Handbook of Solvents - George Wypych - ChemTech - Ventech!

14.21.2 Predicting cosolvency 1009
Figure 14.21.2.2b. Deviations from log-linear model (equation [14.21.2.2], triangle) and the extended log-linear
model (equation [14.21.2.10], circle) for solute benzocaine in various water cosolvent systems. Experimental data
are from Refs. 84 and 85.
ii ( Sm Sm) Sm ( Sw ∑σififiγi) log / = log − log + + 2. 303 ∑ log [14.21.2.11]
Results of equation [14.21.2.11] for naphthalene, benzocaine, and benzoic acid in selected
binary solvent mixtures are also included in Figure 14.21.2.2. A few other examples
can be found in Pinal et al. 75
The extended log-linear model outperforms the log-linear model in more than half of
the cases tested for the three solutes in Figure 14.21.2.2. The improvement occurs mostly in
regions with relatively high f values. In the low f regions, negative deviations of solubilities
from the log-linear pattern are often observed as discussed above, but are not accounted for
by the extended log-linear model as presented by equation [14.21.2.10]. In some cases, such
as naphthalene in methanol and propylene glycol, and benzoic acid in ethylene glycol, the
negative deviations occur over the entire f range of 0~1. In these cases, the extended log-linear
model does not offer better estimates than the original log-linear model. With the activity
coefficients listed in Table 14.21.2.3, the extended log-linear model generates worse
estimates of log (S m/S w) than the log-linear model for systems containing
dimethylacetamide, dimethylsulfoxide, or dimethylformamide. There is a possibility that