Controlled Release by Solute Diffusion

Finally, in Section 19.4 we describe problems involving diffusion and phase change. In many of these systems, diffusion causes solid dissolution. In others, diffusion can produce precipitation or spontaneous emulsification, even when the initial and final solutions are only a single phase. These unusual situations extend our understanding of diffusion. 

In most cases, we want the release rate of a solute, like a drug, to be constant with time. This constant release rate will often give a constant concentration when in use, as suggested by Fig. 19.0-1. Often, we will test for this type of release rate by placing this solute, or a device containing solute, in a beaker of stirred solvent. We will measure the solute concentration versus time. We hope for a linear variation, for this would mean a constant release rate. Such a variation is called zero-order release, as shown in Fig. 19.1-1. 

We usually do not get this constant release rate without careful work. If we directly place a drug in water, its dissolution rate will often be mass-transfer controlled. As a result, the amount released M will vary with time t according to the 

We would prefer a system where we could alter the release rate more systematically. Three systems where we can systematically alter drug dissolution are shown schematically in Fig. 19.1 -2. In the first example, we could make small microcapsules containing a solution of the drug. We would find that the drug s release followed the same mathematical form as Eq. 19.1 -1. Before, however, the mass transfer coefficient k would refer to the drug itself in the stirred solvent. Now, the mass transfer coefficient would be given by 

As an alternative, we could disperse the drug evenly in a slab of water-swollen polymer. When this slab is dropped into water, the drug will diffuse out through the polymer. If the drug is initially dissolved at concentration ci, as shown in the center of Fig. 19.1-2, the amount released M will change at small times according to 

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