Limits of the Co-crystal Concept and Beyond
In this chapter, views of co-crystals are 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.
Co-crystals are nothing other than intermediate compounds. Nevertheless, the concept suffers from intrinsic uncertainties due to the numerous situations in which they can occur such as the continuum between solvate and co-crystals, hybrid salt-co-crystals, multi-epitaxy and so on. It is even possible to find the same partners with simultaneously salt-like and co-crystal-like relationships in a given crystal structure. Therefore, rather than creating questionable terms and maintaining endless semantic debates with poor added value, the scientific community should concentrate more on the three long lasting problems related to intermediate compounds listed below:
1. Prediction of intermediate compounds: indeed, apart from statistical trends (
2. Prediction of the attributes of these intermediate compounds: to date the connection between structures and properties is very weak despite the evident interest.
3. How to design a process to obtain those intermediate phases with a good yield and minimum side products? Moreover these processes should fulfil the specifications for the final products such as crystallinity, crystal size distribution, morphology, and so on.
These questions are really meaningful from an industrial perspective and for advancing knowledge in the organic solid state.