Chemistry Reference
In-Depth Information
8.2.1. Coacervation
One of the first methods for making capsules involved polymer coacervation. In this
method, macromolecules are dissolved in either the dispersed or continuous phase of
an emulsion and are induced to precipitate as a shell around the dispersed phase.
Coacervation can be brought about in several ways, such as changes in temperature
or pH, addition of salts or a second macromolecular substance, or solvent evaporation
(Bungenberg de Jong 1949).
Coacervates are either “simple” or “complex.” Simple coacervations generally
involve nonionized groups on the precipitated macromolecule. With complex coacer-
vations, the precipitation is brought about by the formation of salt bridges. The dis-
tinction is nebulous, however, because both mechanisms can occur in the same
system. An example is when an electrolyte and an alcohol are added to an emulsion
containing gum arabic, which is made up of saccharides and glycoproteins. The
alcohol serves to reduce the solubility of the nonionized groups on the macromolecule
(simple), but it also strengthens the interaction between the cations of the electrolyte
and the gum arabic carboxyl groups (complex; Bungenberg de Jong 1949).
Capsules made with this method must be optimized for whichever application they
are intended. One example is the work of Barry Green at the National Cash Register
Company. Often considered the father of microencapsulation, Green first developed
microcapsules by coacervation for use in multiple-use carbonless paper (Lim 1984).
The project began in 1939 with two goals: to produce better and cheaper NCR sales
topics (made of one-time use paper) and to manufacture paper for cash register
receipts that contained all the chemicals needed to form images on impact (Thies
1999). Generally speaking, these materials were loosely based on Bungenberg de
Jong's coacervation studies at Utrecht University in The Netherlands (Bungenberg
de Jong 1949).
8.2.2. Interfacial Polymerization
A second classical method for making capsules from emulsions is to form the shell
polymer in situ using interfacial polymerization (Morgan and Kwolek 1959;
Wittbecker and Morgan 1959). This method is similar to the “nylon rope trick”
often used as a demonstration, where a solution of diacid chloride in organic
solvent (such as adipoyl chloride in hexanes) is layered in a beaker with a diamine
aqueous phase (such as 1,6-hexadiamine in water; Friedli et al. 2005). Because the
two monomers meet only at the interface of the two phases, the condensation
polymerization to form the polyamide occurs only at the interface.
If the organic solution of diacid chloride is recast as a dispersed phase in an
emulsion with the aqueous solution of diamine as the continuous phase, the
polymer membrane forms around the dispersed phase droplets, effectively making
polyamide shell capsules around the organic phase. Of course, the relative volumes
could be reversed so that the aqueous phase was encapsulated if so desired.
The encapsulation of hemoglobin developed by Chang in the late 1950s is one
excellent example of this process (Chang 1964, 1966, 1972). Chang's initial goal
