Essential Materials Science of Crystalline and

2 Essential Materials Science of Crystalline and Amorphous Products 

The production of crystalline materials is governed by the laws of crystal nucleation and growth, which are reasonably well established (see Chapter 4). Complexities arise in multicomponent mixtures, where such 

In the production of solid pharmaceutical preparations, it is important to be aware of the identities of polymorphs that might be formed during the drying process, or during any later processing steps, e.g. milling, compression, extrusion. The pressure-induced formation of specific solvates with superior stabilities has also been reported, e.g. for paracetamol.  

Compared to crystalline materials, the production and handling of amorphous substances are subject to serious complexities. Whereas the formation of crystalline materials can be described in terms of the phase rule, and solid-solid transformations (polymorphism) are well characterised in terms of pressure and temperature, this is not the case for glassy preparations that, in terms of phase behaviour, are classified as unstable . Their apparent stability derives from their very slow relaxations towards equilibrium states. Furthermore, where crystal structures are described by atomic or ionic coordinates in space, that which is not possible for amorphous materials, by definition, lack long-range order. Structurally, therefore, positions and orientations of molecules in a glass can only be described in terms of atomic or molecular distribution functions, which change over time the rates of such changes are defined by time correlation functions (relaxation times). 

Here N is the Avogadro number and Sc is the critical conhgurational entropy, the minimum value of which is given by k In 2, corresponding to only two possible orientations, such as those existing in imperfect crystals of carbon monoxide (C-0 and 0-C) and nitric oxide. 

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