Chapter 8
Investigations of Solute—Cosolvent Interactions in Supercritical Fluid Media
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A Frequency-Domain
Fluorescence Study
Thomas A . Betts and Frank V . Bright Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, N Y 14214
Static and dynamic fluorescence spectroscopy are used to investigate the local compositional changes in binary supercritical fluids on a picosecond time scale. A fluorescent solute molecule whose emission characteristics are sensitive to solvent polarity is used to probe the composition of the local solvent environment. The systems investigated were supercritical CO with the addition of small amounts of the polar cosolvents, C H O H and C H C N . These systems exhibit a reorganization of the local solvent shell(s) about the fluorescent probe following optical excitation, a process known as solvent relaxation. Average rates for this dynamic solvation process were determined, and an Arrhenius analysis performed. It is shown that the binary supercritical fluid composed of C O and C H O H can reorganize more rapidly than the C O - C H C N system. 2
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Solvation in supercritical fluids depends on the interactions between the solute molecules and die supercritical fluid medium. For example, in pure supercritical fluids, solute solubility depends upon density (7-3). Moreover, because the density of supercritical fluids may be increased significantly by small pressure increases, one may employ pressure to control solubility. Thus, this density-dependent solubility enhancement may be used to effect separations based on differences in solute volatilities (4,5). Enhancements in both solute solubility and separation selectivity have also been realized by addition of cosolvents (sometimes called entrainers or modifiers) (6-9). From these studies, it is thought that the solubility enhancements are due to the increased local density of the solvent mixtures, as well as specific interactions (e.g., hydrogen bonding) between the solute and the cosolvent (10). Cosolvent-modified supercritical fluids are also used routinely in supercritical fluid chromatography (SFC) to modify solute retention times (11-20). In these reports, cosolvents are used to alter the mobile and stationary phase chemistries (16,17,20). However, distinguishing between such effects in a chromatography
0097^156/92/0488-0092$06.00A) © 1992 American Chemical Society
Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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BETTS & BRIGHT
Frequency-Domain Fluorescence Study
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experiment is difficult at best unless one knows the specific effects and their magnitude. Kim and Johnston (27), and Yonker and Smith (22) have used solute solvatochroism to determine the composition of the local solvent environment in binary supercritical fluids. In our laboratory we investigate solute-cosolvent interactions by using a fluorescent solute molecule (a probe) whose emission characteristics are sensitive to its local solvent environment. In this way, it is possible to monitor changes in the local solvent composition using the probe fluorescence. Moreover, by using picosecond time-resolved techniques, one can determine the kinetics of fluid compositional fluctuation in the cybotactic region. In this paper we focus on: 1) the kinetics of cosolvent solvation in supercritical media, and 2) determine how the nature of the cosolvent affects the solvation process.
Theory To investigate changes in solvent composition about a fluorescent probe with time, one studies the time evolution of the probe emission spectrum. The key segments of data in these experiments are the so called time-resolved emission spectra. These are obtained by acquiring the time-resolved intensity decays at a series of emission wavelengths (X^) spanning the entire fluorescence spectrum. In the simplest cases, if the system is accurately described by a two-state, excited-state model, the intensity decay at each wavelength will be bi-exponential. In addition, the apparent decay times (rj will remain constant across the emission spectrum and the pre-exponential amplitude factors (c*i(X)) will be wavelength dependent (25). Thus, the wavelengthdependent time course of the fluorescence intensity decay is described by: n
(i)
where c^X) and t are recovered using frequency-domain fluorescence techniques (24). In the frequency-domain, the experimentally measured quantities are the frequency- (w) and wavelength- (X) dependent phase shift (0 (X,oj)) and demodulation factor (MnXX,^)). For any assumed decay model (equation 1), these values are calculated from the sine (S(X,w)) and cosine (C(X,w)) Fourier transforms. If we assume the decay kinetics are described by a simple sum of exponential decay times we have (24): {
m
(2)
Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.
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SUPERCRITICAL FLUID T E C H N O L O G Y
/
(3)
C(A,G>) =
M (1 + G) xf) 2
and for any set of a (k) and r the calculated phase shift and demodulation factor are given by: {
i}
6 (X,(o) = arctan[5(A.,(o)/C(X,G))]
(4)
M (X,co) = [S(X,u>) + C ^ G ) ) ] .
(5)
c
2
2
172
c
The decay parameters O A ) and r j are recovered from the experimentally measured phase shift and demodulation factor by the method of non-linear least squares (24,25). The goodness-of-fit between the assumed model (c subscript) and the experimentally measured (m subscript) data is determined by the chi-squared (x ) function: 2
e ( « , x ) - e (w,A.)f M
(6)
c
X -—
*fcf where D is the number of degrees of freedom, and a and a are the uncertainties in the measured phase angle and demodulation factor, respectively. Time-resolved emission spectra are reconstructed from the normalized impulse response functions (26): Q
u
where N(X) is the wavelength-dependent normalization factor (26): (8)
and F(X) is the normalized steady-state fluorescence intensity at wavelength X. Once the time-resolved emission spectra are so generated, it is informative to monitor the time evolution of the emission spectra. To this end, it is convenient to focus on the time course of the emission center of gravity (v(t)) (27): v(f) = (|~ J'(A,0v
