title: 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.
authors.affiliation: Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom.
authors.forename: Jerry Y Y
authors.initials: JY
authors.lastname: Heng
authors.forename: Alexander
authors.initials: A
authors.lastname: Bismarck
authors.forename: Adam F
authors.initials: AF
authors.lastname: Lee
authors.forename: Karen
authors.initials: K
authors.lastname: Wilson
authors.forename: Daryl R
authors.initials: DR
authors.lastname: Williams