Handbook of Solvents - George Wypych - ChemTech - Ventech!

772 Roland Schmid
simulations. 188-191 It should be emphasized, however, that the present discussion might be
confined to water as the solvent. Recent theoretical treatments advise the cavity radius not
to be considered as an intrinsic property of the solute, but instead to vary with solvent polarity,
with orientational saturation prevailing at low polarity and electrostriction at high polarity.
191,192 It would appear that the whole area of nonaqueous ion solvation deserves more
methodical attention. It should in addition be emphasized that the cavity radius is sensitive
to temperature. Combining eq. [13.1.29] with
o ⎛∂ΔG
* ⎞
ΔH = ΔG
* + T⎜
⎟
⎝ ∂T
⎠
one obtains 177
⎡
o
ΔH = ΔG
* ⎢1+
⎣⎢
T ⎛ ⎞ T r
⎜ ⎟ −
⎝ T ⎠ r T
⎛ ∂ε ∂ ⎞
⎜ ⎟
ε ε ∂ ⎝∂⎠
( −1)
P P
⎤
⎥
⎦⎥
[13.1.33]
[13.1.34]
The derivation of (∂r/∂T) P from reliable values of ΔH° and ΔG* is interesting, in that
nominally the dielectric effect (-0.018 for water) is smaller than the size effect (-0.069 for
chloride), a result that has not given previously the attention due to it. Consequently, as
Roux et al. 177 stated, unless a precise procedure for evaluating the dependence of the radius
on the temperature is available, the Born model should be restricted to the free energy of solvation.
Notwithstanding this, beginning with Voet, 172 the Born model has usually been
tested by considering the enthalpies of hydration. 193 The reason for the relative success lies
in the fact that ion hydration is strongly enthalpy controlled, i.e., ΔH° ~ ΔG*.
The discussion of radii given here should have implications to all calculations involving
aqueous ionic radii, for instance the solvent reorganization energy in connection with
eq. [13.1.26]. Thus, treatments using the crystal radii as an input parameter 194-197 may be revisited.
13.1.8 THE FUTURE OF THE PHENOMENOLOGICAL APPROACH
Originally, the empirical solvent parameters have been introduced to provide guidelines for
the comparison of different solvent qualities and for an orientation in the search for an understanding
of the complex phenomena in solution chemistry. Indeed, the choice of the right
solvent for a particular application is an everyday decision for the chemist: which solvent
should be the best to dissolve certain products, and what solvent should lead to increased reaction
yields and/or rates of a reaction?
In the course of time, however, a rather sophisticated scheme has developed of quantitative
treatments of solute-solvent interactions in the framework of LSERs. 198 The individual
parameters employed were imagined to correspond to a particular solute-solvent
interaction mechanism. Unfortunately, as it turned out, the various empirical polarity scales
feature just different blends of fundamental intermolecular forces. As a consequence, we
note at the door to the twenty-first century, alas with melancholy, that the era of combining
empirical solvent parameters in multiparameter equations, in a scientific context, is beginning
to fade away. As a matter of fact, solution chemistry research is increasingly being occupied
by theoretical physics in terms of molecular dynamics (MD) and Monte Carlo (MC)
simulations, the integral equation approach, etc.