چکیده (انگلیسی):

The purpose of this study was to establish the link between
the wetting behavior of crystalline pharmaceutical solids and
the localized surface chemistry. A range of conventional
wetting techniques were evaluated and compared with a novel
experimental approach: sessile drop contact angle measurements
on the individual facets of macroscopic (91 cm) single
crystals. Conventional measurement techniques for determining
surface energetics such as capillary rise and sessile
drops on powder compacts were found not to provide reliable
results. When the macroscopic crystal approach was
used, major differences for advancing contact angles, θa, of
water were observed—as low as 16° on facet (001) and as
high as 68° on facet (010) of form I paracetamol. θa trends
were in excellent agreement with X-ray photoelectron spectroscopy
surface composition and known crystallographic
structures, suggesting a direct relationship to the local surface
chemistry. Inverse gas chromatography (IGC) was further
used to probe the surface properties of milled and
unmilled samples, as a function of particle size. IGC experiments
confirmed that milling exposes the weakest attachment
energy facet, with increasing dominance as particle
size is reduced. The weakest attachment energy facet was
also found to exhibit the highest θa for water and to be the
most hydrophobic facet. This anisotropic wetting behavior
was established for a range of crystalline systems: paracetamol
polymorphs, aspirin, and ibuprofen racemates. θa was
found to be very sensitive to the local surface chemistry. It
is proposed that the hydrophilicity/hydrophobicity of facets
reflects the presence of functional groups at surfaces to form
hydrogen bonds with external molecules.