Handbook of Solvents - George Wypych - ChemTech - Ventech!

144 Abraham Nitzan
ion centers. These results are for the p ′ = 0.019 system; the other systems with p ′ < ∞ show
qualitatively similar behavior. Generally, the time evolution of the angular motion is similar
to that of the solvation energy. Typical to the present system that represents simple polar
solvents, the fast relaxation component is associated with the initial relaxation of the
orientational structure in the solvation layers close to the solute.
4.3.6 SOLVATION IN COMPLEX SOLVENTS
The previous sections have focused on a generic model of a very simple solvent, in which
solvation dynamics is determined by molecular translations and reorientations only. These
in turn are controlled by the solvent molecular mass, moment of inertia, dipole moment and
short-range repulsive interactions. When the solvent is more complex we may expect specific
structures and interactions to play significant roles. Still, numerical simulations of solvation
dynamics in more complex systems lead to some general observations:
(a) In large molecular solvents, solvation may be associated with binding of the solute
to particular solvents sites. As seen in Figure 4.3.1, deviations from linearity in the solvent
response potential are associated with the fact that the fraction of polar binding sites constitutes
a relatively small fraction of the solvent molecule.
This deviation from linearity shows
itself also in the solvation dynamics. Figure
4.3.7 shows the linear response functions
and the non-equilibrium solvation function,
C(t) and S(t), respectively, computed as before,
for the di-ether H(CH2OCH2) 2CH3 solvent. Details of this simulations are
given in Ref.11b. If linear response was a
valid approximation all the lines in Figure
4.3.7: The two lines for C(t) that correspond
to q=0 and q=1, and the two lines for S(t)
for the processes q=0→q=1 and the process
q=1→q=0, would coalesce. The marked
differences between these lines shows that
linear response theory fails for this system.
(b) Linear response theory was also
shown to fail for low-density solvents (e.g.
near and above the liquid-gas critical point 11c,24 ). In this case the origin of the non-linearity is
the large rearrangement in the solvent structure near the solute during the solvation process.
This rearrangement is associated with a local density change in such highly compressible
low-density solvents.
(c) Similarly, solvation in mixed solvents usually involve large rearrangement of solvent
structure near the solute because the latter usually have a higher affinity for one of the
solvent components. Solvation in electrolyte solutions provides a special example. 25,26 In
this case the solvent dynamics about the newly created charge distribution is not much different
than in the pure dielectric solution, however in addition the mobile ions rearrange
about this charge distribution, and on the timescale of this process linear response theory
fails. 27
Figure 4.3.7. The solvation and response functions, S(t)
and C(t), respectively, for solvation of a spherical ion in a
model for the solvent 1,2-methoxy ethoxy ethane,
H(CH2OCH2) 2CH3. Full line: S0→ 1(t);
dotted line: S1→ 0(t);
dashed line: C(t)| q=0 and dotted-dashed line: C(t)| q=1.
[From Ref. 11b].
(d) In the situations discussed in (b) and (c) above, new dynamical processes exist:
While the dielectric response in normal simple polar solvents is dominated by molecular ro-