Wettability of pharmaceutical solids: estimates of solid surface polarity

Contact angle measurements of water and methylene iodide with various organic solids were used to estimate the surface free energy per square centimeter, gammaS, and the contributions of nonpolar and polar forces, gammaSd and gammaSp, respectively. The ratio of gammaSp to gammaS was used as an estimate of solid surface polarity, and values ranging from 0 to 42% polarity were calculated for materials of pharmaceutical interest. Surface free energies per mole were calculated to compare the influence of different substituent groups on wettability. This approach offers a convenient means to quantitate the polarity of organic pharmaceutical solid surfaces.