The earliest known appearance of the word, phase, was in 1812 (Onions, 1933). An appropriate definition of the word for this chapter is 'a homogeneous, physically distinct, and mechanically separable portion of matter that is present in a non-homogeneous, physical-chemical system and that may be a single compound or a mixture,' (Gove, 1963).
Phase separation is a broad term that may be applied to various processes such as the formation of a solid or liquid phase from a solution. Examples are the crystallization of a salt or the precipitation of a polymer as the result of the removal of some solvent from solution. The new phase is physically distinct and has different properties compared with its solution and can be separated by mechanical means.
The word coacervate was first noted in 1623 (Onions, 1933). This word may be defined in the present context as 'an aggregate of colloidal droplets (as of two hydrophilic sols or of a sol and ions of opposite charge) held together by electrostatic attractive forces,' (Gove, 1963).
Coacervation is a term used to describe the formation of a coacervate related to phase separation and has been applied to the separation of a colloid from a solution into a phase rich in the colloid called the coacervate and the remaining phase which is poor in colloid (Bungenberg de Jong, 1949a). The coacervate has certain properties that distinguish it from the original solution. The coacervate will form a separate liquid layer, but with stirring may form droplets suspended in the polymer-poor phase; furthermore it is usually more viscous, more concentrated and often has the property of binding or adsorbing onto, or engulfing a solid or liquid which
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Coacervation has also been defined as the partial miscibility of two or more optically isotropic liquids, at least one of which is in the colloidal state. It may also be defined as the production by coagulation of a hydro-philic sol of a liquid phase which often appears as viscous drops instead of forming a continuous liquid phase (Considine, 1983). Luzzi (1976) indicated that a solid precipitate is not formed, but a polymer phase consisting of liquid droplets. Bungenberg de Jong (1949a) indicated that simple coacervation is concerned with non-ionized groups on the macromolecule and complex coacervation is concerned with the charges on the macromolecule and the formation of salt bonds. Luzzi (1976) and Deasy (1984a) indicate that simple coacervation usually deals with systems containing only one colloidal solute and complex coacervation involves systems containing more than one colloid, the solvent being water. Arshady (1990a) uses the term coacervation to include the use of both water and non-aqueous systems and describes simple coacervation and complex coacervation in terms of one or two polymers in aqueous systems, respectively. Another definition of a coacervate proposed by Ecanow etal. (1990) is that phase most dissimilar to water in its physical chemical properties. Deasy (1984a) has pointed out that the solvent is continuous on both sides of the interface and that the polymer or macromolecule is able to diffuse between the two phases.
Some authors separate the discussion of microencapsulation by coacervation, which includes the use of temperature change or incompatible solvent addition, from other phase separation methods such as the use of emulsi-fication and/or solvent removal to prepare a polymer-rich phase (Kondo, 1979a; Arshady 1990a,b; Watts etal., 1990). Other authors include all these processes in the term coacervation-phase separation to indicate that a new phase, a polymer-rich phase, is being formed (Luzzi, 1976; Deasy, 1984a,b).
In any case, a process is applied to a polymer solution, usually dilute, to reduce solubility in the system to such a degree that appropriate phase separation of the polymer takes place, that is the formation of a polymer-rich phase. This polymer-rich phase may be used to treat pharmaceuticals and other substances. Alternatively, the formation of the colloid-rich phase may result in flocculation and the colloid is present in a higher dispersed state and is usually not satisfactory for encapsulation (Bungenberg de Jong, 1949a).
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