Handbook of Solvents - George Wypych - ChemTech - Ventech!

62 George Wypych
Thermal conductivity of solvents is an important property which determines the heat
transfer in a solvent or solution and influences the evaporation rate of solvents as a solution
is being dried.
Activity coefficients may be applied to different processes. In one application, the activity
coefficient is a measure of the escaping tendency from liquid to another liquid or a
gaseous phase (in the liquid to gas phase they can be quantified using Henry’s Law coefficient).
These activity coefficients are derived from distillation data at temperatures near the
boiling point or from liquid-liquid extraction calculations. In another application as defined
by Hildebrand and Scratchard solvent activity to dissolve a non-electrolyte solute is given
by equation:
lnf V
=
( δ − δ )
m solute sovent
RT
2
[2.3.15]
where:
f activity coefficient
Vm molar volume of solute
δsolute solubility parameter of solute
δsolvent solubility parameter of solvent
RT gas constant x temperature
This coefficient is used to express rate constants of bimolecular reactions.
Azeotropes. One solvent may form azeotropes with another solvent due to molecular
association. This physical principle can be exploited in several ways. The most important in
solvent applications is the possibility of reducing the boiling temperature (some azeotropes
have lower boiling point) therefore an applied product such as a coating may lose its solvents
and dry faster. The formation of such azeotrope also lowers flash point by which it increases
hazards in product use. The formation of an azeotrope is frequently used to remove
water from a material or a solvent. It affects the results of a distillation since azeotrope formation
makes it difficult to obtain pure components from a mixture by distillation.
Azeotrope formation can be suppressed by lowering the boiling point (distillation under
vacuum). One benefit of azeotropic distillation is the reduction in the heat required to evaporate
solvents.
Henry’s constant is a measure of the escaping tendency of a solvent from a very dilute
solution. It is given by a simple equation: Henry’s constant = p ×φ, where p is the pressure
of pure solvent at the solution temperature and φ is the solvent concentration in the
liquid phase. A high value of Henry’s constant indicates that solvent can be easily stripped
from dilute water solution. It can also be used to calculate TLV levels by knowing concentration
of a solvent in a solution according to the equation: TLV (in ppm) = [18 H (concentration
of solvent in water)]/ molecular weight of solvent.
pH and corrosivity. The pH of solvents is of limited value but it is sometimes useful if
the solvent has strong basic or acidic properties which could cause corrosion problems.
The acid dissociation constant is the equilibrium constant for ionization of an acid
and is expressed in negative log units.
The color of a solvent may influence the effect of solvent on the final product and allow
the evaluation of solvent quality. The colorless solvents are most common but there are
many examples of intrinsically colored solvents and solvents which are colored because of
an admixture or inadequate storage conditions or too long storage.