Handbook of Solvents - George Wypych - ChemTech - Ventech!

44 George Wypych
ject in the Section 2.3.19). As heat of vaporization increases, the solubility parameter also
increases.
Molar volume is a rather speculative, theoretical term. It can be calculated from Avogadro’s
number but it is temperature dependent. In addition, free volume is not taken into
consideration. Molar volume can be expressed as molecular diameter but solvent molecules
are rather non-spherical therefore diameter is often misrepresentation of the real dimension.
It can be measured from the studies on interaction but results differ widely depending on the
model used to interpret results.
2.3.2 BOILING AND FREEZING POINTS
Boiling and freezing points are two basic properties of solvents often included in specifications.
Based on their boiling points, solvents can be divided to low (below 100 o C), medium
(100-150 o C) and high boiling solvents (above 150 o C).
The boiling point of liquid is frequently used to estimate the purity of the liquid. A
similar approach is taken for solvents. Impurities cause the boiling point of solvents to increase
but this increase is very small (in the order of 0.01 o C per 0.01% impurity). Considering
that the error of boiling point can be large, contaminated solvents may be
undetected by boiling point measurement. If purity is important it should be evaluated by
some other, more sensitive methods. The difference between boiling point and vapor condensation
temperature is usually more sensitive to admixtures. If this difference is more
than 0.1 o C, the presence of admixtures can be suspected.
The boiling point can also be used to evaluate interactions due to the association
among molecules of solvents. For solvents with low association, Trouton’s rule, given by
the following equation, is fulfilled:
ΔS
o
bp
o
ΔHbp
= = 88Jmol
K
T
bp
−1 −1
[2.3.1]
where:
o
ΔSbp molar change of enthalpy
o
ΔHbp molar change of entropy
Tbp boiling point
If the enthalpy change is high it suggests that the solvent has a strong tendency to form associations.
Boiling point depends on molecular weight but also on structure. It is generally lower
for branched and cyclic solvents. Boiling and freezing points are important considerations
for solvent storage. Solvents are frequently stored under nitrogen blanket and they contribute
to substantial emissions during storage. Freezing point of some solvents is above temperatures
encountered in temperate climatic conditions. Although, solvents are usually very
stable in their undercooled state, they rapidly crystallize when subjected to any mechanical
or sonar impact.
Figures 2.3.2 to 2.3.6 illustrate how the boiling points of individual solvents in a group
are related to other properties. Figure 2.3.2 shows that chemical structure of a solvent affects
the relationship between its viscosity and the boiling point. Alcohols, in particular,
show a much larger change in viscosity relative to boiling point than do aromatic hydrocarbons,
esters and ketones. This is caused by strong associations between molecules of alcohols,
which contain hydroxyl groups. Figure 2.3.3 shows that alcohols are also less volatile