Handbook of Solvents - George Wypych - ChemTech - Ventech!

2.3 Basic physical and chemical properties 49
Vapor density relative to air
6
5
4
3
2
alcohols
ketones
1
20 40 60 80 100 120 140 160
Molecular weight, Daltons
Figure 2.3.10. Vapor density relative to air of alcohols
and ketones vs. their molecular weight.
The vapor pressure at the boiling point of a pure solvent is equal to atmospheric pressure.
When solvents are used in mixtures or solutions, the vapor pressure is affected by
other components present in the mixture. For example, if a solvent is hydrophilic, the addition
of a hydrophilic solute decreases the vapor pressure. The addition of a hydrophobic solute
to a hydrophilic solvent increases the vapor pressure. Alcohols have hydrophobic
chains, therefore addition of small quantities of alcohol to water increases vapor pressure of
resulting solution. Because of these phenomena and other types of associations between
solvents in their mixtures, theory cannot be used to accurately predict the resulting vapor
pressure.
Raoult’s Law has limited prediction capability of the vapor pressure of two miscible
solvents:
p = m p + ( 1−m
) p
[2.3.6]
12 1 1 1 2
0
1 2 3 4 5 6 7 8
where:
p12 vapor pressure of the mixture
m1 molar fraction of the first component
p1,p2 vapor pressures of the components
If associations exist between molecules in the mixture, the vapor pressure of the mixture is
lower than that predicted by the law.
2.3.6 SOLVENT VOLATILITY
The evaporation rate of solvents is important in many applications. This has resulted in attempts
to model and predict solvent volatility. The evaporation rate of a solvent depends on
its vapor pressure at the processing temperature, the boiling point, specific heat, enthalpy
and heat of vaporization of the solvent, the rate of heat supply, the degree of association between
solvent molecules and between solvent and solute molecules, the surface tension of
the liquid, the rate of air movement above the liquid surface, and humidity of air surrounding
the liquid surface.
Vapor pressure at 25 o C, mm hg
140
120
100
80
60
40
20
normal
branched
Number of carbon atoms in alcohol
Figure 2.3.11. Vapor pressure at 25 o C of normal and
branched alcohols vs. number of carbon atoms in their
molecule.