Jw. Zhang et al., FLUORESCENCE SPECTROSCOPY AND INTEGRAL-EQUATION STUDIES OF PREFERENTIAL SOLVATION IN SUPERCRITICAL-FLUID MIXTURES, Journal of physical chemistry, 99(22), 1995, pp. 9268-9277
We present both fluorescence spectroscopy and integral equation studie
s of supercritical and subcritical fluid mixtures that demonstrate the
possibility of a solute being preferentially solvated by either of th
e components in the mixture dependent on the pressure, along a given i
sotherm. The fluorescence studies use the relatively nonpolar probe py
rene to estimate the degree of preferential solvation in mixtures of C
O2 and CHF3. In a mixture of 15 mol % CHF3 in CO2, the CHF3 preferenti
ally solvates the probe at reduced densities greater than about 1.3. H
owever, for reduced densities between 0.7 and 1.3 the solvent strength
is very close to that of pure CO2, indicating a depletion of the CHF3
in the solvent shell around the solute. This general behavior is conf
irmed by integral equation calculations for ternary mixtures. Local co
mpositions of the solvents around a dilute solute are calculated from
the pair correlation functions. These are obtained using the numerical
method of Labik-Gillan for solution of the Ornstein-Zernike equation
with the Percus-Yevick closure. We investigate the influence of the si
ze and energy parameters of the solvents on the degree and nature of t
he preferential solvation and demonstrate the interesting solvation be
havior that can occur as one traverses from low-density gas to high-de
nsity supercritical fluid.