Coacervation and phase separation

Is a partial desolvation of a homogeneous polymer solution into a polymer-rich phase (coacervate) and the poor polymer phase (coacervation medium). Currently, two methods for coacervation are available, namely simple and complex processes. The mechanism of microcapsule formation for both processes is identical, except for the way in which the phase separation is carried out. In simple coacervation a desolvation agent is added for phase separation, whereas complex coacervation involves complexation between two oppositely charged polymers.

The three basic steps in complex coacervation are:
(i) formation of three immiscible phases
(ii) deposition of the coating
(iii) rigidization of the coating

First step : formation of three immiscible phases; liquid manufacturing vehicle, core material, coating material. The core material is dispersed in a solution of the coating polymer. The coating material phase, an immiscible polymer in liquid state is formed by :
  1. changing temperature of polymer solution, e.g. ethyl cellulose in cyclohexane
  2. addition of salt, e.g. addition of sodium sulphate solution to gelatine solution in vitamin encapsulation
  3. addition of nonsolvent, e.g. addition of isopropyl ether to methyl ethyl ketone solution of cellulose acetate butyrate
  4. addition of incompatible polymer to the polymer solution, e.g. addition of polybutadiene to the solution of ethylcellulose in toluene
  5. inducing polymer – polymer interaction, e.g. interaction of gum Arabic and gelatine at their iso-electric point. 
Second step : deposition of liquid polymer upon the core material.

Finally, the prepared microcapsules are stabilized by crosslinking, desolvation or thermal treatment. Crosslinking is the formation of chemical links between molecular chains to form a three-dimensional network of connected molecules. The vulcanization of rubber using elemental sulfur is an example of crosslinking, converting raw rubber from a weak plastic to a highly resilient elastomer. The strategy of covalent crosslinking is used in several other technologies of commercial and scientific interest to control and enhance the properties of the resulting polymer system or interface, such as thermosets and coatings.

Schematic representation of the 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; (d) coalescence of coacervate to form continuous shell around core particles.

Polymer Encapsulation by Rapid Expansion of Supercritical Fluids

Supercritical fluids are highly compressed gasses that possess several advantageous properties of both liquids and gases. The most widely used being supercritical carbon dioxide(CO2), alkanes (C2to C4), and nitrous oxide (N2O). A small change in temperature or pressure causes a large change in the density of supercritical fluids near the critical point.

Supercritical CO2 is widely used because of its advantages :
  • it has low critical temperature value
  • nontoxic, non flammable
  • readily available
  • highly pure
  • cost-effective
The most widely used methods are as follows:
  • Rapid expansion of supercritical solution (RESS)
  • Gas anti-solvent (GAS)
  • Particles from gas-saturated solution (PGSS)
Microencapsulation by rapid expansion of supercritical solutions (RESS)

Supercritical fluid containing the active ingredient and the shell material are maintained at high pressure and then released at atmospheric pressure through a small nozzle. The sudden drop in pressure causes desolvation of the shell material, which is then deposited around the active ingredient (core) and forms a coating layer.

Active ingredient and shell material must be very soluble in supercritical fluids. In general, very few polymers with low cohesive energy densities are soluble in supercritical fluids such as CO2. The solubility of polymers can be enhanced by using co-solvents. In some cases nonsolvents are used; this increases the solubility in supercritical fluids, but the shell materials do not dissolve at atmospheric pressure.

Gas anti-solvent (GAS) process

Gas anti-solvent (GAS) process also called supercritical fluid anti-solvent (SAS). Saupercritical fluid is added to a solution of shell material and the active ingredients and maintained at high pressure. This leads to a volume expansion of the solution that causes super saturation such that precipitation of the solute occurs. Thus, the solute must be soluble in the liquid solvent, but should not dissolve in the mixture of solvent and supercritical fluid. On the other hand, the liquid solvent must be miscible with the supercritical fluid. This process is unsuitable for the encapsulation of water-soluble ingredients as water has low solubility in supercritical fluids. It is also possible to produce submicron particles using this method.

Particles from a gas-saturated solution (PGSS)

This process is carried out by mixing core and shell materials in supercritical fluid at high pressure. During this process supercritical fluid penetrates the shell material, causing swelling. When the mixture is heated above the glass transition temperature (Tg), the polymer liquefies. Upon releasing the pressure, the shell material is allowed to deposit onto the active ingredient. In this process, the core and shell materials may not be soluble in the supercritical fluid.

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