Chemistry Reference
In-Depth Information
Being recent research arrivals to the field ourselves, we wrote this chapter in a style
that we hope will entice others to enter it as well. We begin with a brief overview of
methods used to make capsules, making sure to reference other, more comprehensive
sources in each case. We also survey the catalytic capsule field and capsules as
drug-delivery vehicles, highlighting recent innovations in both areas. We hope this
chapter will convince you that polymeric capsules are exciting materials with
equally exciting applications.
8.2. METHODS OF ENCAPSULATION
Hollow polymeric
capsules
are
usually
produced
using
one
of
three
methods (Fig. 8.1).
1. Emulsion-templated encapsulation: A polymeric shell is self-assembled or
formed over a liquid emulsion droplet. This typically includes capsules made
by coacervation and interfacial polymerization as well as many colloidosomes.
2. Solid-templated encapsulation: A solid or gel bead replaces the liquid droplet
as the template. This typically includes colloidal templating, and it is not as
widespread for hollow capsules because the solid template must be broken
down and removed without harming the shell.
3. Vesicle-based capsules: The shell is self-assembled into a hydrophobic shell or
bilayer surrounded inside and outside by solution. These materials are formed
using either small amphiphiles or amphiphilic block copolymer vesicles.
We will discuss each of these general methods in turn, using examples that are
historically significant or particularly representative. We typically mention only
hollow capsules in which the core material (be it liquid or solid) is encapsulated
by a polymeric shell that has a composition different from the core.
“Microcapsule” diameters range from 1 to 1000 mm, where any capsules with diam-
eters smaller than 1 mm are considered “nanocapsules.” Because we intend for this
discussion to be merely instructive for the general reader new to this field, it
should not be regarded as comprehensive. In addition, although not all of the
methods discussed here have been used for drug delivery and catalyst encapsulation
(some are rather new); we offer them as possible candidates for such applications.
Many processes for forming polymeric capsules use emulsion droplets as tem-
plates for shell self-assembly. An emulsion is a mixture of two immiscible liquids
where the minor (dispersed) phase forms small droplets in the major (continuous)
phase. We generally think of emulsions as being oil-in-water (where the oil is the dis-
persed phase in the aqueous continuous phase) or water-in-oil (vice versa), although
any two immiscible phases form emulsions under the appropriate conditions.
Mayonnaise is a common example of an emulsion, in this case a water-in-oil
emulsion of vinegar or lemon juice in up to 80 vol% oil. Emulsification is nonspon-
taneous, requiring shaking, stirring, or homogenization. Once formed, emulsions
