The effective use of a drug is achieved by its controlled release, and film-coated pellets, tablets, or capsules are used for this purpose. Consider the case in which a drug (species A) is dissolved uniformly in a semi-spherical matrix with radii Ri and R0 as shown in the figure below. The thick lines in the figure represent the impermeable coating, and transfer of the drug takes place only through the surface of the tablet located at r = Ri. You may assume that the drug concentration on this surface is zero. This is known as the perfect sink condition. a) If the initial concentration of species A, cA0, is below solubility, then the release of the drug is governed by diffusion. Derive the governing equation, together with the initial and boundary conditions. Use dimensionless quantities in the form: θ is dimensionless concentration, r* is dimensionless radial distance and τ is dimensionless time. Also, define these dimensionless variables to formulate the dimensionless PDE. b) Derive an expression for the molar flux of species on the tablet surface.
The effective use of a drug is achieved by its controlled release, and film-coated
pellets, tablets, or capsules are used for this purpose. Consider the case in which a drug (species
A) is dissolved uniformly in a semi-spherical matrix with radii Ri and R0 as shown in the figure
below.
The thick lines in the figure represent the impermeable coating, and transfer of the drug takes
place only through the surface of the tablet located at r = Ri. You may assume that the drug
concentration on this surface is zero. This is known as the perfect sink condition.
a) If the initial concentration of species A, cA0, is below solubility, then the release of the drug is
governed by diffusion. Derive the governing equation, together with the initial and boundary
conditions. Use dimensionless quantities in the form: θ is dimensionless concentration, r* is
dimensionless radial distance and τ is dimensionless time. Also, define these dimensionless
variables to formulate the dimensionless PDE.
b) Derive an expression for the molar flux of species on the tablet surface.
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