Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.21.2 Predicting cosolvency 1003
1-pentanol acting on PCB congeners 30 and naphthalene. 26 and butanone on anthracene and
fluoranthene. 75
Even more hydrophobic organic solvents produce little or even negative influence on
the solubility of HOCs. For instance, the presence of benzene does not increase the aqueous
solubility of PCBs up to their saturation concentration. 29 Solubility of a few PCB congeners
in water were found to be depressed by dissolved dichloromethane and chloroform. 73 On the
other hand, PCB solubility showed little change when cosolvent benzyl alcohol, 1-hexanol,
1-heptanol, or 1-octanol was present. 29,30 Similar “no change” observations were made for
naphthalene with cosolvents dichloromethane and chloroform, 73 and for solutes benzene
and hexane with cosolvent MTBE. 25 Much of the complexity with hydrophobic cosolvents,
or rather, cosolutes, can be explained by the fact that these cosolvents may partition into the
solute phase, thus the physical state of the solute is no longer the same as is in pure water. Instead
of a basically pure crystalline or liquid phase of solute, the solute and the cosolvent
form an organic mixture, and the composition and ideality of this mixture will very much
determine the concentrations of its components in the aqueous phase. Such a situation may
be better investigated along the line of phase partitioning, where Raoult’s law defines an
ideal system.
14.21.2.5 Predicting cosolvency in homogeneous liquid systems
The log-linear model
Yalkowsky and Roseman introduced the log-linear model in 1984 to describe the phenomenon
of the exponential increase in aqueous solubility for nonpolar organic compounds as the
cosolvent concentration is increased. 61 They showed that
( )
i
logS = flogS + 1 −f
logS
[14.21.2.1]
m
c w
Rearranging equation [14.21.2.1] results in
i ( m w) ( c w)
log S / S = flog S / S =σ f
[14.21.2.2]
The left side of equation [14.21.2.2] reflects the extent of solubilization; f defines how
much cosolvent is required to reach the desired solubilization. The constant σ is the
end-to-end slope of the solubilization curve and defined by:
( )
σ= logS − logS = log S / S
c w c w [14.21.2.3]
The model can be extended to systems containing a number of cosolvents:
i ( m )
log S / S =∑σ f
[14.21.2.4]
w i i
where the subscribe i denotes the i th component of the solvent mixture.
Two measured solubilities will define the value of σ that is specific to a solute/cosolvent
pair. The value of σ is also dependent of the solubility unit selected and on
whether 10-based or e-based logarithm is used. The magnitude of σreflects the difference in
molecular interactions between solute/cosolvent and solute/water. When applied to describe
cosolvency, σ is like a microscopic partition coefficient if water and cosolvent are
thought of as two independent entities. There had been other definitions of σ, such as the