Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.3 Basic physical and chemical properties 43
is returned. In the second case, a list of solvents and their values for the selected property are
given in tabular form in ascending order of the property in question.
2.3.1 MOLECULAR WEIGHT AND MOLAR VOLUME
The molecular weight of a solvent is a standard but underutilized component of the information
on properties of solvents. Many solvent properties depend directly on their molecular
weights. The hypothesis of Hildebrand-Scratchard states that solvent-solute interaction occurs
when solvent and polymer segment have similar molecular weights. This is related to
the hole theory according to which a solvent occupying a certain volume leaves the same
volume free when it is displaced. This free volume should be sufficient to fit the polymer
segment which takes over the position formerly occupied by the solvent molecule.
Based on this same principle, the diffusion coefficient of a solvent depends on its molecular
mass (see equations [6.2] and [6.3]). As the molecular weight of a solvent increases
its diffusion rate also increases. If there were no interactions between solvent and solute, the
evaporation rate of the solvent would depend on the molecular weight of the solvent. Because
of various interactions, this relationship is more complicated but solvent molecular
weight does play an essential role in solvent diffusion. This is illustrated best by membranes
which have pores sizes which limit the size of molecules which may pass through. The resistance
of a material to solvents will be partially controlled by the molecular weight of the
solvent since solvent molecules have to migrate to the location of the interactive material in
order to interact with it.
The chemical potential of a solvent also depends on its molecular weight (see eq.
[6.6]). If all other influences and properties are equal, the solvent having the lower molecular
weight is more efficiently dissolving materials, readily forms gels, and swells materials.
All this is controlled by the molecular interactions between solvent and solute. In other
words, at least one molecule of solvent involved must be available to interact with a particular
segment of solute, gel, or network. If solvent molecular weight is low more molecules
per unit weight are available to affect such changes. Molecular surface area and molecular
volume are part of various theoretical estimations of solvent properties and they are in part
Hildebrand solubility parameter,δ, calcm -3
25
20
15
10
5
0 500 1000 1500 2000 2500
Heat of vaporization, kJ kg -1
Figure 2.3.1. Hildebrand solubility parameter vs. heat of
vaporization of selected solvents.
dependent on the molecular mass of the solvent.
Many physical properties of solvents
depend on their molecular weight, such as
boiling and freezing points, density, heat of
evaporation, flash point, and viscosity. The
relationship between these properties and
molecular weight for a large number of solvents
of different chemical composition is
affected by numerous other influences but
within the same chemical group (or similar
structure) molecular weight of solvent correlates
well with its physical properties.
Figure 2.3.1 gives an example of interrelation
of seemingly unrelated parameters:
Hildebrand solubility parameter and
heat of vaporization (see more on the sub-