D

where y is the velocity of the moving front, ô(t) is the thickness of the rubbery layer (the infiltration and gel layers represented in Fig. 8.7) at time t, and D is the diffusion coefficient of the drug in the matrix.4 When Sw » 1, Fickian drug diffusion predominates, whereas when Sw « 1, zero-order release kinetics are observed. The amount of drug released from a thin slab is expressed as an exponential:

n varying from ~0.5 to >1.0 where MjM^is the fraction of drug and k is a constant.

It has been seen in Chapter 7 that the use of macromolecules as dispersion stabilisers depends in part on the osmotic forces arising from the interaction of solvated polymer chains as neighbouring particles approach (see Fig. 7.7). It is thus important to know how factors such as temperature and additive affect this interaction. Flory has given the free energy of dilution (the opposite process to the concentration effect discussed in section 7.2)

NaCl Na2SO3

1000 100 10 1 0.1 0.01 Concentration (mmol dm~3)

0 0

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