Nucleation, Surface Energies and Directed Polymorphism

This concept of a size-dependent phase stability in polymorphic materials is addressed here by confronting this model and testing it against the well-characterised H-bonded compound L-glutamic acid (l-GA) which is a material with two mono-tropically related polymorphic modifications the metastable a-form that transforms into the stable p-form through a solvent-mediated transformation [66]. 

The energies of the facetted molecular clusters of different sizes were plotted as a function of cluster size for both polymorphic forms of l-GA (Fig. 8.6c). The results were fitted with a power law function which enabled calculation of cluster energy for any molecular cluster size. The results revealed that the metastable a-form is a more thermodynamically stable form than the stable p-form at smaller cluster sizes with a crossover point of 240 molecules. 

Therefore, it is concluded that the first nucleating form (when the cluster size is small), according to Ostwald rule, would be the metastable a-form which has become surface stabilised due to its small size hence making it more stable than the stable p-form. hi this respect stable and metastable as used here refer to the bulk crystallographic structures. 

FIGURE 8.6 Facetted molecular clusters of a- (a) and (I-forms (b) and energy minimisation (c) of relaxed facetted clusters of l-GA. Source Adapted from Hammond et al. [70]. Reproduced from Elsevier. 

This work [71] also examines ihe impact of facetted versus non-facetted clusters as well as the variation in the conformation of the glutamic acid molecule as a function of distance from the centre of the nanoparticle. This shows the impact of relaxation and illustrates the possibility of mapping the molecular conformation from solution to surface and eventually bulk conformations. 

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