Investigation of the surface free energy of pharmaceutical materials from contact angle, sedimentation, and adhesion measurements

The surface free energy of pharmaceutical solids, although not directly measurable, controls processes such as interparticulate bonding during tableting and powder flow and plays an important role in suspension stability. In addition to the traditional contact angle methods, the indirect sedimentation method was used to evaluate the surface free energies of acetaminophen and adipic acid. The effects of pH and solubility of the drug in the liquid systems used in the sedimentation test are discussed. The results were used to predict whether the materials would adhere to hydrophilic and hydrophobic surfaces. While the adhesion of adipic acid was predicted, that of acetaminophen was not. Possible explanations for the failure of the theory are explored.