Decoupling the contribution of surface energy and surface area on the cohesion of pharmaceutical powders.

PURPOSE: Surface area and surface energy of pharmaceutical powders are affected by milling and may influence formulation, performance and handling. This study aims to decouple the contribution of surface area and surface energy, and to quantify each of these factors, on cohesion.
METHODS: Mefenamic acid was processed by cryogenic milling. Surface energy heterogeneity was determined using a Surface Energy Analyser (SEA) and cohesion measured using a uniaxial compression test. To decouple the surface area and surface energy contributions, milled mefenamic acid was "normalised" by silanisation with methyl groups, confirmed using X-ray Photoelectron Spectroscopy.
RESULTS: Both dispersive and acid-base surface energies were found to increase with increasing milling time. Cohesion was also found to increase with increasing milling time. Silanised mefenamic acid possessed a homogenous surface with a surface energy of 33.1 ± 1.4 mJ/m(2) , for all milled samples. The cohesion for silanised mefenamic acid was greatly reduced, and the difference in the cohesion can be attributed solely to the increase in surface area. For mefenamic acid, the contribution from surface energy and surface area on cohesion was quantified to be 57% and 43%, respectively.
CONCLUSIONS: Here, we report an approach for decoupling and quantifying the contribution from surface area and surface energy on powder cohesion.