Handbook of Solvents - George Wypych - ChemTech - Ventech!

4.3 Polar solvation dynamics 143
Figure 4.3.5.(a) The solvation energy E(t) and (b) the
solvation function S(t) associated with the three solvation
shells defined in the text, plotted against time after
the ion charge is switched on, for the system with
p ′ =0.019. Solid line, nearest shell; dotted line, second
shell, dashed line, outer shell. [From Ref. 11a].
Figure 4.3.6. The time dependence of the average angle
between the molecular dipoles and between the corresponding
radius vectors to the ion center, associated
with molecules in the three different solvation shells
defined in the text, plotted against time following the
switching on of the ionic charge. [From Ref. 11a].
Next we examine the relative contributions
of different solvation shells to the
solvation process. This issue is important
for elucidating the solvation mechanism,
and has been under discussion since an
early remark made by Onsager 23 that the
shorter time scales are associated mostly
with solvent layers further away from the
solute, and that the longer ~ τ D times are associated
with the individual response of sol-
vent molecules nearest to the solute. From the structure of the solute-solvent radial
distribution function of the simulated system one can estimate 11a that the first solvation shell
about the solute consists of the eight nearest neighbor solvent particles at distance closer
than 5.5 Å from the solute center, and the second solvation shell encompasses the next nearest
26 solvent particles at distance smaller than ~10 Å from the solute center. Taking the rest
of the solvent particles in the simulation box as the “third solvation shell”, Figure 4.3.5
shows the contributions of these layers to the time evolution of the solvation energy and of
the solvation function. It seems that the fast component in these time evolutions is faster for
the contribution from the first solvation shell. The same shell also shows a distinct slow
component which is much smaller or absent in the contribution from the further shells. Also
note that the solvation energy is dominated by the first solvation shell: the first, second, and
third shells contribute ~67%, 24%, and 9%, respectively, to the solvation energy. The fast
relaxation component accounts for ~80% of the solvation energy. It should be kept in mind,
however, that the contribution from outer shell molecules is suppressed by the finite size of
the simulated system.
Finally, the nature of the motion that gives rise to the fast relaxation component is seen
in Figure 6, which depicts as functions of time the average angles between the molecular dipoles
in the different solvation shells and between the corresponding radius vectors to the