title: Determination of surface heterogeneity of D-mannitol by sessile drop contact angle and finite concentration inverse gas chromatography.
abstract: The sensitivity of two techniques in tracking changes in surface energetics was investigated for a crystalline excipient, D-mannitol. Macroscopic crystals of D-mannitol were grown from saturated water solution by slow cooling, and sessile drop contact angle was employed to measure the anisotropic surface energy. The facet-specific surface energy was consistent with localised hydroxyl group concentrations determined by X-ray photoelectron spectroscopy (XPS), and was also in excellent agreement with the surface energy distribution of the powder form of mannitol measured via a new methodology using inverse gas chromatography (IGC) at finite concentrations. The gamma(SV)(d) was found to vary between 39.5 mJ/m(2) and 44.1 mJ/m(2) for contact angle and between 40 mJ/m(2) and 49 mJ/m(2) for IGC measurements. We report here, a high level of surface heterogeneity on the native mannitol crystal surfaces. When the surfaces of both D-mannitol samples (powder and large single crystals) were modified by dichlorodimethylsilane to induce surface hydrophobicity, both IGC and contact angle revealed a homogeneous surface due to functionalisation of mannitol crystal surface with methyl groups resulting in gamma(SV)(d) of approximately 34 mJ/m(2). It was shown that both IGC and contact angle techniques are able to detect surface chemical variations and detailed surface energetic distribution.
authors.affiliation: Surfaces and Particle Engineering Laboratory, Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, UK.
authors.forename: Raimundo
authors.initials: R
authors.lastname: Ho
authors.forename: Steven J
authors.initials: SJ
authors.lastname: Hinder
authors.forename: John F
authors.initials: JF
authors.lastname: Watts
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Dilworth
authors.forename: Daryl R
authors.initials: DR
authors.lastname: Williams
authors.forename: Jerry Y Y
authors.initials: JY
authors.lastname: Heng
mesh_terms: D002626:Chemistry, Pharmaceutical / Q000379:methods; D002849:Chromatography, Gas / Q000379:methods*; D003460:Crystallization; D005079:Excipients / Q000737:chemistry*; D057927:Hydrophobic and Hydrophilic Interactions; D008353:Mannitol / Q000737:chemistry*; D056951:Photoelectron Spectroscopy / Q000379:methods; D011208:Powders; D012821:Silanes / Q000737:chemistry; D013678:Technology, Pharmaceutical / Q000379:methods
chemical_list: D005079:Excipients; D011208:Powders; D012821:Silanes; D008353:Mannitol; C040863:dichlorodimethylsilane