Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.1 Solvent effects on chemical systems 11
whose molecules interact superficially, as in the case of the hydrocarbons. At all events,
there is no alternative to meeting the challenge face to face.
If we mix a solute and a solvent, both being constituted by chemically saturated molecules,
their molecules attract one another as they approach one another. This interaction can
only be electrical in its nature, given that other known interactions are much more intense
and of much shorter range of action (such as those which can be explained by means of nuclear
forces) or much lighter and of longer range of action (such as the gravitational force).
These intermolecular forces usually also receive the name of van der Waals forces, from the
fact that it was this Dutch physicist, Johannes D. van der Waals (1837-1923), who recognized
them as being the cause of the non-perfect behavior of the real gases, in a period in
which the modern concept of the molecule still had to be consolidated. The intermolecular
forces not only permit the interactions between solutes and solvents to be explained but also
determine the properties of gases, liquids and solids; they are essential in the chemical transformations
and are responsible for organizing the structure of biological molecules.
The analysis of solute-solvent interactions is usually based on the following partition
scheme:
ΔE = ΔE + ΔE + ΔE
[2.1.1]
i ij jj
where i stands for the solute and j for the solvent.This approach can be maintained while the
identities of the solute and solvent molecules are preserved. In some special cases (see below
in specific interactions) it will be necessary to include some solvent molecules in the
solute definition.
The first term in the above expression is the energy change of the solute due to the
electronic and nuclear distortion induced by the solvent molecule and is usually given the
name solute polarization. ΔE ij is the interaction energy between the solute and solvent molecules.
The last term is the energy difference between the solvent after and before the introduction
of the solute. This term reflects the changes induced by the solute on the solvent
structure. It is usually called cavitation energy in the framework of continuum solvent models
and hydrophobic interaction when analyzing the solvation of nonpolar molecules.
The calculation of the three energy terms needs analytical expressions for the different
energy contributions but also requires knowledge of solvent molecules distribution around
the solute which in turn depends on the balance between the potential and the kinetic energy
of the molecules. This distribution can be obtained from diffraction experiments or more
usually is calculated by means of different solvent modelling. In this section we will comment
on the expression for evaluating the energy contributions. The first two terms in equation
[2.1.1] can be considered together by means of the following energy partition :
ΔE + ΔE = ΔE + ΔE + ΔE
− [2.1.2]
i ij el pol d r
Analytical expressions for the three terms (electrostatic, polarization and dispersion-repulsion
energies) are obtained from the intermolecular interactions theory.
2.1.2.1 Electrostatic
The electrostatic contribution arises from the interaction of the unpolarized charge distribution
of the molecules. This interaction can be analyzed using a multipolar expansion of the
charge distribution of the interacting subsystems which usually is cut off in the first term