Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.21.2 Predicting cosolvency 1011
Both the hydrophobicity and hydrogen bonding property of the solutes seem to be important
in influencing the extent of the deviation from the ideal log-linear pattern.
Additional deviations related to the solute’s behavior may occur. For organic electrolytes,
the acid dissociation constant K a may decrease as cosolvent fraction f increases. 40,75,92
This, in turn, will affect the patterns of solubilization by cosolvents. Furthermore, a high
concentration of solutes may invalidate the log-linear model, which presumes negligible
volume fraction of solute and no solute-solute interactions. For solid solutes, solvent induced
polymorphism may also bring additional changes in their solubilization profile.
Another approach to quantitatively address the deviations of solubilization from the
log-linear model makes use of an empirical parameter β:
i
( Sm Sw) ( Sm Sw
)
β=log / / log /
The modified log-linear equation then takes the form:
( ) =∑
[14.21.2.12]
log S / S β σ f
[14.21.2.13]
m w i i
Table 14.21.2.3. UNIFAC derived activity coefficients for selected binary
water-cosolvent systems
f 0.1 0.2 0.4 0.6 0.8 0.9
Methanol MW = 32.04 Density = 0.7914
mol/L 2.4700 4.9401 9.8801 14.8202 19.7603 22.2303
x 0.0471 0.1000 0.2286 0.4001 0.6401 0.8001
γ, cosolvent 1.972 1.748 1.413 1.189 1.052 1.014
γ, water 1.003 1.013 1.055 1.14 1.298 1.424
Ethanol MW = 46.07 Density = 0.7893
mol/L 1.7133 3.4265 6.8530 10.2796 13.7061 15.4194
x 0.0331 0.0716 0.1705 0.3163 0.5523 0.7351
γ, cosolvent 5.550 4.119 2.416 1.564 1.152 1.050
γ, water 1.005 1.022 1.097 1.256 1.57 1.854
1-Propanol MW = 60.1 Density = 0.8053
mol/L 1.3399 2.6799 5.3597 8.0396 10.7195 12.0594
x 0.0261 0.0569 0.1385 0.2657 0.4910 0.6846
γ, cosolvent 12.77 8.323 3.827 2.001 1.248 1.077
γ, water 1.006 1.024 1.111 1.301 1.706 2.093
2-Propanol MW = 60.1 Density = 0.7848
mol/L 1.3058 2.6116 5.2233 7.8349 10.4466 11.7524