An aqueous suspension containing 15% w/w 14C-DEET
(7.8 lCi/mg) in the form of permeable, walled polysaccharide
microcapsules enclosing DEET and adjuvant material
was prepared by an interfacial precipitation method
(Speaker, 2005). Briefly, microcapsule walls are formed in
two steps. First, a charged amphiphilic macromolecule
(CMC) is introduced into an emulsion. The macromolecule
is rapidly driven to the emulsified phase interface by polar
solvent interaction forces. Subsequently, the second, wallforming
step is to introduce a second, reactive agent
(BZK) to the system, precipitating the macromolecule to
form a wall structure surrounding each droplet. DEET
was dissolved in a waxy adjuvant base at 60 C to form a
core phase consisting of approximately 60% DEET, 30%
Cetyl Esters Wax, and 10% Emulsifying Wax. This base
has a specific gravity of approximately 0.92 g/cm3 and gels
below about 40 C, and is understood to retard the diffusion
and vaporization of DEET. The resulting core material
was subsequently dispersed in an aqueous solution of
1% CMC at 60 C using a magnetic stirrer turning at
approximately 2000 rpm, using 2 g CMC solution to 1 g
core phase. After formation of a stable oil-in-water dispersion,
the microcapsule wall was formed by adding 1%
aqueous BZK solution, using 2 g BZK solution to every
5 g CMC solution. The encapsulated suspension thus
formed was diluted with distilled water to a final DEET
concentration of 15% w/w, and cooled to ambient temperature.
The control formulation was 15% w/w 14C-DEET
(11.1 lCi/mg) in 95% ethanol.