Front swelling

Fig. 13. Proposed mechanism explaining slow swelling in unbuffered solutions and faster swelling in buffered solutions. Free H (actually hydronium ion) is Donnan excluded from gel, but protons attached to acidic buffers can be carried to the swelling front, where proton transfer to amine groups occurs.

Fig. 15. Conceptualization of processes leading to amine protonation and gel swelling at swelling front. Initially proton attached to carrier diffuses from outer solution (I) to vicinity of front (II). Transfer of proton to amine occurs when amine is still in unhydrated region (III) this represents a transition state. Upon protonation the amine moves into hydrated portion of gel (IV). Plotted is the free energy G at different stages. Activation free energy is AG. This figure illustrates case where proton is attached to a monoacidic buffer. Proton can also be in form of hydronium ion, with accompanying counterion.

The swelling front between the gel phase and the glassy (or semicrystalline) phase of the matrix... 

Harland et al.12 developed a model for drug release based on mass balances of the drug and the solvent at the swelling front and the erosion front. The release profile was found to be a combination of Fickian and zero order, as shown by Eq. (5.16) ... 

The drug and polymer concentrations at the swelling front R are given by ... 

The linear dependence of uptake on time results from the existence of a cleaily dis-cernable propagating solvent-induced swelling front which moves thrmi the pconstant velocity. The polymer ahead of the front is largely free of penetrant, while the polymer behind the front has essentially reached its equilibrium swelling value corresponding to the temperature and pressure of the experiment. For Fickian... 

The rate and characteristics of swelling play an important role in the characterization of polymers. However, in traditional swelling experiments, detection of the swelling front and investigations of its influence on the polymer dynamics are difficult, while the characterization of these features plays a significant role in distinguishing between so-called Case I or Fickian and Case 11 diffusion [134]. 

The synchronization of the two fronts is rapidly achieved with PVAL and soon produces a zero-order release. In the system containing HPMC, the synchronization is still not reached when, at approximately 300 min, the swelling front stops. Nevertheless, from that moment, drug release is constant because the system behaves as a conventional erodible system. 

Fig. 8. Diagrammatic representation of a swelling polymeric system in contact with a penetrant indicating the initial position of the tablet surface a, the dissolution medium/swollen polymer front (eroding front) S, and the rubbery-glassy front (swelling front) R. The coordinate 0 is the center of the tablet and x is the distance coordinate [78]...

---