Spontaneous Crystalline-to-Amorphous Phase Transformation of Organic or Medicinal Compounds in the Presence of Porous Media, Part 2: Amorphization Capacity and Mechanisms of Interaction

Amorphization of crystalline compounds using mesoporous media is a promising technique to improve the solubility and dissolution rate of poorly soluble compounds. The objective of this paper is to determine the capacity of amorphization and understand the mechanisms of phase transformation. Commercial grades of mesoporous silicon dioxide (SiO2) samples (5- to 30-nm mean pore diameters) with either constant surface area or constant pore volume were used. The amorphization capacity of naphthalene was not proportional to either the surface area or the pore volume measured using adsorption chambers. Instead, the amorphization capacity correlated with surface curvature, that is, the smaller the pore diameter and the higher the surface curvature, the greater the amorphization capacity. The change in surface chemistry due to a highly curved surface may be responsible for the enhanced amorphization capacity as well. The amorphization of crystalline compounds was facilitated through capillary condensation, with the decrease in pore volume as the direct experimental evidence. The amorphization capacity was also enhanced by the dipole-dipole or dipole-induced dipole interaction, promoted by the hydroxyl groups on the surface of SiO2. The enthalpy of vapor-solid condensation of crystalline compounds was a useful indicator to predict the rank order of amorphization capacity. (C) 2011 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 100:4674-4686, 2011