The surface energy of solids is important in governing a range of solid-liquid and solid-solid interactions, influencing processing-ability of powders to performance of a formulation. This study reports on the determination of facet specific surface energy of crystalline pharmaceutical solids and correlating this to bulk powder properties. Surface energy was determined by; (i) liquid sessile drop contact angle measurements on macroscopic (>1cm) single crystals, (ii) adhesion force measurements by atomic force microscopy on (~100 m), and (iii) infinite dilution inverse gas chromatography measurements (powder). For sessile drop measurements; water, diiodomethane, ethylene glycol and formamide were used as probe liquids [1]. Adhesion force measurement was conducted in a liquid environment and surface energies were calculated from the JKR and DMT models [2]. The retention time for a series of n-alkanes (hexane to decane), with methane as a dead-time probe, was used to calculate retention volumes to determine dispersive component of the surface energy as described by the Schultz approach [3]. Aspirin and paracetamol were used as model pharmaceutical solids. Macroscopic single crystals of aspirin and paracetamol were grown by solvent evaporation from acetone and methanol solutions, respectively. Sessile drop contact angles revealed the anisotropic nature of pharmaceutical solids with the contact angle of water varying from 43^o to 61^o for aspirin and from 16^o to 68^o for paracetamol. Similarly for diiodomethane, ethylene glycol and formamide, the contact angles were found to be facet specific, agreeing very well with literature values [4, 5]. Using the Owens-Wendt model [6], surface energies of aspirin and paracetamol was calculated. The surface energies obtained by sessile drop contact angle was in good agreement with the AFM surface energetics, whilst the IGC value was representative of the higher energetic sites due to preferential interactions (infinite dilution). The surface energy for aspirin and paracetamol reported here, is facet specific. This anisotropicity is attributed to localised surface chemistry and structural morphology. Using the approach here, we are able to correlate single crystal data to powder/bulk properties. These complimentary approaches will provide a detailed description of surface properties of crystalline pharmaceutical solids.

References

[1] C. J. van Oss, Interfacial Forces in Aqueous Media. 1994, New York: Marcel Dekker Inc. [2] H. J. Butt, B. Cappella, M. Kappl, Force measurements with the atomic force microscope: Technique, interpretation and applications, Surface Science Reports (2005), 59, 1–152.
[3] J. Schultz, L. Lavielle, C. Martin, The role of the interface in carbon fibre-spoxy composites. Journal of Adhesion (1987), 23, 45-60.
[4] J. Y. Y. Heng, A. Bismarck, A. F. Lee, K. Wilson and D. R. Williams, Anisotropic Surface Chemistry of Aspirin Single Crystals, Journal of Pharmaceutical Sciences (2007), 96 (8), p2134-2144.
[5] J. Y. Y. Heng, A. Bismarck, A. F. Lee, K. Wilson and D. R. Williams, Anisotropic Wettability of Macroscopic Form I Paracetamol Crystals, Langmuir (2006), 22 (6), p2760-2769.
[6] Owens, D.K. and R.C. Wendt, Estimation of the surface free energy of polymers. Journal of Applied Polymer Science (1969), 13, 1741-1747.

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