A two-dimensional Hartman-Perdok analysis of polymorphic fat surfaces observed with atomic force microscopy

The surfaces of pure triacylglycerol (TAG) crystals of polymorphic type bet a (16.16.16) and beta' (16.18.16) are studied ex situ using atomic force mi croscopy. Molecular contrast images show no reconstructed surfaces. On both types of TAG crystals, the overall 3D morphology is reflected in the shape of the monomolecular step patterns on the large {0 0 1} faces. The shape o f the observed two-dimensional (2D) nuclei and growth spirals on these face s can be explained by a combination of a 2D Hartman-Perdok analysis based o n the topology of the crystallographic surface structure, and the theory of Burton. Cabrera and Frank. This method shows that steps having high bondin g energies sometimes give rise to high kink densities, which may explain th eir absence on the morphology. For the beta polymorph, extended sub-steps w ith half the molecular height were found, which are related to planar fault s in the crystal and do not act as growth source. Melt grown beta' crystals sometimes show vicinal {1 0 1} faces, which indicates a strong interaction between closely separated steps. (C) 2001 Elsevier Science B.V. All rights reserved.