Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.3 Polar solvation dynamics 145
tations, the motions that change the local structure about the solute are usually dominated by
solvent translation. This gives rise to a new, usually slower, relaxation components. Solvation
in electrolyte solutions clearly shows this effect: In addition to the dielectric response
on the picosecond timescale, a much slower relaxation component is observed on the nanosecond
scale. 25 Numerical simulations have identified the origin of this relaxation component
as the exchange between a water molecule and an ion in the first solvation shell about
the solute. 26
Finally, it is intuitively clear that in large molecule complex solvents simple molecular
rotation as seen in Figure 4.3.6 can not be the principal mode of solvation. Numerical simulations
with polyether solvents show that instead, hindered intramolecular rotations that distort
the molecular structure so as to bring more solvating sites into contact with the ion
dominate the solvation dynamics. 11b The bi-modal, and in fact multi-modal, character of the
solvation is maintained also in such solvents, but it appears that the short time component of
this solvation process is no longer inertial as in the simple small molecule solvents. 11
4.3.7 CONCLUSIONS
Numerical simulations of solvation dynamics in polar molecular solvents have been carried
out on many models of molecular systems during the last decade. The study described in
sections 4.3.4-4.3.5 focused on a generic model for a simple polar solvent, a structureless
Stockmayer fluid. It is found that solvation dynamics in this model solvent is qualitatively
similar to that observed in more realistic models of more structured simple solvents, including
solvents like water whose energetics is strongly influenced by the H-bond network. In
particular, the bimodal nature of the dynamics and the existing of a prominent fast Gaussian
relaxation component are common to all models studied.
Such numerical simulations have played an important role in the development of our
understanding of solvation dynamics. For example, they have provided the first indication
that simple dielectric continuum models based on Debye and Debey-like dielectric relaxation
theories are inadequate on the fast timescales that are experimentally accessible today.
It is important to keep in mind that this failure of simple theories is not a failure of linear response
theory. Once revised to describe reliably response on short time and length scales,
e.g. by using the full k and ω dependent dielectric response function ε(k,ω), and sufficiently
taking into account the solvent structure about the solute, linear response theory accounts
for most observations of solvation dynamics in simple polar solvents.
Numerical simulations have also been instrumental in elucidating the differences between
simple and complex solvents in the way they dynamically respond to a newly created
charge distribution. The importance of translational motions that change the composition or
structure near the solute, the consequent early failure of linear response theory in such systems,
and the possible involvement of solvent intramolecular motions in the solvation process
were discovered in this way.
We conclude by pointing out that this report has focused on solvation in polar systems
where the solvent molecule has a permanent dipole moment. Recently theoretical and experimental
work has started on the dynamics of non-polar solvation. 28 This constitutes another
issue in our ongoing effort to understand the dynamics of solvation processes.
REFERENCES
1 For recent reviews see M. Maroncelli, J. Mol. Liquids, 57, 1 (1993); G.R. Fleming and M. Cho, Ann. Rev.
Phys. Chem., 47, 109 (1996).