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

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 γSVd was found to vary between 39.5 mJ/m2 and 44.1 mJ/m2 for contact angle and between 40 mJ/m2 and 49 mJ/m2 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 γSVd of ∼34 mJ/m2. It was shown that both IGC and contact angle techniques are able to detect surface chemical variations and detailed surface energetic distribution.