Polymorphism; Basic principles and characterization methods

In his book "The Logic of Living Things" , François Jacob quotes Jean Perrin as saying that "in the world of living things, as elsewhere, it is always a question of explaining the complicated visible by the simple invisible". But in beings as in things, it's an invisible with "drawers". There is no single organization in the living world, but rather a series of interlocking organizations, like Russian dolls.

While biology has revealed the infinite diversity of living forms over the last few centuries, and particularly over the one that has just ended, even though they all originate from a limited number of atoms, physics and chemistry are not to be outdone. They have revealed to us other Russian dolls included in those representing the living world and concerning atoms and molecules.

A spectacular and often-quoted example of how a (relatively simple) entity can, in its solid state, crystallize in different spatial arrangements, is that of carbon. Remember that, depending on how the atoms are distributed, this can give rise to diamond or graphite. It's hardly worth emphasizing just how different the physicochemical properties of these two entities are, simply because the same atom is spatially distributed differently in the solid state.

In the case of simple bodies, the ability of atoms to exist in the solid state in different crystalline forms is called allotropy. The same is true of inorganic and organic molecules. This is known as polymorphism. A more precise definition of this phenomenon is given below.

In most cases, the different crystalline forms obtained for the same molecule will have more or less different physical and chemical properties.

As soon as a field of technical activity involves atomic or molecular entities (mineral or organic), the consequences of polymorphism need to be considered at an early stage, if we are to avoid problems as different as the withdrawal of a drug from the market or the ageing of chocolate or margarine, for example.

For a more theoretical study of polymorphism, readers can consult the articles "Crystallization and polymorphism" [AF 3 640] ,