A. Kamlet-Taft Pi* Scale for Polarity/Polarizability

Structure 1



Copyright 2002 by Marcel Dekker. All Rights Reserved.



Structure 1



(Continued)



Copyright 2002 by Marcel Dekker. All Rights Reserved.



SF6 (22–25). Under subcritical (liquid) conditions, a wide variation in π∗ was

found among the solvents: 0.8 (NH3 ), 0.04 (CO2 ), −0.03 (N2 O), −0.21 (CClF3 ),

−0.22 (ethane), and −0.36 (SF6 ) (22,23). These π∗ values correlate well with

the Hildebrand solubility parameters of the solvents. The same variation in π∗

was observed for the solvents under supercritical conditions when compared at

a single reduced density (Figure 2) (22). For a given supercritical fluid, the π∗

values were again found to increase with increasing fluid density; however, the

solvent strength was clearly nonlinear with density, especially in the low-density

region (Figure 2). This was particularly true for supercritical CO2 , ethane, and

Xe, for which characteristic three-density-region solvation model behavior was

observed. The apparent linear dependence of the π∗ values on fluid density in

supercritical NH3 and SF6 was attributed to specific solute–solvent interactions

that represent the two extremes—unusually high polarity in NH3 and a general

lack of sensitivity due to the nonpolar nature of SF6 (22).

Kim and Johnston made a similar observation of nonlinear density dependence for the shift in the absorption spectral maximum of phenol blue in



Figure 2



Plot of π∗ vs. reduced density (ρ/ρc ) for the five fluids. (From Ref. 22.)



Copyright 2002 by Marcel Dekker. All Rights Reserved.



supercritical ethylene, CClF3 , and fluoroform (26). Quantitatively, the stabilization of the photoexcited probe molecule in solution is linearly related to the

intrinsic solvent strength, E 0 T .

E 0 T = A[(n2 − 1)/(2n2 + 1)]

+ B[(ε − 1)/(ε + 2) − (n2 − 1)/(n2 + 2)] + C



(1)



where A, B, and C are constants specific to the solvent, n is the solvent refractive index, and ε is the solvent dielectric constant. According to Kim and

Johnston (26), the plot of the absorption spectral maximum of phenol blue vs.

E 0 T deviates from the linear relationship [Eq. (1)] in the near-critical density

region; this deviation can be attributed to the clustering of solvent molecules

about the solute probe (Figure 3).

A similar deviation was observed by Yonker et al. in the plot of π∗ values

as a function of the first term in Eq. (1), (n2 − 1)/(2n2 + 1); the deviation was

also discussed in terms of solute–solvent clustering (Figure 4) (23–25).

The use of similar molecular probes in various supercritical fluids has been

reported (27–34), e.g., 9-(α-perfluoroheptyl-β,β-dicyanovinyl)julolidine dye for

supercritical ethane, propane, and dimethyl ether (27); nile red dye for 1,1,1,2tetrafluoroethane (28); 4-nitroanisole and 4-nitrophenol for ethane and fluorinated ethanes (29); 4-aminobenzophenone for fluoroform and CO2 (30); phenol

blue for CO2 , CHF3 , N2 O, and ethane (31); and coumarin-153 dye for CO2 ,



Figure 3 Transition energy (ET ) and isothermal compressibility vs. density for phenol

blue in ethylene: (᭺) 25◦ C, (᭝) 10◦ C, (–––) calculated E0 T . (From Ref. 26.)



Copyright 2002 by Marcel Dekker. All Rights Reserved.



Figure 4

Ref. 23.)



π∗ vs. Onsager reaction field function (L(n2 )) for CO2 at 50◦ C. (From



fluoroform, and ethane (32,33). The results of these studies showed the characteristic density dependence of solvation in supercritical fluids, supporting the

solute–solvent clustering concept.

B. Pyrene and the Py Scale

The molecular probe pyrene is commonly employed to elucidate solute–solvent

interactions in normal liquids (18,35). Because of the high molecular symmetry,

the transition between the ground and the lowest excited singlet state is only

weakly allowed, subject to strong solvation effects (36–39). As a result, in the

fluorescence spectrum of pyrene the relative intensities of the first (I1 ) and third

(I3 ) vibronic bands vary with changes in solvent polarity and polarizability. The

ratio I1 /I3 serves as a convenient solvation scale, often referred to as the Py

solvent polarity scale. Py values for an extensive list of common liquid solvents

have been tabulated (15,16).



Structure 2



Copyright 2002 by Marcel Dekker. All Rights Reserved.



Several research groups have used pyrene as a fluorescent probe in the

study of supercritical fluid properties (2,3,40–48). In particular, the density dependence of the Py scale has been examined systematically in a number of

supercritical fluids such as CO2 (2,3,40–43,45,46), ethylene (40,41,47), fluoroform (3,40,41,43,47), and CO2 -fluoroform mixtures (43). The Py values obtained

in various supercritical fluids correlate well with the polarity or polarizability

parameters of the fluids (3,40,41,43,47). For example, Brennecke et al. (40)

found that the Py values obtained in fluoroform were consistently larger than

those obtained in CO2 , which were, in turn, consistently larger than those found

in ethylene over the entire density region examined. In addition, the Py values

obtained in the liquid-like region (reduced density ∼1.8) indicate that the local polarity of fluoroform is comparable to that of liquid methanol, CO2 with

xylenes, and ethane with simple aliphatic hydrocabons (15,16).

For the density dependence of solute–solvent interactions in supercritical

fluids, the Py values were found to increase with increasing density in a nonlinear

manner (2,3,40–43). For example, Sun et al. reported Py values in supercritical

CO2 over the reduced density (ρr ) range 0.025–1.9 at 45◦ C (Figure 5) (2). At

low densities (ρr < 0.5), the Py values are quite sensitive to density changes,

increasing rapidly with increasing density. However, at higher densities, the Py

values exhibit little variation with density over the ρr range ∼0.5–∼1.5, followed

by slow increases with density at ρr > 1.5. The nonlinear density dependence

was attributed to solvent clustering effects in the near-critical region of the



Figure 5 Py values in the vapor phase (᭿) and CO2 at 45◦ C with excitation at 314 nm

(᭺) and 334 nm (᭝). (From Ref. 2.)



Copyright 2002 by Marcel Dekker. All Rights Reserved.