Perspectives on the Wetting of Solids in Pharmaceutical Systems

The ability of water and aqueous solutions to wet relatively nonpolar pharmaceutical solids during the processing and administration of solid dosage forms is an important part of development.

Various factors, both fundamental and technological, which are important to wettability are reviewed and analyzed. Initially, the ideal thermodynamic importance of liquid surface tension and solid surface energetics, determined by the contact angle and the polarity of the solid surface, are established. Then, emphasis is placed on various factors that change the surface energetics due to crystal defects, polymorphism, varying Miller Indices, crystal habit, amorphous structure, variable surface concentration of components in a formulation mixture, surface roughness, and complex pore structure. Case studies cover single component systems (APIs and excipients), binary mixtures (amorphous solid dispersions and physical mixtures), multicomponent systems (granules and tablets), as well as disintegration and dissolution of solid oral dosage forms.

This perspective and analysis indicates the primary importance of understanding and modifying solid surface energetics, surface chemical and physical heterogeneities, and pore structure to promote wettability in pharmaceutical systems.