Probing the dynamics of matrix hydration in the presence of electrolytes

The aim of our work was to probe the mechanisms associated with induced matrix stiffening via textural analysis as a consequence of in situ electrolyte interactions within hydroxypropyl-methylcellulose (HPMC) and polyethylene oxide (PEO) matrices in relation to their role in controlling the release of highly soluble drugs such as diltiazem hydrochloride (>50% water soluble at 25 degrees C). The dynamics of HPMC and PEO matrix swelling during hydration in the presence of appropriate electrolytes intended to induce constant drug release rates from simple monolithic systems are influenced by continuously shifting peripheral matrix stiffening toward the matrix core in a manner dependent on electrolyte content and hydration time. Matrix erosion for HPMC and PEO controls (i.e., without electrolyte) follow linear dissolution kinetics (r2 > 0.97), while formulations with electrolyte characteristically undergo a square root of time decline in weight. The swelling potential of the electrolyte-containing matrices, influenced by the boundary infiltration process, reflected considerable suppression during the first 2 hr of exposure to medium, while subsequent events differed in both polymers. In view of these differences, simultaneous measurements in textural transitions and electrolyte conductivity showed that PEO has a higher affinity for water molecules than does HPMC.