Characterization of supercritical fluid solvents using solvatochromic shifts

Abstract

fit in this cavity better than in other parts of the lipid bilayer. The proximity of 02 would then enhance the effect of paramagnetic relaxation on the fluorine atoms and raise qF. In the meantime, <7h of the aliphatic protons near the cavity would also increase.However, this would only contribute to a very small fraction of the weighted average of all protons. Thus, we observe qF > qH in the bilayers. Since the cavity may have a better chance to be located near the middle part of the PFD molecule (Figure 4B), the increase in qF of the la, le, and 2a fluorine atoms would be larger than that in qF of the 2e fluorine (Table I). This is in contrast to the situation in bulk PFD liquid, in which qF of the 2e fluorine (2.45 ± 0.05 s™1 atm™1 at 75.38 MHz) is slightly larger than qF of other fluorine atoms (2.17 ± 0.08 for la, 2.15 ± 0.04 for le, and 2.20 ± 0.03 for 2a, all in s™1 atm™1 at 75.38 MHz),10 because the small end of an ellipsoid is sterically more favorable for the approach of 02 in an isotropic liquid. The values of qF at 250 MHz (2.05-2.25 s™1 atm™1) are smaller, and the differences are also less. 19 The qF values of PFD droplets decrease further at 282.23 MHz (Table I). However, the differences in the qF values for the PFD droplets as well as the decane solution of PFD are too small to be significant, possibly due to the change in medium and larger experimental uncertainties. Since the corresponding F-19 signals of m-PFD are very broad and unresolved at 298 K,12,13 the interaction between lipid bilayers cannot be determined by NMR under these conditions. Because of the low solubility of PFD in the lipid bilayers,9 molecules of the two isomers are not likely to either interact or interfere with each other.PFTPA is a flexible molecule. Therefore, a schematic representation like Figure 4 may not be entirely adequate. However, the lipid bilayer would still experience a local perturbation by a dissolved PFTPA molecule. It is well-known that solutions of fluorocarbons in hydrocarbons exhibit nonideal behavior.24™27 The (24) Hildebrand,

Fundamental Studies and Applications of Supercritical Fluids

“…Additional work has appeared on Xe, SF 6 , C^, and NH 3 (24,25). For all fluids, spectral shifts were observed with fluid density.…”

“…The majority of the reports on supercritical fluid solvation have used steady-state solvatochromic absorbance measurements (21)(22)(23)(24)(25)(26)(27)(28). The original aim of these experiments was to determine the solvating power of supercritical fluids for chromatography and extraction (SFC and SFE) (26,28).…”

“…Yonker, Smith and co-workers (24-26,28) compared their results to the McRae continuum model for dipolar solvation (56,57), which is based on Onsanger reaction field theory (58). Over a limited density range, there was agreement between the experimental data and the model (24)(25)(26)28), but conditions existed where the predicted linear relationship was not followed (28). At low fluid densities, this deviation was attributed (qualitatively) to fluid clustering around the solute (28).…”

“…Steady-state solvatochromic techniques provide a reasonable means to study solvation processes in supercritical media (5,(17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(43)(44)(45)(59)(60)(61)(62)(63)(64)(65)(66)(67)(68). But, unless the interaction rates between the solute species and the supercritical fluid are slow, these "static" methods cannot be used to study solvation kinetics.…”

“…Some of these efforts have used optical spectroscopy (17)(18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(43)(44)(45) and others have used chromatographic methods as a tool to probe the aforementioned interactions. Many of these experimental studies have been advanced by theoretical calculations and modelling (35-42).…”