MODELING MUCOADHESION BY USE OF SURFACE-ENERGY TERMS OBTAINED FROM THE LEWIS-ACID LEWIS BASE APPROACH .2. STUDIES ON ANIONIC, CATIONIC, ANDUNIONISABLE POLYMERS

Surface energies of carbopol, chitosan, hydroxypropyl cellulose (HPC) and poly(HEMA) were assessed from contact angle and surface tension ex periments. The surface energy was considered in terms of an apolar Lif shitz-van der Waals term and a polar acid-base term, which in turn is divided into electron donor and electron receptor (Lewis acid-Lewis ba se) contributions. Using these surface energy terms the interaction of dry and hydrated polymer with mucin in the presence of either artific ial gastric or intestinal fluid, or saline was predicted. The predicti ons were related to measured forces of detachment. There was a signifi cant difference between the surface energy on dry and hydrated HPC and also for carbopol; for the other polymers either the surface energy o f the hydrated material was not detectable, or the effect of hydration was minimal. There were good correlations between mucoadhesive streng th and the calculated free energies of interaction between mucin and p olymer in the presence of each of the fluids, for each individual poly mer. Thus, two trends were observed, one for unionisable and the other for ionisable polymers. It is argued that the increased mucoadhesion seen with ionisable polymers (compared with the predicted value based on results of unionisable polymers) is a direct result of the ionic in teraction. No attempt has been made to correct for the ionisation effe ct, but the surface energy predictions provide insight into the mechan ism of the mucoadhesion process. This approach is useful for understan ding and predicting interactions between different materials and biolo gical components.