Effect of crystal habits on the surface energy and cohesion of crystalline powders

Abstract

The role of surface properties, influenced by particle processing, in particle-particle interactions (powder cohesion) is investigated in this study. Wetting behaviour of mefenamic acid was found to be anisotropic by sessile drop contact angle measurements on macroscopic (>1cm) single crystals, with variations in contact angle of water from 56.3° to 92.0°. This is attributed to variations in surface chemical functionality at specific facets, and confirmed using X-ray photoelectron spectroscopy (XPS). Using a finite dilution inverse gas chromatography (FD-IGC) approach, the surface energy heterogeneity of powders was determined. The surface energy profile of different mefenamic acid crystal habits was directly related to the relative exposure of different crystal facets. Cohesion, determined by a uniaxial compression test, was also found to relate to surface energy of the powders. By employing a surface modification (silanisation) approach, the contribution from crystal shape from surface area and surface energy was decoupled. By "normalising" contribution from surface energy and surface area, needle shaped crystals were found to be ∼2.5× more cohesive compared to elongated plates or hexagonal cuboid shapes crystals.

Effect of milling temperatures on surface area, surface energy and cohesion of pharmaceutical powders

“…Recently, we have presented surface energy versus surface coverage plots to demonstrate level of surface heterogeneity of pharmaceutical materials as a function of milling intensity (Shah et al, 2014a). Furthermore, data for comparing surface energy heterogeneity before and after silanisation is reported, evidently suggesting that surfaces are homogeneous post silanisation (Shah et al, 2014a;Shah et al, 2014b;Shah et al, 2014c ).…”

“…For determining dispersive surface energy, a series of dispersive n-alkane probes were injected at a range of concentrations to achieve fractional surface coverages (n/nm) ranging from 0.007 to 0.1. For surface energy measurements to be representative of the entire material surface properties, typical fractional coverages used for analysis ranges from n/nm=0.02 to 0.05 (Gamble et al, 2013;Gamble et al, 2012;Shah et al, 2014a;Shah et al, 2014b). Considering the range of fractional surface coverages typically used to provide material representative surface energy, n/nm=0.04 was selected for analysis of both silanised and unsilanised materials.…”

“…A uniaxial compression test, established for determining the unconfined yield stress for soil mechanics application (Head, 1994) and recently adapted for pharmaceutical powders (Shah et al, 2014b), was used for determination of powder cohesion. BA powder was poured into a 5 mm diameter evacuable IR die (Specac Ltd., Slough, UK) and consolidated to prepare a loosely packed powder compact.…”

“…The angle of internal friction can be calculated from the gradient of regression line. Theoretical principles of this method can be found in the literature (Head, 1994;Shah et al, 2014b).…”

“…Even though surface area for the sample material milled at room temperature is significantly lower compared to BA milled at cryogenic temperatures, cohesion for BA milled at room temperature and cryogenic temperature are very similar. An approach for decoupling contributions from surface energy, particle shape and surface area on cohesion has been reported recently (Shah et al, 2014a;Shah et al, 2014b) and thought to provide further insight into contribution of different surface attributes on the BA milled at different temperatures.…”