Anisotropic surface energetics and wettability of macroscopic form I paracetamol crystals

Advancing (θ_A) and receding (θ_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 γ^d calculated from the contact angles were found to be similar (34 ± 1 mJ/m²), while the polar components varied significantly. Cleaving the crystals exposed a more apolar (010) surface with very different surface properties, including γ^d = 45 ± 1 mJ/m². The relative surface polarity (γ^p/ γ) 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 ls 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.