Chemistry Reference
In-Depth Information
solvate, that is, to enter the solvation shells of Et, THF, and cyH. Regarding cyH as
solvaton, we see that its solvation shell is strongly depleted with Et and enriched with
cyH, but almost unchanged as to THF.
The comparison of Figures 4.4 and 4.5 reveals strong similarities, as expected
on the basis of the
G
ij
analogies. Practically, all surfaces are qualitatively identical,
but not for all values. Looking at the pictures in columns 1 and 3, which show the
behavior and capability of Et, MEt, and cyH as solvatants, we can see that the values
in system II are much larger in magnitude, by about 100%, than in system I. On the
contrary, the behavior of THF as solvatant (central column) shows not only the same
surface shapes as system I, but also the same values, as if THF could not distinguish
the different strength of the MEt-MEt and Et-Et associations, which is instead felt by
cyH. Moreover, the same values of δ
x
21
in systems I and II are again an indication
that THF does not interact via H-bond with MEt and Et.
Let us now examine the nine pictures in Figure 4.6 for system III. First, the range
of all values are in the same order of magnitude, and the maximum effects are shown
in the solvation shells of TCM and THF. They are characterized by positive values
of δ
x
21
and δ
x
12
, respectively, and correspondingly negative values of δ
x
31
and δ
x
32
,
which demonstrate the preference of each of these components to be solvated by
the other in the whole domain except, of course, in the binary systems where the
solvatant is not available. In fact, in the area near the binary 1+3, δ
x
11
is positive, and
similarly, near the binary 2+3, δ
x
22
is positive. In the surrounding of the solute cyH
(third row of pictures), the values of the excess local compositions are smaller than
those around solvatons TCM and THF, and are positive for δ
x
33
and negative for the
other two.
The fact that δ
x
33
is positive over the whole domain for all three systems sug-
gests that cyH is always preferentially solvated by other cyH molecules. However, the
absence of polar functional groups in its structure indicates that is the responsibility
of those interactions that are effective among the other components. A small (in sys-
tem III) or large (in systems I and II) fraction of the number of molecules of species 1
or 2 that would be present in the solvation shell of cyH if it had the bulk composition,
due to the interactions between their functional group are attracted in the solvation
shells of 1 or 2 and replaced by other cyH molecules.
4.6.3 P
reFerenTial
s
olvaTion
Since our definition of preferential solvation is simply a difference of excess local
compositions whose behavior has been shown in previous figures, we have not
reported figures with the pictures of all Δpsi
i
j,k
surfaces. From Figure 4.4 through
Figure 4.6, the necessary information can be obtained. Moreover, a large degree of
the insight into the local structure that can be inferred from the discussion of Δpsi
i
j,k
has already been obtained from the analysis of the behavior of δ
x
ji
. For this reason,
we will limit the discussion of Δpsi
i
j,k
to the most salient features.
In system I, the most evident characteristics is the behavior of Δps
1,3
. As can be
realized by examining the first and third surface in the upper row of Figure 4.4,
