The characterization of heterogeneous surfaces using modified immersional calorimetry

Abstract

The present work describes the development and validation of a sensitive modified immersional calorimeter for assessing the integral interfacial energetics and wettability of tablet surfaces by aqueous dispersions of hydroxypropyl methylcellulose and its usefulness in predicting film coating results. Tablets manufactured from microcrystalline cellulose, dibasic calcium phosphate, lactose, and magnesium carbonate gelatin granulations were analyzed. Tablet surfaces were modified by varying the tablet porosity (by changing compression force) and specific surface energy (by adding various levels of magnesium stearate). A thermal gravimetric technique utilizing first derivative plots of drying curves of coating droplets on tablets as well as a kinematic assessment of tablet wettability are also described. Coating of tablets manufactured from the same excipient was performed in a 30 cm diameter Hi-Coater{dollar}\sp{lcub}\rm R{rcub}{dollar} side-vented coating apparatus using a competitive coating operation. The coating parameters were set to a level in which overwetting occurred thus "stressing" the coatability of the tablets. Tablet coating success was assessed by visual analysis, diametrical hardness increases, and with a size exclusion chromatographic (SEC) technique. This technique allowed the amount of coat applied to individual tablets to be determined at coat amounts of less than 1 mg. Immersional results were directly related to the surface area of interaction which depended not only on the porosity and penetration of the coating dispersion but, with low energy tablets, on the relative surface area of the immediate tablet surface. It was hypothesized that an immersional response "window" exists within which tablets will ultimately coat well. Those with larger immersional results will show rapid wetting of the tablet by the coating droplets which will dehydrate the droplets too rapidly for the coating to coalesce, and those with low responses will not allow enough spreading of the droplets to allow adequate coalescence. The comparison of the immersional results with the results of the tablet coating operation to some extent support such an hypothesis although the measurement of the surface area of interaction is essential in estimating the true surface energies of the interface from immersional results.