Chapter 19 Controlled release and related phenomena page 19-1
Chapter 19 Controlled release and related phenomena
1. Spherical microcapsule
From Eqn. of Contuity, at steady state, in spherical coordinates,
1
integrating once, we have c = A/r2 (A = constants)
integrating again, we have c = –A/r + B (B = constant)
Boundary conditions are c(Ri) = csat and c(Ro) = 0
At r = Ro, c = –A/Ro + B = 0 => B = A/Ro => c = –A (1/r – 1/Ro)
(b)
Ro – Ri
(c) A thicker shell will reduce the concentration gradient, thus lowering the releasing rate.
(d) Water may diffuse into the core of the pill, and dilute the concentration, thus results a
decrease in the releasing rate. On the other hand, the diffusion coefficients of ternary
system are concentration dependent. This will also affect the releasing rate.
2. Parallel mechanisms for controlled release
a) Total flux by parallel routes:
Chapter 19 Controlled release and related phenomena page 19-2
f1
D1
DH
l
c10 0
b) Wall resistance is
D1
DH
Note: If the wall is very thin, then the resistance across the pore may be small compared to the
resistance first reaching the pore. In this case, a better model may be that of an orifice,
dexcribed in Example 3.5-2.