SURFACE CHEMICAL ASPECTS OF AQUEOUS POLYMER FILM COATING (CONTACT ANGLES, ADHESION, CELLULOSIC ETHERS, SURFACTANTS, SOLUTE ADSORPTION)

The surface chemical aspects of polymer film coating of pharmaceuticals have been studied by examining the equilibrium wetting properties of aqueous polymer solutions and adhesion with various well-defined solids having different surface polarities, paraffin, polyethylene, polymethylmethacrylate, and polyethylene terephthalate.;Using a well-controlled adhesion test, a relatively linear relationship of adhesive failure with dry film thickness was obtained for thin films. An adhesion parameter, A(,o), in units of force per unit area, was determined by extrapolating to zero film thickness and was shown to be proportional to the calculated work of adhesion at the film-substrate interface.;Randomly roughened solid surfaces were shown to produce significant contact angle hysteresis by affecting the receding angle much more than the advancing angle, suggesting that advancing angles in such systems can be taken as reasonable estimates of the intrinsic contact angle. Receding angles of several compressed drugs were found to be zero degrees, implying that in pharmaceutical systems the aqueous coating solution will not recede over a "rough" tablet surface during solvent evaporation. Advancing contact angles and surface tensions of aqueous solutions were analyzed using the Gibbs and Young equations to determine relative adsorption of solute to the solid-liquid and liquid-vapor interfaces, assuming no solute adsorption to the solid-vapor interface. Equal adsorption was observed for all solutions studied on paraffin and for "hydrophilic" polymers, polyvinyl pyrrolidone, polyethylene glycol, and hydroxyethyl cellulose, on the more polar solids. In contrast, solutions of "hydrophobic" polymers, hydroxypropyl cellulose and hydroxypropyl methylcellulose, and surfactants wetted semipolar solids less than would be predicted by assuming equal adsorption. Solute adsorption to paraffin was nonspecific and readily reversible as comparable hysteresis to that for pure liquids was obtained. Similar behavior was observed for "hydrophilic" polymer solutions. "Hydrophobic" polymer solutions produced zero degree receding angles, as did surfactants above a certain concentration. When zero degree receding angles were observed, there was evidence that a residual "adsorbed" film was being left behind.