Chemistry Reference
In-Depth Information
7.2 Theory
The calculation of the release rate of an ingredient from a capsule or sphere is
based on the Crank model.
18
The underlying assumption is that ingredients
that diffuse out of the capsule are instantaneously removed, and so there is no
external resistance in the surrounding phase to the mass transfer. The short-
time approximation of the Crank model is given by Baker:
19
D
ep
t
p
R
2
r
M
t
M
0
3
D
ep
t
R
2
¼
6
ð
1
Þ
where t is the time, M
0
the initial total mass of compound in the capsule, M
t
the
mass of compound diffused out of the capsule, R the diameter of the capsule,
and the quantity D
ep
is defined by
D
e
1
j
þ
j
K
dc
D
ep
¼
ð
2
Þ
The effective diffusion coefficient D
e
depends on the microstructure of the
capsule material. It may be estimated by empirical correlation, e.g., by the
general Maxwell equation
20
for spheres,
D
e
¼
D
c
2D
c
1
j
ð
Þ
K
dc
D
d
1
þ
2
j
ð
Þ
ð
3
Þ
D
c
2
þ
j
ð
Þ
K
dc
D
d
1
j
ð
Þ
where
j
is the volume fraction of the inner phase of the capsule, K
dc
the
partition coefficient, and D
d
and D
c
are diffusion coefficients of the compound
in, respectively, the capsule (wall) material and the continuous phase.
7.3 Materials and Methods
7.3.1 Preparation of Wax Capsules
The following ingredients were used in the making of the various wax-based
encapsulates: beeswax (Fagron, Cera Alba #7323), polyglycerol polyricinoleate
(PGPR, Admul WOL, Quest International), acids (Merck), fluorescein iso-
thiocyanate (FITC, Janssen Chimica), rhodamine B (Aldrich), and refined
sunflower oil from Albert Heijn ('Zonnebloemolie').
Two preparation techniques were employed as illustrated schematically in
Figure 2. Method 1, the 'solid' preparation technique, is elaborately explained
elsewhere.
15
In preparation technique 2, water containing FITC (total volume
20%) was added to a solution of oil containing 1.6 wt% PGPR and 2 wt%
beeswax. The mixture was gently stirred at 651C for 10 min, and then was
cooled down to room temperature or 51C within 1 h while still stirring. The
resulting liquid-like viscous W/O emulsion consisted of small water droplets
(with dye) of size 1-10 mm stabilized by Admul WOL and wax.
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