Chemistry Reference
In-Depth Information
cross-linking, or ionic interactions (Dinsmore et al. 2002). This class of capsules is
notable because, unlike the previous two methods of capsule preparation, colloido-
somes inherently have uniform pores, defined by the interstices between the
colloid particles, a parameter tuned by varying the particle diameter.
Orlin Velev and colleagues are considered to have reported the first colloidosomes
in the mid-1990s (Velev et al. 1996; Velev and Nagayama 1997). In this system,
1-mm diameter polystyrene (PS) latex beads were surface functionalized with
sulfate groups to give them a negative charge and were dispersed in an oil-in-water
emulsion of octanol in water. The authors found that if the surface charges on the
PS beads were optimized, the beads could be made just hydrophobic enough to sit
at the phase boundary of the oil and water. Addition of casein served to stabilize
the capsules by mitigating adsorption of PS beads onto the surface of the templates
or flocculation of the colloidosomes. Addition of HCl and CaCl
2
, which act as
coagulants for the latexes, provided more shell stabilization. As with previous
methods, the size of the resulting microcapsules was dictated by the size of the orig-
inal disperse-phase droplets, which in this case were generated by homogenization
and were on the order of 10-100 mm.
Other groups have used alternate methods for stabilizing colloidosome shells,
including chemical cross-linking (Cayre et al. 2004) and sintering (Dinsmore et al.
2002). Dinsmore and coworkers heated PS latex colloidosomes to 105 8C, just
beyond the glass transition of PS, for 5 min and found by scanning electron
microscopy that this caused 150-nm diameter bridges to form between the latex
beads. Heating for 20 min caused the interstitial spaces to close completely.
Colloidosome formation can be generalized as long as the surface energies among
the dispersed and continuous phases, and the colloid particles, are optimized exper-
imentally. Thus far, colloidosome compositions have included unfunctionalized
poly(methyl methacrylate) (Dinsmore et al. 2002), magnetite (Fe
3
O
4
) nanoparticles
(Duan et al. 2005), polymeric microrods (Cayre et al. 2004), and temperature-
responsive gels (Berkland et al. 2007).
A subcategory of colloidosomes and the colloidal templating of the next section is
the use of “gel-trapping” methods for generating colloidosomes (Paunov 2003;
Paunov and Cayre 2004). Gel trapping is similar to the general methods except
that the disperse phase of the emulsion is an agarose (1.5%) aqueous gel, where
the emulsion is stabilized by the PS latexes used to form the capsule shells (Binks
and Lumsdon 2001). This gives the capsules more mechanical stability as they are
washed and as the colloid shells are strengthened by cross-linking (Cayre et al.
2004). The encapsulated gel can be left
in the microcapsules if so desired or
heated out after shell stabilization.
8.2.4. Colloidal Templating
We can think of the previous methods for generating capsules as “liquid templating,”
where self-assembly occurs around a liquid. An alternative is to use a solid template,
usually called colloidal templating. The shell is generated around a solid particle
that
is later digested,
leaving a hollow sphere (Caruso 2000). Deposition of
