Co-crystal Concept and Beyond

Unite de Cristallogenese, EA3233 IMR 4114, Universite de Rouen, F-76821 Mont Saint Aignan, France 

In this chapter, views of co-crystals will be broadened. Indeed, any classification has its limits and, clearly, the thermodynamics of heterogeneous equilibria can help to replace the concept of the co-crystal in a more general perspective. 

1 Most Frequent Heterogeneous Equilibria Related to Co-crystals 

When a solid phase is characterized, the independent components of the related system must be defined together with the physical variable(s) applicable to the system. For instance, let us suppose that A, B and C are three independent components - C stands for a solvent - and any new crystallized entities dilferent from the polymorphs of A, B and C are called compounds. These chemical entities, 1-1-0 for AB and in general n-m-p for any ternary compound A BmCp , can have different attributes such as those listed below  

The series of examples given below have been selected for their systematic and general aspects. 

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In recent times the concept of crystal polymorphism has expanded beyond its original boundaries to encompass crystal forms of the same molecule with different molecular partners. These may be solvent molecules in solvates [20], or counterions if the molecule can be made non-neutral (by say proton or electron transfer [21]), or other molecules in co-crystals [22]. It is worth noting that the formation of solvate and hydrated forms is commonly observed during polymorph screening, therefore the use of the term pseudopolymorphism to describe solvate forms of a given molecular crystal ought to be discouraged, at least because solvates may, in turn, be polymorphic [23-25]. 

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