MODELING MUCOADHESION BY USE OF SURFACE-ENERGY TERMS OBTAINED BY THE LEWIS-ACID LEWIS-BASE APPROACH

The surface energies of carbopol and mucin have been assessed from con tact angle and surface tension measurements. The surface energy has be en considered in terms of an apolar Litshitz-van der Waals term and a polar acid-base term, which in turn is divided into electron donor and electron acceptor (Lewis acid-Lewis base) contributions. Using these surface energy components, the interaction of dry polymer, and hydrate d polymer in the presence of simulated gastric fluid, intestinal fluid and saline, with mucin gel has been predicted. The predicted behaviou r has been compared with measured mucoadhesion performance. It was fou nd that the surface energy of the carbopol changed when hydrated. The surface energy of the dry polymer gave a better correlation with mucoa dhesive strength after a short contact time. The surface energy of the hydrated polymer was found to give the best correlation with mucoadhe sion after a longer contact time. This implies that the interpenetrati on stage of mucoadhesion has a driving force which is related to inter facial phenomena. The carbopol, but not the mucin, was found to show c hanges in surface energy as a function of pH. These differential chang es correlated with changes in mucoadhesive behaviour in the different model media.