A comparison of morphology and surface energy characteristics of sulfathiazole polymorphs based upon single particle studies

The morphological, adhesion and surface energetic properties of three sulfathiazole polymorphs (III, IV and polymorph I prepared from both acetone and methanol, designated I-ace and I-met, respectively) produced using Nektar supercritical fluid (SCF) technology have been characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM).Surface roughness values for each polymorph were determined at different length scales. At sample sizes less than 1 μm × 1 μm the polymorphs rank in terms of roughness as follows: I-met > I-ace ≅ IV > III. At the larger scales the polymorphs rank in terms of roughness as follows: I-met > III > I-ace ≅ IV.The surface energies for polymorphs determined against graphite (HOPG) and particles of the same polymorph were, respectively, I-met: 0.99 mJ m−2 (S.D. 1.25 mJ m−2), 3.09 mJ m−2 (S.D. 2.67 mJ m−2); I-ace: 309 mJ m−2 (S.D. 329 mJ m−2), 16 mJ m−2 (S.D. 11 mJ m−2); III: 1.17 mJ m−2 (S.D. 1.5 mJ m−2), 5.4 mJ m−2 (S.D. 3.6 mJ m−2); IV: 20.35 mJ m−2 (S.D. 28.5 mJ m−2), 16.8 mJ m−2 (S.D. 9.6 mJ m−2). In terms of surface energies the polymorphs hence rank I-ace > IV > III ≅ I-met (HOPG adhesion measurements) and IV ≅ I-ace > III > I-met (particle cohesion measurements).Consideration of contacting asperities and surface roughness was shown to have limited effect on the surface energies, and instead the differences were ascribed to variations in the surface chemistry as a result of changes in crystallization mechanisms.