Elucidation of the variability in consistency of pharmacopoeia quality petrolatum

Abstract

The Pharmacopeia monograph for petrolatum poorly defines the material’s physical properties. Indeed, differences between petrolatum grades can be substantial; yield stress varies between 65 and 280 Pa which can be compared with the consistency of respectively thin cream or thick ointment. This variation is not only due to differences in composition or refining process but also as a result of different processing; for example, thermal history influences petrolatum structure considerably. Slow cooling of petrolatum resulted in a yield stress of 26 Pa and fast cooling in 79 Pa. X-ray showed that crystallinity was 0.7% for the first cooling case and 1.5% for the second one. Crystallite size was estimated to be 20–50 nm. To investigate if this relatively small difference in crystallinity may induce the difference in consistency, 15 nm SiO₂ particles were added to petrolatum. Indeed, a small increase in SiO₂ concentration led to a major increase in yield stress. This was argued to be due to the small size of the particles, resulting in a large increase in absolute number of particles. The Pharmacopeia does not unambiguously define the pharmaceutical excipient petrolatum. As a consequence, the formulator has to take care of selecting the appropriate grade as well as to carefully control the processing of the material in order to achieve a consistent pharmaceutical product.

Keywords:

• Petrolatum
• crystallinity
• X-ray
• yield stress
• thermal history

Acknowledgements

This work was supported by the R&D tax credit of the Ministry of Economic Affairs, The Netherlands (Grant no. S016053527).

Disclosure statement

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of this article.

Novelty statement

Rheological differences between petrolatum grades have been described before, however these have not been elucidated. Therefore, this work shows how not only composition but also thermal history can influence petrolatum rheological properties significantly.

Additional information

Funding

This work was supported by the R&D tax credit of the Ministry of Economic Affairs, The Netherlands (Grant no. S016053527).