Handbook of Solvents - George Wypych - ChemTech - Ventech!

Interactions in Solvents and
Solutions
Jacopo Tomasi, Benedetta Mennucci, Chiara Cappelli
Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Italy
8.1 SOLVENTS AND SOLUTIONS AS ASSEMBLIES OF INTERACTING
MOLECULES
A convenient starting point for the exposition of the topics collected in this chapter is given
by a naïve representation of liquid systems.
According to this representation a liquid at equilibrium is considered as a large assembly
of molecules undergoing incessant collisions and exchanging energy among colliding
partners and among internal degrees of freedom. The particles are disordered at large scale,
but often there is a local order that fades away. Solutions may be enclosed within this representation.
The collection of particles contains at least two types of molecules − those having a
higher molar fraction are called the solvent −the others the solute. Collisions and exchange
of energy proceed as in pure liquids, local ordering may be different, being actually dependent
on the properties of the molecule on which attention is focused, and on those of the
nearby molecules.
Liquids at a boundary surface require more specifications to be enclosed in the representation.
There is another phase to consider which can be either a solid, another liquid, or a
gas. Further specifications must be added to characterize a specific boundary system (see
Section 8.8), but here it is sufficient to stress that the dynamic collision picture and the occurrence
of local ordering is acceptable even for the liquid portion of a boundary.
This naïve description of liquid systems actually represents a model −the basic model
to describe liquids at a local scale. As it has been here formulated, it is a classical model: use
has been made of physical classical concepts, as energy, collisions (and, implicitly, classical
moments), spatial ordering (i.e., distribution of elements in the space).
It is clear that the model, as formulated here, is severely incomplete. Nothing has been
said about another aspect that surely has a remarkable importance even at the level of naïve
representations: molecules exert mutual interactions that strongly depend on their chemical
composition.
To say something more about molecular interactions, one has to pass to a quantum description.
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