Preferential Solvation in Mixed Solvents - Fluctuation Theory of Solutions - page 74

Trifluoroethanol, hexafluoro-2-propanol (Figure 3.3). These fluoroalkanols have
rather unsymmetric preferential solvation parameter curves with extrema near x S =
0.2 to 0.25. The δ x ′ WW (max) are 0.31 and 0.28 and the δ x ′ WS (min) are -0.20 and -0.13,
respectively, for the trifluoro- and hexaluoro-alkanol. In both cases water self-
associates more for the former, having an exposed hydrophobic methylene group.
2-Methoxy-, 2-ethoxy-, and 2-butoxyethanol (Figure 3.4). At 313 K the preferen-
tial solvation curve for aqueous 2-methoxyethanol is symmetric with δ x ′ WW (max) =
0.10 and δ x ′ WS (min) = -0.008, similar to the values for 1,2-ethanediol at 298 K (see
above, the temperature effect appears to be small, since the 343 K data are similar to
the 313 K ones). As the alkyl substituent becomes longer, the curves become skewed,
with the extrema near x S = 0.2 for δ x ′ WW (max) = 0.18 and 0.075 and near x S = 0.15 for
δ x ′ WS (min) = -0.06 and -0.019 for the ethoxy and butoxy components. The param-
eters for the latter, being smaller than those for the former, are unexpected.
Ethanolamines (Figure 3.4). Data for 2-ethanolamine, N - methyl-2-ethanolamine,
N , N -dimethyl-2-ethanolamine, diethanolamine, and triethanolamine have been
dealt with in detail (Marcus 1995), and the results are summarized here. KBIs for
2-ethanolamine have also been reported previously (Matteoli and Lepori 1984) in
substantial agreement with the later ones. For this aqueous system mutual asso-
ciation dominates over the self-association: δ x ′ WW (min) = -0.046 and δ x ′ WS (max) =
0.007, the latter (or - δ x SS for self-association of the ethanolamine) being insignifi-
cant. The results for N -methyl-2-ethanolamine resemble closely those for the free
amine group. When another methyl group is substituted on the amine the system
becomes even more ideal, the preferential solvation parameters do not exceed 0.02
in absolute value, but in water-rich mixtures δ x ′ WW is slightly positive. For di- and tri-
ethanolamines, data are available only for 273 K as are data for monoethanolamine,
that is, 2-aminoethanol. For all three systems the (nonvolume-corrected) preferential
solvation curves are skewed, δ x WW (min) being near x S = 0.75 and δ x WS (max) being
near x S = 0.25, the former extremum is ≥ -0.02, the latter is ≤ 0.04. The more ethanol
groups are substituted on the amine, the more ideal the mixtures become, see also
Matteoli (1997).
Tetrahydrofuran, 1,4-dioxane (Figure 3.5) and 1,3-dioxolane . Here appreciable
preferential solvation occurs—for the former mixtures with δ x ′ WW (max) = 0.33 near
x S = 30 and δ x ′ WS (min) = -0.20 near x S = 0.25 in the first solvation shell at 298 K.
The water-water preference and disfavored water-tetrahydrofuran interactions per-
sist to the second solvation shell. For the aqueous dioxane mixtures with two ether
linkages and the same number (four) of hydrophobic methylene groups, the self-
association of the water is even larger with δ x ′ WW (max) = 0.57 near x S = 0.65 in the
first solvation shell at 298 K and persists with δ x ′ WW (max) = 0.20 in the second shell.
However, the disfavored mutual interaction is much smaller than for tetrahydrofuran,
being only δ x ′ WS (min) = -0.052 in the first shell and hardly significant in the second.
For aqueous 1,3-dioxolane (Marcus 2002a), with only three methylene groups, the
preferential solvation curves show less preferences than for aqueous tetrahydrofuran
and 1,4-dioxane (data are available at 323 K): δ x ′ WW (max) = 0.17 near x S = 0.30 and
δ x ′ WS (min) = -0.075 near x S = 0.20.
Acetone (Figure 3.5). Here, again, the KBIs were reported previously (Matteoli
and Lepori 1990). The self-association of the water, δ x ′ WW (max) = 0.31 near x S = 0.50