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AAPS PharmSciTech. 2006 December; 7(4): E12–E20.
Published online 2014 March 30. doi:  10.1208/pt070484
PMCID: PMC2750321

Anisotropic surface chemistry of crystalline pharmaceutical solids


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 (>1 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 0 waterwere 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 0 most hydrophobic facet. This anisotropic wetting behavior was established for a range of crystalline systems: paracetamol polymorphs, aspirin, and ibuprofen racemates. θa was 0 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.

Keywords: Wetting, contact angles, surface chemistry, crystals, physicochemical properties

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Selected References

These references are in PubMed. This may not be the complete list of references from this article.
1. Williams D. Inverse gas chromatography. In: Ishida H, editor. Characterisation of Composite Materials. London, UK: Butterworth-Heinemann; 1994. pp. 80–104.
2. Good RJ. Contact angles and the surface free energy of soolids. In: Good RJ, Stromberg RR, editors. Surface and Colloid Science. New York, NY: Plenum Press; 1979. pp. 1–29.
3. Roselman IC, Tabor D. The friction of carbon fibres. J Phys D Appl Phys. 1976;9:2517–2532. doi: 10.1088/0022-3727/9/17/012. [Cross Ref]
4. Zhang D, Flory JH, Panmai S, Batra U, Kaufman MJ. Wettability of pharmaceutical solids: its measurement and influence on wet granulation. Colloids Surf A. 2002;206:547–554. doi: 10.1016/S0927-7757(02)00091-2. [Cross Ref]
5. Etzler FM. Characterization of surface free energies and surface chemistry of solids. In: Mittal KL, editor. Contact Angle, Wettability and Adhesion. Utrecht, The Netherlands: VSP; 2003. pp. 1–46.
6. Kwok DY, Neumann AW. Contact angle techniques and measurements. In: Milling AJ, editor. Surface Characterization Methods: Principles. Techniques and Applications. New York, NY: Marcel Dekker; 1999. pp. 37–86.
7. Buckton G. Assessment of the wettability of pharmaceutical powders. In: Mittal KL, editor. Contact Angle, Wettability and Adhesion. Utrecht. The Netherlands: VSP; 1993. pp. 437–451.
8. Duncan-Hewitt W, Nisman R. Investigation of the surface free energy of pharmaceutical materials from contact angle, sedimentation, and adhesion measurements. In: Mittal KL, editor. Contact Angle, Wettability and Adhesion. Utrecht, The Netherlands: VSP; 1993. pp. 791–811.
9. Hooton JC, German CS, Allen S, et al. Characterization of particle-interactions by atomic force microscopy: effect of contact area. Pharm Res. 2003;20:508–514. doi: 10.1023/A:1022684911383. [PubMed] [Cross Ref]
10. Hooton JC, German CS, Allen S, et al. An atomic force microscopy study of the effect of nanoscale contact geometry and surface chemistry on the adhesion of pharmaceutical particles. Pharm Res. 2004;21:953–961. doi: 10.1023/B:PHAM.0000029283.47643.9c. [PubMed] [Cross Ref]
11. Grimsey IM, Osborn JC, Doughty SW, York P, Rowe RC. The application of molecular modelling to the interpretation of inverse gas chromatography data. J Chromatogr A. 2002;969:49–57. doi: 10.1016/S0021-9673(02)00898-1. [PubMed] [Cross Ref]
12. Schultz J, Lavielle L, Martin C. The role of the interface in carbon fibre-epoxy composites. J Adhes. 1987;23:45–60. doi: 10.1080/00218468708080469. [Cross Ref]
13. Bartell FE, Osterhof HJ. Determination of the wettability of a solid by a liquid. Ind Eng Chem. 1927;19:1277–1280. doi: 10.1021/ie50215a026. [Cross Ref]
14. Labajos-Broncano L, Gonzalez-Martyn ML, Bruque JM, Gonzalez-Garcya CM, Janczuk B. On the use of Washburn's equation in the analysis of weight-time measurements obtained from imbibition experiments. J Colloid Interface Sci. 1999;219:275–281. doi: 10.1006/jcis.1999.6492. [PubMed] [Cross Ref]
15. Shi SQ, Gardner DJ. A new model to determine contact angles on swelling polymer particles by the column wicking method. J Adhes Sci Technol. 2000;14:301–314. doi: 10.1163/156856100742564. [Cross Ref]
16. Prestidge CA, Ralston J. Contact angle studies of ethyl xanthate coated galena particles. J Colloid Interface Sci. 1996;184:512–518. doi: 10.1006/jcis.1996.0646. [PubMed] [Cross Ref]
17. Prestidge CA, Ralston J. Contact angle studies on galena particles. J Colloid Interface Sci. 1995;172:302–310. doi: 10.1006/jcis.1995.1256. [Cross Ref]
18. Conder JR, Young CL. Physicochemical Measurement by Gas Chromatography. Chichester, UK: Wiley; 1979.
19. Kiselev AV, Yashin YI. Gas-Adsorption Chromatography. London, UK: Plenum Press; 1969.
20. Grimsey IM, Feeley JC, York P. Analysis of the surface energy of pharmaceutical powders by inverse gas chromatography. J Pharm Sci. 2002;91:571–583. doi: 10.1002/jps.10060. [PubMed] [Cross Ref]
21. Newell HE, Buckton G, Butler DA, Thielmann F, Williams DR. The use of inverse phase gas chromatography to measure the surface energy of crystalline, amorphous, and recently milled lactose. Pharm Res. 2001;18:662–666. doi: 10.1023/A:1011089511959. [PubMed] [Cross Ref]
22. York P, Ticehurst MD, Osborn JC, Roberts RJ, Rowe RC. Characterisation of the surface energetics of milled dl-propranolol hydrochloride using inverse gas chromatography and molecular modelling. Int J Pharm. 1998;174:179–186. doi: 10.1016/S0378-5173(98)00247-6. [Cross Ref]
23. Heng JYY, Pearse DF, Wilson DA, Williams DR. Characterization of solid state materials using vapor sorption methods. In: Zakrzewski A, Zakrzewski M, editors. Solid State Characterization of Pharmaceuticals. Danbury, CT: ASSA International; 2006.
24. Heng JYY, Thielmann F, Williams DR. The effects of milling on the surface properties of form I paracetamol crystals. Pharm Res. 2006;23:1918–1927. doi: 10.1007/s11095-006-9042-1. [PubMed] [Cross Ref]
25. Heng JYY, Bismarck A, Lee AF, Wilson K, Williams DR. Anisotropic surface energetics and wettability of macroscopic form I paracetamol crystals. Langmuir. 2006;22:2760–2769. doi: 10.1021/la0532407. [PubMed] [Cross Ref]
26. Gao LC, McCarthy TJ. Contact angle hysteresis explained. Langmuir. 2006;22:6234–6237. doi: 10.1021/la060254j. [PubMed] [Cross Ref]
27. Heng JYY, Williams DR. Wettability of paracetamol polymorphic forms I and II. Langmuir. 2006;22:6905–6909. doi: 10.1021/la060596p. [PubMed] [Cross Ref]
28. Heng JYY, Bismarck A, Lee AF, Wilson K, Williams DR. Anisotropic surface chemistry of aspirin crystals. J Pharm Sci. In press. [PubMed]
29. Carvajal MT, Staniforth JN. Interactions of water with the surfaces of crystal polymorphs. Int J Pharm. 2006;307:216–224. doi: 10.1016/j.ijpharm.2005.10.006. [PubMed] [Cross Ref]
30. Muster TH, Prestidge CA. Face specific surface properties of pharmaceutical crystals. J Pharm Sci. 2002;91:1432–1444. doi: 10.1002/jps.10125. [PubMed] [Cross Ref]

Articles from AAPS PharmSciTech are provided here courtesy of American Association of Pharmaceutical Scientists