual ionic interaction. When chitosan and xanthan are mixed, a common pH is estabhshed, and the polymer chains associate to each other. These first two steps are rapid processes, followed by slow phase separation that is referred to as coacervation. In the coacer-vation step, water molecules orient in between the polymer chains and form hydrogen bonds. When all surrounding water molecules are oriented in this fashion, a polyelectrolyte hydrogel is obtained. Reprinted from Dumitriu et al. [141]. Copyright 2004 Elsevier...

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