Chemistry Reference
In-Depth Information
Abstract
: A complete procedure, starting from the experimental techniques
to obtain accurate thermodynamic
G
E
and
V
E
data for mixing, is accurately
described for the calculation of Kirkwood-Buff integrals (KBIs) and local
compositions in the whole mole fraction domain of a ternary mixture of
organic compounds. Proper equations to represent
G
E
and
V
E
are suggested,
and these are used in a computer program, which calculates the values of all
the intermediate quantities necessary to obtain the KBIs. The computer rou-
tines allow an easy and fast calculation of the KBIs, and of the local compo-
sitions, in correspondence with a lattice of mole fraction values that is then
used to plot the KBIs as continuous surfaces. This treatment is applied to three
ternary mixtures containing components of different polarity, purposely cho-
sen to examine the roles played by different types of interactions (H-bond,
dipole-dipole, dipole-induced dipole, dispersion) in the determination of the
local composition around each solute species. The mixture of ethanol (Et) +
tetrahydrofuran (THF) + cyclohexane (cyH) (system I) is investigated and the
capability of THF to perturb the strong Et self-association characteristics of the
alcohol-hydrocarbon binary mixtures is examined. In system II, 2-methoxy-
ethanol (MEt) + THF + cyH, the KBIs and local composition behaviors are
compared with system I to examine the effects of the additional ether oxygen
of MEt on the molecular interactions among the components. Finally, the mix-
ture of trichloromethane (TCM) + THF + cyH (III) is studied to indicate the
degree to which the strength of the TCM-THF interaction competes with the
TCM and THF respective self-associative interactions shown in their binary
mixtures with cyH.
4.1 INTRODUCTION
Among the various applications of the Kirkwood-Buff (KB) theory exploited so far,
its use in solvation studies was the first one to be considered by investigators of chemi-
cal and physical processes in solution. The KB theory was devised to offer the pos-
sibility of predicting, from the knowledge of the
g
ij
s and through their integrals, the
KBIs, the excess thermodynamic properties of mixtures (Kirkwood and Buff 1951).
Due to the difficulties of obtaining
g
ij
for actual systems, the theory was practically
ignored for many years, until Ben-Naim developed a procedure for its inversion, in
this way providing the equations for obtaining the KBIs from excess thermodynamic
properties (Ben-Naim 1977).
Knowledge of the KBIs for a mixture allows one to calculate the excess or defi-
ciency of the number of moles of component
j
(solvatant) in the neighborhood of the
central molecule (solvaton)
i
, and therefore, under certain approximations, informa-
tion on the local composition in the solvation shell of
i
can be obtained. Despite the
numerous developments that this approach offered concerning solvation studies, 7
years passed before a systematic and accurate study of the KBIs for a number of
mixtures appeared (Matteoli and Lepori 1984). In the following years, other papers
by different authors were published on this subject and a variety of aqueous and non-
aqueous mixtures of nonelectrolytes and of electrolytes was considered. Chapter 3 in
