Chemistry Reference
In-Depth Information
3.4 PREFERENTIAL SOLVATION IN TERNARY SOLVENT MIXTURES
Ternary solvent systems were dealt with by Matteoli and Lepori (1995) following an
earlier paper in which a binary solvent system (water + 1-propanol) containing urea
at substantial concentrations was treated (Matteoli and Lepori 1990). The general
expression for relating the KBIs of the A + B + C ternary solvent system, where
i
and
j
are any two among the three solvents, to thermodynamic data are (see Equation 1.62)
ij
A
-
δ
ij
G
=
(3.10)
ij
cc
A
c
ij
j
This equation is treated in more detail in Section 1.2.4 in Chapter 1 and Section 4.3.1
in Chapter 4. There was no way for the estimation of the correlation volume for the
application of expressions such as Equation 3.2, so that the preferential solvation
coefficients, for example, for solvation of component C by the components A and B
is simply defined as
psc
ABC
=∆
GG
-
(3.11)
(
)
AC
BC
where the Δ
G
ij
=
G
ij
-
G
i
ij
are the volume corrected KBIs as explained in Section 3.2.
For a more detailed treatment of preferential solvation in ternary systems see Sections
4.3.2 and 4.3.3 in Chapter 4.
There are six KBIs for each ternary system that could be presented in a three-
dimensional triangular prism. It was expedient, however, to show the results in terms
of sections parallel to a side of the triangular base. In such sections, for example, the
mole fraction
x
C
is kept constant and
x
A
is varied between 0 and 1 -
x
C
while
x
B
=
1 -
x
C
-
x
A
.
These equations were applied to four ternary solvent systems: water + 1-propanol
+
N
,
N
-dimethylformamide at 313 K, and water + 1,2-ethanediol + ethanol, water +
1,2-ethanediol + acetonitrile, and ethanol + 1,4-dioxane + chloroform at 323 K.
Addition of component C =
N
,
N
-dimethylformamide to the water (A) + 1-propanol
(B) system decreases the self-association of each of these components drastically,
until it practically disappears at
x
C
= 0.2. The psc
(A,B)C
in this system changes sign
when
x
C
= 0.1, being positive but turning negative in 1-propanol-rich mixtures, where
water loses its structure and individual water molecules are preferentially solvating
the
N
,
N
-dimethylformamide by its better hydrogen bonding ability and smaller size.
Acetonitrile (C) is strongly self-associated in both binary mixtures with water (A)
and with 1,2-ethanediol (B), and addition of B to A + C mixtures or of A to B + C
mixtures has little effect on this. On the other hand, a definite preference for water-
1,2-ethanediol association in the presence of acetonitrile is manifested. If ethanol
replaces acetonitrile as component C in the mixtures, the same trends are observed,
although to a smaller degree. Thus, mixtures of A + B in the last two examples act
as if they were a single component, irrespective of their composition, with regard to
solvation of the component C.
The fourth example dealt with has a protic component C = ethanol and two
aprotic ones, of which one is (mildly) dipolar, A = chloroform, and the other not,
