Handbook of Solvents - George Wypych - ChemTech - Ventech!

746 Roland Schmid
turns out, polar solvation has traditionally been overemphasized relative to nonpolar solvation
(dispersion and induction), which is appreciable even in polar solvents.
The conceptual problems of the empirical solvent parameters summarized:
• The solvent acceptor number and other “polarity” scales include appreciable,
perhaps predominant, contributions from solvent structure changes rather than
merely measuring anion solvation.
• Care is urged in a rash interpretation of solvent-reactivity correlations in enthalpic
terms, instead of entropic, before temperature-dependence data are available.
Actually, free energy alone masks the underlying physics and fails to provide
predictive power for more complex situations.
• Unfortunately, the parameters used in LSER’s sometimes tend to be roughly related
to one another, featuring just different blends of more fundamental intermolecular
forces. Not seldom, fortuitous cancellations make molecular behavior in liquids
seemingly simple (see below).
Further progress would be gained if the various interaction modes could be separated
by means of molecular models. This scheme is in fact taking shape in current years giving
rise to a new era of tackling solvent effects as follows.
13.1.6 THE PHYSICAL APPROACH
There was a saying that the nineteenth century was the era of the gaseous state, the twentieth
century of the solid state, and that perhaps by the twenty-first century we may understand
something about liquids. 33 Fortunately, this view is unduly pessimistic, since theories of the
liquid state have actively been making breath-taking progress. In the meantime, not only
equations of state of simple liquids, that is in the absence of specific solvent-solvent interactions,
34-36 but also calculations of simple forms of intermolecular interactions are becoming
available. On this basis, a novel approach to treating solvent effects is emerging, which we
may call the physical approach. This way of description is capable of significantly changing
the traditionally accepted methods of research in chemistry and ultimately will lay the foundations
of the understanding of chemical events from first principles.
A guiding principle of these theories is recognition of the importance of packing effects
in liquids. It is now well-established that short-ranged repulsive forces implicit in the
packing of hard objects, such as spheres or dumbbells, largely determine the structural and
dynamic properties of liquids. 37 It may be noted in this context that the roots of the idea of
repulsive forces reach back to Newton who argued that an elastic fluid must be constituted
of small particles or atoms of matter, which repel each other by a force increasing in proportion
as their distance diminishes. Since this idea stimulated Dalton, we can say that the very
existence of liquids helped to pave the way for formulating modern atomic theory with
Newton granting the position of its “grandfather”. 38
Since the venerable view of van der Waals, an intermolecular potential composed of
repulsive and attractive contributions is a fundamental ingredient of modern theories of the
liquid state. While the attractive interaction potential is not precisely known, the repulsive
part, because of changing sharply with distance, is treatable by a common formalism in
terms of the packing densityη, that is the fraction of space occupied by the liquid molecules.
The packing fraction is a key parameter in liquid state theories and is in turn related in a simple
way to the hard sphere (HS) diameter σ in a spherical representation of the molecules
comprising the fluid: