Chemistry Reference
In-Depth Information
A
Solvation
C
Pure solvent
Lewis-Randall's
ideal solution
Infinitely dilute solution
nonideal solution
FIGURE 8.1
Schematic of the solvation process described in terms of the mutation of two
solvent molecules labeled “A” and “C” into the anion and cation, where for simplicity we
assume ν
+
= ν
-
= 1.
mechanism underlying these phenomena; a short-ranged (SR) density perturbation
induced by the insertion of a solute into the solvent medium, and the long-ranged
(LR) propagation of this perturbation up to a distance dictated by the medium's
correlation length (See Figure 8.1) (Chialvo and Debenedetti 1992; Debenedetti and
Chialvo 1992).
The coexistence of these two length scales made difficult the early attempts to
interpret the experimental evidence from, and the modeling of, systems characterized
by highly compressible media, and gave rise to heated debates (Brennecke et al.
1990; Economou and Donohue 1990; McGuigan and Monson 1990). The source
of controversy at the time was the failure to discriminate the solvation contribu-
tions from the overwhelming compressibility-driven contributions to the mechanical
properties of the infinite dilute solutes. The debate was eased after the develop-
ment of a rigorous solvation formalism for infinitely dilute solutions that identified
and isolated the individual contributions from the two length scales (Chialvo and
Cummings 1994; Chialvo, Kalyuzhnyi, and Cummings 1996; Chialvo et al. 1999,
2001). In fact, the coexistence of solvation and compressibility-driven phenomena,
that is, what makes supercritical solutions challenging to model, becomes also the
key to characterizing their thermodynamic properties in terms of the two distinctive
length scales (Chialvo and Cummings 1994).
In Section 8.2 we discuss the main ideas behind the formalism and illustrate some
of the features based on predictions from integral equation calculations involving
simple binary mixtures modeled as Lennard-Jones systems (Section 8.2.1), to guide
the development of, and provide molecular-based support to, the macroscopic model-
ing of high-temperature dilute aqueous-electrolyte solutions (Section 8.2.2), as well
as to highlight the role played by the solvation effects on the pressure dependence of
the kinetic rate constants of reactions in near-critical solvents (Section 8.2.3).
Nonvolatile species solubilities in highly compressible solvents might be sig-
nificantly low, but usually much higher than the corresponding ideal gas solvent
counterparts (typically referred to as the solubility enhancement); consequently,
the actual systems are not infinitely dilute, that is, we must deal with finite
