Characterization by molar mass determination

When it comes to analyzing and characterizing chemical substances, elemental chemical analysis and, above all, the determination of molar mass are essential for identifying the molecule. In classical organic (or inorganic) chemistry, compounds are made up of atoms of a well-defined number and nature. They also have well-defined physical properties: density, melting temperature, boiling temperature, etc., and are characterized by a unique and generally low molar mass (less than 1,000 or 2,000). Determining their molar mass or mass per mole, i.e. 6.022 × 10 ²³ molecules, is easily done by analyzing the thermodynamic properties of binary solutions of the substance under study in solution (at low concentration) in a given solvent. Methods derived from these thermodynamic properties (so-called colligative methods), such as cryoscopy, ebulliometry or tonometry, are ideally suited to this task.

In macromolecular chemistry (plastics, rubber, water-soluble polymers or biopolymers: proteins, nucleic acids, viruses, etc.), the problem of determining molar mass is more complex. In most cases, these macromolecular substances have relatively large dimensions and relatively high molar masses, so that the colligative methods mentioned above are practically unusable. What's more, with rare exceptions, macromolecular compounds exhibit significant mass heterogeneity, defined by the polymolecularity of the substance. In contrast to well-defined chemical species, there is a more or less broad distribution of molar masses, hence the notion of distribution curves and average molar mass values. In order to access these different molecular parameters, it has been necessary to develop a number of methods more suited to this type of substance, such as hydrodynamic methods (viscosity, steric exclusion chromatography, etc.) or radiation methods: interaction between matter and radiation (multi-angular light scattering, etc.). These two groups of techniques are generally combined to deliver better results.

The aim of this article is not to describe in detail the principle, let alone the practice, of these different methods, but to give a general overview, in particular to specify the type of molecular parameters and the accessible mass range, as well as their advantages, disadvantages and difficulties. Some practical examples of application to specific macromolecular substances widely used in macromolecular chemistry are given.