Physicochemical attributes of white petrolatum from various sources used for ophthalmic ointment formulations

Excipients from different sources may lead to significant differences in the performance of drug products, posing challenges in product quality control. A previous report showed that the drug release rates from ophthalmic ointments were affected by source variation of white petrolatum. To understand the physicochemical properties including the microstructure of white petrolatum and the impact of this on the performance of finished products, the following were investigated: rheological properties, thermal analysis (differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA)), as well as structural characteristics (polarized light microscopy (PLM), small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS)). The rheological parameters may be indicative of the drug release rate of the finished ophthalmic ointments when using a process involving hot-melting with immediate cooling. The white petrolatum from the four different sources also showed different thermal behavior. According to SAXS and WAXS, all of the petrolatum showed semi-crystalline behavior with different extents of crystallinity (from 9.5% to 16.9%). The crystalline domains of the petrolatum are ordered in an orthorhombic structure forming lamellar sheets with a periodicity from 9.1 nm to 9.9 nm. An in-depth understanding of the semi-crystalline structure was obtained and it provided valuable information for formulation optimization and characterization of petrolatum-based products.