2. Discussion of the physical chemistry of aggregates

Aggregates appear as a particular state of matter, in that they present both similarities and fundamental differences with other states of matter. It is therefore legitimate to ask whether the physical quantities characteristic of matter (such as the notion of temperature, the definition of a melting temperature, etc.) correctly describe aggregates or whether, on the contrary, new quantities need to be defined. The answer is not immediate and remains a matter of debate, despite a great deal of work, some of which is presented in the rest of this article.