Anisotropic Surface Energetics and Wettability of Macroscopic Form I Paracetamol Crystals

Abstract

Advancing (theta(A)) and receding (theta(R)) contact angles were measured with several probe liquids on the external facets (201), (001), (011), and (110) of macroscopic form I paracetamol crystals as well as the cleaved (internal) facet (010). For the external crystal facets, dispersive surface energies gamma(d) calculated from the contact angles were found to be similar (34 +/- 1 mJ/m(2)), while the polar components varied significantly. Cleaving the crystals exposed a more apolar (010) surface with very different surface properties, including gamma(d) = 45 +/- 1 mJ/m(2). The relative surface polarity (gamma(p)/gamma) of the facets in decreasing order was (001) > (011) > (201) > (110) > (010), which agreed with the fraction of exposed polar hydroxyl groups as determined from C and O 1s X-ray photoelectron spectroscopy (XPS) spectra, and could be correlated with the number of non-hydrogen-bonded hydroxyl groups per unit area present for each crystal facet, based on the known crystal structures. In conclusion, all facets of form I paracetamol crystals examined exhibited anisotropic wetting behavior and surface energetics that correlated to the presence of surface hydroxyl groups.

The Effects of Milling on the Surface Properties of Form I Paracetamol Crystals

“…In complex crystalline materials it is very likely that chemical groups on each crystal facet are dissimilar and thus have very different surface properties. It has been recently reported that the individual crystal facets of paracetamol form I exhibit anisotropic contact angle wettability and some significant variations in both dispersive and polar surface energy for facets (201), (001), (011) and (110) (26). These results were ascribed to variations in the surface hydroxyl group concentrations which were verified with XPS.…”

“…These d SV values are however very similar of those obtained by liquid sessile drop contact angle measurements on the external facets of a macroscopic single crystal. d SV of the external facets of paracetamol crystal as measured by contact angles was 34.4 mJ/m 2 (26).…”

“…Differences have been observed for the liquid sessile drop contact angles on the different crystal facets of paracetamol. The detailed anisotropic wetting behaviour for these single crystals is reported elsewhere (26).…”

Surface Wetting Characteristics of Rubbed Polyimide Thin Films

“…Heng et al (Heng et al, 2006) confirmed that the effect of surface roughness on contact angle hysteresis for single crystalline paracetamol was negligible. For rubbed JASL-9800 films displayed here, the surface roughness is less than 1 nm.…”

Determination of surface heterogeneity of d-mannitol by sessile drop contact angle and finite concentration inverse gas chromatography

“…Using macroscopic crystals, Heng et al (10) were able to measure experimentally the anisotropic surface energetics and wettability of APIs. The facet-specific surface energetics were found to be mainly due to chemical heterogeneities present on the surfaces, with negligible influence on the data due to surface roughness, dissolution, swelling and non-equilibrium effects (11). The approach greatly enhances the quality and quantity of surface energetic and chemical information one can obtain from crystalline pharmaceuticals using a liquid probe technique such as contact angle.…”