Wettability and surface chemistry of crystalline and amorphous forms of a poorly water soluble drug

Abstract

The present study compares energetics of wetting behavior of crystalline and amorphous forms of a poorly water soluble drug, celecoxib (CLB) and attempts to correlate it to their surface molecular environment. Wettability and surface free energy were determined using sessile drop contact angle technique and water vapor sorption energetics was measured by adsorption calorimetry. The surface chemistry was elucidated by X-ray photoelectron spectroscopy (XPS) and crystallographic evaluation. The two solid forms displayed distinctly different wetting with various probe liquids and in vitro dissolution media. The crystalline form surface primarily exhibited dispersive surface energy (47.3 mJ/m 2), while the amorphous form had a slightly reduced dispersive (45.2 mJ/m 2) and a small additional polar (4.8 mJ/m 2) surface energy. Calorimetric measurements, revealed the amorphous form to possess a noticeably high differential heat of absorption, suggesting hydrogen bond interactions between its polar energetic sites and water molecules. Conversely, the crystalline CLB form was found to be inert to water vapor sorption. The relatively higher surface polarity of the amorphous form could be linked to its greater oxygen-to-fluorine surface concentration ratio of 1.27 (cf. 0.62 for crystalline CLB), as determined by XPS. The crystallographic studies of the preferred cleavage plane (0 2 0) of crystalline CLB further supported its higher hydrophobicity. In conclusion, the crystalline and amorphous forms of CLB exhibited disparate surface milieu, which in turn can have implications on the surface mediated events.