Structural and Thermal Characterization of Mono- and Diacyl Polyoxyethylene Glycol by Infrared Spectroscopy and X-ray Diffraction Coupled to Differential Calorimetry

Physical and thermal properties of polyoxyethylene glycol glycerides (Gelucire 50/13) used as sustained-release matrix forming agent in pharmaceutical applications are studied by coupled time-resolved synchrotron X-ray diffraction and differential scanning calorimetry combined with infrared spectroscopy. With these techniques, all polymorphs formed after various thermal treatments from quenching to slow crystallization are characterized. In particular, the fatty esters of polyoxyethylene glycol (PEG) polymorph conformations are all provided by the same lamellar phase with the PEG chains under a helical conformation but with a more or less important tilt of the PEG chains. The rate of crystallization was crucial for the formation of these polymorphs. A slow crystallization rate (0.1 °C/min) will induce the formation of a 120 Å lamellar phase presenting a higher melting point and consequently a better stability in time and in temperature. Inversely, a rapid crystallization rate will induce the formation of a 90 Å lamellar phase. We show, however, the possibility of a rapid transition toward the most stable form. Possible interdigitation of fatty acid chains is also discussed.