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 β (16.16.16) and β′ (16.18.16) are studied ex situ using atomic force microscopy. 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 of the observed two-dimensional (2D) nuclei and growth spirals on these faces can be explained by a combination of a 2D Hartman–Perdok analysis based on the topology of the crystallographic surface structure, and the theory of Burton, Cabrera and Frank. This method shows that steps having high bonding energies sometimes give rise to high kink densities, which may explain their absence on the morphology. For the β polymorph, extended sub-steps with half the molecular height were found, which are related to planar faults in the crystal and do not act as growth source. Melt grown β′ crystals sometimes show vicinal {1 0 l} faces, which indicates a strong interaction between closely separated steps.