J. Phys. Chem. 1994, 98, 8793-8800
8793
Solvation in Mixed Supercritical Fluids: TICT Spectra of Bis(4,4'-aminophenyl) Sulfone in Ethanol/COz Richard D. Schulte and John F. Kauffman' Department of Chemistry, University of Missouri-Columbia,
Columbia, Missouri 6521 1
Received: March 29, 1994"
W e present an examination of the fluorescence spectra of bis(aminopheny1) sulfone (APS) dissolved in mixed ethanol/C02 supercritical fluid solvents having cosolvent concentrations in the 1-10 mol % range. Peak shifts and areas of the locally excited (LE) and charge-transfer (CT) components of the spectra are determined from two component spectral fits and are compared to similar measurements in a series of liquid n-alcohols. Measures of the supercritical fluid solvent polarity are obtained by correlation of the spectral parameters of the fluid samples with parameters of the liquid samples. Simple mixing rules are proposed for the estimation of the mixed fluid refractive index and the dielectric constant and are used to calculate the values of two different solvent polarity functions under various fluid conditions. It is found that calculated values of the solvent polarity functions match the measured values over a wide range of fluid conditions only when the local fluid environment is assumed to be enriched in the ethanol concentration by up to an order of magnitude over the bulk concentration. Because APS is insoluble in pure C 0 2 under the conditions of our study, these results provide important information regarding cosolvent-enhanced solubility in supercritical fluid solutions.
I. Introduction Numerous compounds have been identified which undergo twisted intramolecular charge transfer (TICT) in the excited electronic state.' The flusrescencespectrum of a TICT compound will generally exhibit strong solvatochromaticity, which is attributed to the existence of two fluorescent isomers of the compound, one being the so-called locally excited (LE) state and the second being the charge-transfer (CT) state. The LE state geometry and dipole moment do not differ significantly from the ground state, and the LE spectral component is less sensitive to the solvent polarity than is the CT emission. Shifts in the center wavelength of the LE component of a few nanometers can be anticipated over a broad range of solvent polarity. In contrast, the CT state has a larger dipole moment than the ground-state molecule, and therefore, the CT reaction only occurs in polar solvents, in which the charge-transfer electronic configuration results in a significant energetic stabilization of the solvent-solute system. Thus, the contribution of the CT spectral component to the static fluorescence spectrum of a TICT compound is highly sensitive to solvent polarity. The CT band center frequency is expected to be strongly red shifted from the LE band center, and the observed shift as well as the quantum yield of the CT spectral feature will also depend strongly on 'solvent polarity.2 These properties make TICT compounds good candidates as probes of the local dielectric constant in microheterogeneous media.3 Bi~(4,4~-aminophenyl) sulfone (APS) is one such TICT compound which has been studied in a variety of polar solvents by both static2 and dynamic methods.66 The structure of APS is shown in Figure 1. Experimental investigations with geometrically restricted analogues of APS have demonstrated7 that a relative twisting of one of the aniline moieties accompanies the CT reaction. This contrasts the behavior of many TICT compounds,lJ in which only the alkylaminogroup twists following excitation, and is thought to result from d-orbital participation. Rettig and ChandrossZhave studied the spectroscopy of the bis(N,N'-dimethyl) derivativeof APS (DMAPS) and its dependence on temperature, in order to explore the kinetic and thermodynamic aspects of the TICT reaction. They demonstrate that at low temperature the shape of the spectrum is determined largely by
* Author to whom correspondence should be addressed.
Abstract published in Aduance ACS Abstracts, August 1, 1994.
0022-365419412098-8793%04.50/0
I
R
I
R
Figure 1. Structure of APS (R = H)and DMAPS (R = CH3).
kinetic considerations, implying that the TICT reaction is an activated process. As the sample temperature is increased in the kinetically controlled regime, they observe an increase in the intensityofthe CTcomponent, which they attribute to an increased rate of barrier crossing. As the temperature is increased, they find that the contribution of the CT emission to the spectrum diminishes, in contrast to the low-temperature result. This is taken as evidence that at high temperatures the spectrum is governed by the equilibrium which is established between the CT and LE states following initial excitation, under the assumption that the CT state potential energy well is deeper than the LE state well in polar solvents. The transition temperature between these regimes is around 250-280 K for DMAPS in alcohols, and it is significantly lower in nitriles. Thus, spectra collected at room temperature and above are in the equilibrium-controlled regime. Importantly, this equilibrium-controlled regime only arises when the equilibrium between the two states can be established on a time scale which is short with respect to the emission lifetime of the compound. This has been verified experimentallyfor APS and DMAPS by the time-resolvedstudies of Su and Simon," in which they observe emission from the CT band within a few picoseconds following excitation. They have made a thorough study of the kinetics and solvent dynamics of the TICT reaction of APS and its bis-Nfl'-substituted derivatives, which are largely consistent with the interpretations of Rettig and Chandross. Importantly, however, Su and Simon6 find that the solvent dependence of the reaction rate can be attributed solely to the solvent permittivity and that viscosity plays a minor role, if any, in governing the reaction kinetics of APS derivatives dissolved in solvents of moderate (Le., 1-10 cP) viscosity at room temperature. 0 1994 American Chemical Society
8794
Schulte and Kauffman
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 Computer
QQ
Nd:YAG Laser
Discriminators
0
ReferencePMT
-
Pressure Control
Monochromator
Syringe Pump
Our interest in TICT compounds in general, and APS in particular, stems fromour broader interest in developing a detailed understanding of the solvation and reactivity of solutes dissolved in supercritical fluid solvents. CO1is especially interesting because of its convenient critical properties, its availability, and its lack of toxicity. Its modest solubility parameter9 can be enhanced by adding a small amount of a polar cosolvent (often referred to as a modifier or entrainer) such as ethanol. Such solvent systems are highly complex owing to the large density fluctuations inherent to supercritical fluids as well as the effects resulting from local enrichment of the fluid composition in cosolvent concentration on a microscopic scale. Yet understanding the nature of solvation in such a mixed supercritical fluid is important from a practical point of view, because of the emerging importance of supercritical COZ as an environmentally benign medium for solid extraction.1° From the above discussion, it is clear that a microscopic probe of the local solute environment is required in order to examine the solute-induced solvent structural influences on solubility and reactivity in supercritical fluid solutions and that spectroscopic investigations of TICT compounds provide such a probe. Only a few spectroscopic investigations of mixed fluid systems have been reported to date. For example, Kim and Johnstonll have investigated phenol blue dissolved in mixed fluid systems composed of CO2 and polar liquids and demonstrate that local enrichment of the polar component by a factor of 2-25 occurs at 35 OC over a wide range of modifier bulk concentrations and fluid pressures. Betts and Bright12 have measured the fluorescence spectra and (4AMP) in propanol/ decays of 4-amino-N-methylphthalimide COz mixtures. They observed nanosecond spectral relaxation and conclude that 4AMP is associated with the propanol in the ground state. The TICT spectra of 4-(dimethy1amino)benzonitrilejb8e and 4-(dimethylamino)ethylbenzoate3e have been reported in both neat fluids and in fluid mixtures composed of COz and CFsH, both of which have critical temperatures just above room temperature. Importantly, all of the compounds mentioned above are soluble in both components of the fluid mixtures. In this paper, we present the results of our investigation of the local dielectric properties of mixed supercritical fluid solutions through spectroscopic studies of APS vs fluid pressure and composition. It is important to note that APS is not soluble in pure CO2 under the conditions of this investigation, and these studies therefore speak to the nature of solubility enhancement in mixed fluids.
We examine the spectrum of APS dissolved in mixed COz/ethanol solvents under various conditions. By comparison of the mixed fluid spectra with spectra collected in a series of alcohols, we develop an empirical measure of the mixed fluid solvent polarity. W e discuss the models for the analysis of spectral features and their relevance and implications for investigations of APS in mixed fluids. We also compare our spectroscopic measure of solvent polarity with direct measures of mixed fluid bulk permittivity. This comparison suggests that the local solvent environment of the APS in this mixed fluid system is enriched in ethanol by an order of magnitude over the bulk molar composition. 11. Experimental Section
Spectroscopic measurements of the supercritical fluid solutions were made with a custom-built spectrometer (see Figure 2), utilizing a frequency-doubled, mode-locked dye laser excitation source. The YAG laser (Lee Laser Model 712-ST) is modelocked with a NEOS Model N12041 acoustooptic modulator, and the output is frequency doubled with a KTP crystal. The 532-nm beam is used to pump an extended cavity R6G dye laser (Coherent Laser Inc. Model 599 with modification). The dye laser output is frequency doubled in a KDP crystal to give UV pulses for excitation. The APS sample is excited with 298-nm light, and the fluorescence is collected at a 90° angle to the excitation beam path with f/4 optics, dispersed by a monochromator (ISA Model HlOl), and measured with a photomultiplier tube (Hammatsu Model R928) operating in the photon-counting mode. A second photomultiplier tube (Hammatsu Model R928) collects the reference fluorescence at a fixed wavelength from the same sample through a collection lens and band-pass filter located 180' from the signal optics. Both signals are sent to Philips Model 691 5 discriminators without amplification, and the resulting photon count rates are measured by the counter inputs of an analog-to-digital converter board (Computer Boards Inc. Model CIO-8) installed in a 80386 compatible computer. The monochromator is fitted with a stepper motor which is also controlled by the ADC board, and the spectral intensities are measured every 2 nm. Data collection and display as well as monochromator operation are controlled with custom software of our own design. Supercritical fluid solutions were prepared in a customdesigned, two-channel, 304 stainless steel cell, fitted with quartz
Solvation in Mixed Supercritical Fluids 4500
'
[
s
I
,
,
,
1
,
A
1
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8795 ,
1
1
I
,
Decanol 30000 h
w
c
5
2
8
v
.-x g
3000
. x
: 20000
. a
2500
-
F
C
01
01
a
C
25003
0
v
1
3 2547 0 psi 2 1824 9 psi 1 1471 0 psi
. . 4
C -
2000
01
V
15000
C
%
1500
i
0
5
1000
LL
5000
500 0
300
350
400
450
500
550
0
600
Wavelength (nm)
Figure 3. Corrected spectra of APS in representative n-alcohols.
windows. One channel is available as a reference, and the second channel, with a volume of 2 mL, contains the sample. Window sealing was accomplished by the use of Teflon gaskets and back plates. Pressure was provided by a piston-type pressure generator (High Pressure Equipment Co. Model 50-6-15). Pressure was monitored with a digital pressure transducer (Omega Model PX102 and Model DP-41V digital readout) located inside the sample chamber. The pressure transducer was calibrated with a Heise bourdon tube gauge (Model CCBDGVSOOOC). Stable temperature was maintained by cartridge heaters controlled by a type-T thermocouple (Omega Model TMQSS-062G-6 and Model CN9000A controller), also located inside the sample chamber. The cell contents were under constant mixing using a Tefloncoated magnetic stir bar. APS spectra were also measured in a series of n-alcohols in order to correlate the supercritical fluid solution spectral properties with alcohol solvent properties. Spectra in the pure liquids were measured with the laser fluorometer described above for comparison of the liquid spectral parameters with supercritical fluid spectra and also with a SLM-AMINCO Model 8 100 fluorometer for comparison of the spectral parameters between alcohols. All liquid-phase measurements were collected at room temperature. Mixed fluid solutions were prepared by addition of 20 p L of APS dissolved in ethanol for the 1 vol % mixtures and 100 p L of APS dissolved in ethanol for the 5 vol % mixtures, with an eppendorf pipet. In both cases, the final APS concentration was 1 X le5M. Before the chamber was loaded with the ethanol sample, the cell was purged twice with COz to remove the residual air. The cell was placed in a COz-filled glovebag, where the appropriate sample was introduced. The cell was then sealed, removed from the glovebag, and connected to the pressure generator. The cell was filled with 800 psi of COz and heated to 328 K. Once the set temperature was reached, CO2 was added to achieve thedesired pressure. APS was purchased from Aldrich, and recrystallized twice from absolute ethanol before use. Spectroscopic grade alcohols were purchased from Aldrich and used without further purification. High-purity, supercritical fluid grade COz (Air Products, 99.9999%) was used without further purification. 111. Results and Analysis
Figure 3 shows the representative speetra of APS in a series of liquid alcohols measured with the S L M 8 100 fluorometer. The integrated spectral areas in the liquid solvents differ by only 10% over the range of solvents, and differences are not correlated with
300
350
400
450
500
Wavelength ( n m )
Figure 4. Corrected pressure-dependentspectra of APS dissolved in 1%
(vol) ethanol/COZ.
35000 h
Y
30000
L
3. 2605 psi 2. 1875 psi 1. 1422 psi
i
3 0 V
%
25000
.-c
vl
C
2 -
20000
5000
0 350
400
450
500
Wavelength (nm)
Figure 5. Corrected pressure-dependentspectra of APS dissolved in 5% (vol) ethanol/COZ.
the solvent permittivity, indicating that the total fluorescence quantum yield of APS is not strongly dependent on the solvent in this series of n-alcohols and that the nonradiative rates from the C T and L E states of APS are similar. This is an important consideration when using spectral intensities as a measure of spectral CT character or solubility in supercritical fluids. The spectra exhibit a characteristic blue shift of both the LE and C T components as the solvent permittivity is decreased, and the ratio of the CT component area to the LE component area also diminishes with decreasing permittivity. These observations are consistent with the studies of Rettig and Chandross.2 Note that the CT band is practically undiscernible in decanol, and this is problematic in fitting the spectrum, as discussed below. Figure 4 shows the dependence of the APS spectrum on fluid composition at 5 5 'C in a mixed ethanol/COz supercritical fluid solvent. The composition changes from 2 mol % ethanol at 1400 psi to 1 mol % at 2600 psi as COz is added to a sample of APS dissolved in 1 vol % ethanol/COz. Figure 5 is a similar plot for a 5 vol % ethanol/COz fluid. All spectra have been corrected for spectrometer efficiency but are otherwise unaltered. The 1 vol % spectra exhibit an increased intensity with increasing pressure, but only a small blue shift of the peak maxima and a slight
8796 The Journal of Physical Chemistry, Vol. 98, No. 35, 199'4 337
I
I
I
Schulte and Kauffman
I
I 0
1 %
e
5 %
2 5 f
i
1
i
U
c
al
: -3
303
OC
430
450
50C
W a v e l e n g t h (nm)
I
1 co3
35c
I
1500
2000
1
2500
3000
Pressure
Figure 6. Relativespectra areas vs pressure for APS dissolved in 1%and 5% (vol) EtOH/C02 fluids. Relative spectral area is defined as the ratio of the spectral area at a given pressure to the spectral area of the 2600
psi spectrum. narrowing of the spectra are observed as COz pressure is increased. Measurements at C 0 2 pressures below 1400 psi show almost no fluorescence. The spectral intensity is found to increase monotonically as the pressure is increased, which we attribute to an increase in the solubility of the APS in the mixed fluid as the density increases. The fluid composition of the 1 vol % sample at 55 OC for pressures in excess of 1400 psi is well within the single-phase region of the fluid-phase equilibrium curve,I3 so the observation is not the result of partitioning of the APS between liquid and supercritical fluid phases, as might be anticipated in the two-phase region. The solubility of the APS appears to be strongly dependent on the C 0 2 density when small modifier concentrations are used. The pressure-dependent spectra of the 5 vol % ethanol sample displayed in Figure 5 exhibit a similar increase in the spectral intensity with pressure, but the relative spectral area, defined as the ratio of the area at a given pressure to the area of the highest pressure spectrum for each sample, is greater at any given pressure for the 5% sample than for the 1% sample. This is illustrated in Figure 6 and indicates that the solubility of APS in the compressible region increases significantly as the mole fraction of ethanol is increased. It is interesting to note that at a pressure of 1550 psi the relative spectral area has a value of about 2.5, indicating that the APS concentration in solution has become greater than the high-pressure concentration. At this pressure, the C 0 2 density is about 0.42 g/mL, which corresponds to a 10 mol 7% ethanol fluid. According to phase equilibrium data for this system,13-'s the single-phase pressure is about 1380 psi so that the system at 1550 psi is well into the single-phase region. However, it is also in the center of the compressible regime of the fluid, in which local solvent density enhancement and modifier enrichment are expected to be large. It is possible that large density and compositional fluctuations in the compressible region provide the system with regions which are highly enriched in ethanol and are therefore better able to solvate the APS. Note also that the spectral shift of the 1550 psi spectrum is intermediate between the 1422 and 1719 psi spectra and is consistent with the generally observed trend that the C T feature shifts toward the blue as the COz fraction is increased. Spectra measured at COz pressures below 1400 psi in the 5 vol % sample show moderate fluorescence intensity but also exhibit fluctuations in the spectral profile which we attribute to the existence of two phases. A
Figure 7. Fit of the spectrum of APS dissolved in 5% (vol) EtOH/C02 at 2600 psi. Contributionsfrom the LE and CT spectral components are shown by the dashed lines. See text for details of the fitting procedure.
comparison of the nearly isobaric spectra from Figures 4 and 5 indicates that a significant C T spectral component arises as the ethanol mole fraction is increased. Only a small fraction of the 1 vol % spectra can be attributed to the C T emission, while the 5 vol % spectra exhibit a strong C T component. This indicates that the increase in solubility of APS as the ethanol mole fraction is increased is accompanied by an increase in the local solvent permittivity around the solute, as discussed below. Importantly, this indicates that increased solubility is the result of enrichment of the local environment in the cosolvent. Quantitative comparisons of the spectra are made following fitting of each spectrum by nonlinear least-squares fitting using the commercially available Peakfit software package. Both the liquid and supercritical fluid spectra can be adequately fit to a sum of an exponentially modified Gaussian spectral component and a simple Gaussian spectral component. A spectrum of APS in 0.2 mol % ethanoljhexane is well fit by the exponentially modified Gaussian functional form alone, and this is assumed to represent the shape of the pure LE spectral component. The exponentially modified Gaussian is a convolution of an exponential function with a Gaussian peak shape and has the form
The function is characterized by an area ALE,a width WLE,a center wavelength ALE, and a distortion parameter Ad. In fitting both the liquid and the mixed fluid data presented here, the width and the distortion parameter of the exponentially modified Gaussian component are fixed to the values determined from the spectrum of APS in 0.2 mol % ethanol/hexane. The center wavelength and area are retained as the fitting parameters. The width WCT,area ACT,and center wavelength ACT of the simple Gaussian, which represents the C T contribution to the spectrum, are also varied in the fitting procedure. This procedure is tantamount to subtracting out the LE spectral component from the experimental spectrum16 and allows us to determine the center frequencies of both the LE and C T spectral components, as well as the relative areas of each component in a well-defined manner. We note that when t b C T band is not clearly discernible in the spectrum, as is in the case for APS in decanol or in the 1 vol % samples, the C T peak center parameter determined by the fitting procedure tends to have a very large standard error. In these
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8797
Solvation in Mixed Supercritical Fluids
TABLE 1: Values of Spectral Parameters from Best Fits of Experimental Spectral to the Sum of LE and CT Contributions.cc See Text for Details of the Fitting Procedure alcohol c n ALE, nm 1@ALe XCT,nm wm 106Am CT/LE 332 f 1.0 0.15 f 0.01 422 f 1.0 48 f 0.95 1.05 f 0.02 6.79 33.62 1.326 MeOH 333 f 0.6 1.22 f 0.07 413 f 1.0 48 f 1.11 1.359 4.59 f 0.1 3.77 EtOH 24.3 1.18 f 0.08 409 1.0 46f 1.11 4.39 f 0.1 3.71 1.383 333 f 0.7 PrOH 20.1 1.397 332 f 0.4 2.43 i 0.09 402 f 2.0 45 f 1.24 4.52 f 0.1 1.86 ButOH 17.8 2.76 f 0.09 403 f 2.0 42 f 1.45 3.30 f 0.1 1.19 1.408 331 f 0.3 PentOH 13.9 330 f 0.4 3.37 f 0.13 398 f 2.0 42 f 1.50 4.40 f 0.2 1.30 13.3 1.416 HexOH 329 f 0.2 1.52 f 0.04 394 f 2.0 41 f 1.44 1.36 f 0.05 0.89 10.3 1.4295 OctOH 327 f 0.3 1.14 f 0.03 397 f 2.0 41 f 1.86 0.91 f 0.04 0.80 8.1 1.4372 DecOH
*
*
a The spectra are assumed to be the sum of an exponentially modified Gaussian (LE component) and a simple Gaussian (CT component). The width and distribution parameters of the LE component are fixed at 9.3 and 25.4 nm, respectively. The tabulated uncertainties are standard errors from the fitting procedure.
TABLE 2
Values of Spectral Parameters from Best Fits of Experimental Spectra to the Sum of Le and CT Contributions.*c
See Text for Details of the Fitting Procedure P,psi
mol frac EtOH
ALE, nm
1361 1422 1550 1719 1875 2056 2264 2605
0.110 0.102 0.085 0.074 0.066 0.060 0.057 0.053
326 f 0.5 331 f 0.4 331 f 0.5 327 f 0.2 327 f 0.3 326 f 0.2 326 f 0.2 325 0.2
1471 1550 1626 1726 1824 2018 2256 2547
0.021 0.018 0.017 0.016 0.014 0.013 0.012 0.01 1
327 f 0.4 325 f 0.3 325 f 0.3 324 f 0.2 323 f 0.2 323 f 0.2 322 f 0.1 321 f 0.2
1 0 5 ~ ~ ~ XCT,nm
Wm
106Am
CT/LE
353 f 3 406 f 1 398 f 1 395 f 1 396 f 1 396 f 1 395 f 1 396 f 1
47 f 3 4732 1 44 f 1 45 f 1 46 f 1 46 f 1 44 f 1 44 f 2
0.08 f 0.001 1.4 f 0.02 4.9 f 0.1 1.4 f 0.04 1.3 f 0.04 1.2 f 0.04 1.2 f 0.04 1.1 i 0.05
0.86 4.43 3.76 1.64 1.58 1.34 0.98 0.87
351 f 8 357 f 8 364 f 7 380 f 7 386 f 8 400 f 8 416 f 14 406 f 37
61 f 7 47 f 6 49 f 5 41 i5 41 f 6 34 f 8 2 0 f 15 38 f 39
0.19 f 0.03 0.13 f 0.02 0.16 f 0.02 0.13 f 0.02 0.12 f 0.02 0.07 f 0.03 0.02 f 0.01 0.03 f 0.01
0.41 0.20 0.25 0.17 0.15 0.07 0.01 0.02
5% Case
*
0.91 f 0.03 3.22 f 0.1 13.30 f 0.7 8.82 0.3 8.43 f 0.3 9.55 f 0.3 12.50 f 0.3 13.30 f 0.3 1%Case 4.82 f 0.1 6.16 f 0.1 6.18 f 0.1 7.74 f 0.2 7.63 f 0.2 10.20 f 0.1 13.40 f 0.1 13.60 f 0.2
a The spectra are assumed to be the sum of an exponentially modified Gaussian (LE component) and a simple Gaussian (CT component). The width and distribution parameters of the LE component are fixed at 9.3 and 25.4 nm, respectively. The tabulated uncertainties are standard errors from the fitting procedure.
cases, the fitted CT peak center is not a good measure of the local solvent polarity, as discussed below. Figure 7 shows the fit of the 5 mol %spectrum at 2605 psi, along with the L E and CT spectral components, as an example of a typical fit. A small shift in the LE maximum is observed as the pressure is increased in both 1 and 5 vol % environments, and the average value of the LE maximum in the 5 vol % spectra is red shifted from the 1 vol % average peak center by about 4 nm. Spectral parameters from the best fits to n-alcohols are tabulated in Table 1, and the parameters for the mixed fluid results are presented in Table 2. The observations are consistent with the assumption that the L E emission from APS is not strongly influenced by solvent permittivity. The C T band of APS in alcohols, on the other hand, exhibits a 25-nm spectral shift as well as a 6-fold change in relative area over the range of permittivities in the alcohol series. The liquid solvent results are in agreement with the results of Su and SimonMJ7 and Rettig and Chandrosse2 The magnitude of the CT band spectral shift provides an empirical measure of the local solvent permittivity around the APS molecules dissolved in mixed supercritical fluids. Mataga and Kubota'* have derived an expression based on the Onsager reaction field theory, which predicts that the shift of the fluorescence feature should be linear in the reaction field function given by
Av = A [ where
7 1
-7 n2-1 ) + 2- 1 ~ + 2n + 2
n + 2
(3) and (4) The px refer to the dipole moments of the ground (x = g) and charge-transfer (x = CT) states, e is the bulk solvent permittivity, n is the solvent refractive index, and a is the Onsager cavity radius. Kajimoto et aI.l9have used this approach to examine the supercritical fluid solvation of 4-(dimethylamino)benzonitrile in CF3H. They suggest that the second term can be neglected, though this will severely effect the spectral shifts predicted directly from the molecular parameters. However, this simplification is adequate for the purpose of correlating the supercritical fluid local solvent parameters with parameters measured for the series of liquid n-alcohols examined here, since the same approximation is applied to each series of spectra. This simplification has the advantage that the analysis does not require input regarding the values of the solute dipole moments. The CT spectral shift is expected to be linear in the expression (5)
(2)
A plot of the C T spectral shift in alcohols vs G,which we refer to as the "Mataga solvent polarity function", is shown in Figure 8. The frequency shift is measured in cm-1 relative to 407 nm,
8798
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994
-750
-500
TABLE 3 Mataga Solvent Polarity Parameter for APS Dissolved in 5% (vol) EtOH/C02 at 55 OC under Several Different CO2 Pressures*f Mataga solvent polarity PCO~, mol P,psi d c m 3 fracEtOH measd h = 1 h = 5 h = 10 hSt dh,d
1
1
1
-
L
‘ E -250 c
/
.c _
1
vr
1
0
-
0,
a
‘
0.5
0 4
“
1
~
”
‘
1
0.7
0.6
0.8
Motoga Solvent Polarity Function
Figure8. Spectralshiftvs Mataga solvent polarity function for n-alcohols. See Table 1 for spectral parameters. 1003
l
I
1
l
’
I
t
I
I
, .
I
I
I
,
I
I
1422 1550 1719 1875 2056 2264 2605
0.35 0.43 0.5 0.56 0.62 0.66 0.71
0.12 0.085 0.074 0.066 0.06 0.057 0.053
0.600 0.520 0.480 0.490 0.490 0.480 0.490
0.18 0.16 0.15 0.13 0.12 0.1 0.08
0.58 0.5 0.44 0.39 0.35 0.32 0.28
0.640 0.650 0.650 0.630 0.610 0.580 0.550
5.5 10.2 5.5 7.2 6.0 8.5 7.0 7.2 7.5 6.9 8.3 7.4 9.1 7.7
a Measured alues aredetermined by a correlation of the spectral shifts with spectral shifts measured in liquid n-alcohols at room temperature. Calculated values make use of the permittivity and refractive index calculated by eqs 7 and 8, respectively. Values are calculated for enrichment factors of 1,5,and 10. An estimated enrichment factor, hlt and an estimated local permittivity, €(hat), are also given. See text for details. C02 density is determined by the Pitzer two-parameter correlation method.2s Values at specific pressures and temperatures are found by linear interpolationoftheavailabledata. EtOH mole fractions are based on I .9mL of C02 in the mixture.
1
I
2.0
i -
Schulte and Kauffman
-
/
t
i i
1
w 0
3
0.5
1 1 1
i’
3 i
t
0.0 -0 5
m
1
/
UL->L-d
1
020 022 924 026 028 030 0 3 2
c t,
, , , , , / , , , , I,
j
, , , ~ , , , , ~ , , , , , , , , , ~
1400 1600 1800 2000 2200 2400 2600 Pressure (psi)
Figure 9. Spectral shift vs pressure for A B dissolved in 5% (vol) EtOH/
c02.
which is chosen arbitrarily as the zero shift point of reference. The plot is linear to within the error of the measurement, having a slope of -7550 cm-1 and an intercept of 4678 cm-. (Note that we have not included the decanol data in determining the parameters of this line, though the decanol point is shown on the plot.) Comparison of the spectral shifts in the 5 vol % mixed supercritical fluid solvent with this plot indicates that the APS exists in an environment having an effective solvent polarity similar to butanol at low pressure and similar to octanol at pressures above 1800 psi. No shift is observed between 1800 and 2600 psi. This is illustrated in Figure 9 as a plot of C T shift vs pressure. The solvent polarity defined by eq 5 has been calculated from the observed spectral shifts of the mixed fluid spectra, assuming that the linear correlation between G and the frequency shift observed for the n-alcohols represents the mixed fluid correlation. These results are presented in Table 3. The C T components of the 1 vol % spectra are so small and the standard errors of the fitted C T maximum are so large that the spectral shift is not useful as a measure of solvent polarity in this solvent system. A second method of extracting the local permittivity from the experimental spectra is based on the analysis of the DMAPS spectra in a variety of polar solvents by Rettig and Chandross.2 They have demonstrated in several TICT compounds that the
Rettig S o l v e n t Polarity Function
Figure 10. Natural log of the ratio of the CT component area to the LE
component area vs Rettig solvent polarity function for n-alcohols. See Table 1 for spectral parameters. ratio of the C T to LE quantum yields is related to the solvent permittivity according to the expression
(Ocr)= OLE
In -
[e-1
n2-1] 2e+ 1 2n2+ 1
We present a plot of this function, which we refer to as the “Rettig solvent polarity function”, for APS dissolved in the alcohol series in Figure 10, in which the fitted areas of the LE and and C T components are assumed to be proportional to the quantum yields of the LE and CTcomponent. This plot is in qualitative agreement with the predictions of Rettig and Chandrossz and supports the validity of our fitting procedure. The current study has examined a broader range of solvent polarities than previous studies,2 and a slight deviation from linearity becomes apparent as the solvent polarity becomes small. We have fitted the natural log of the peak ratios from the alcohol data by linear regression to a line having a slope of 30.3 and an intercept of-7.4. The Rettig solvent polarity values determined from the spectra of APS in the mixed supercritical fluid solvents by correlation with the alcohol data are presented in Table 4. We note again that the uncertainty in the CT spectral component parameters in the 1 vol % spectra as indicated by the standard error is sufficiently large that the actual values derived for solvent polarity from them are of limited use.
Solvation in Mixed Supercritical Fluids
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994 8799
TABLE 4 Rettig Solvent Polarity Parameter for APS Dissolved in 5% (vol) EtOH/C02 at 55 OC under Several Different Cot Pressures*c Rettig solvent polarity PCO~, mol P,psi g/cm3 fracEtOH measd h = 1 h = 5 h = 10 hat €(hat) 1422 1550 1719 1875 2056 2264 2605
0.35 0.43 0.5 0.56 0.62 0.66 0.71
0.12 0.85 0.074 0.066 0.06 0.57 0.053
0.290 0.290 0.260 0.260 0.250 0.240 0.240
0.14 0.12 0.11 0.1 0.09 0.07 0.06
0.29 0.27 0.25 0.23 0.21 0.19 0.17
0.280 6.0 0.300 6.0 0.300 6.0 0.300 6.0 0.290 6.5 0.280 7.0 0.270 8.0
11.9 8.5 6.6 5.5 5.4 5.5 6.0
a Measured values are determined by a correlation of In (Am/&) with values measured in liquid n-alcoholsat room temperature. Calculated values make use of the permittivityand refractive index calculated by eqs 7 and 8, respectively. Values are calculated for enrichment factors of 1, 5, and 10. An estimated enrichment factor, h,,, and an estimated local permittivity, €(hat), are also given. See text for details. C02 density is determinedby the Pitzer two-parametercorrelation method.25Values at specific pressures and temperatures are found by linear interpolation of the availabledata. EtOH mole fractions are based on 1.9 mL of CO2 in the mixture.
parameter. These parameters determine the shape of the sigmoidal function, and for the methanol/CO system, their values are a1 = 0.598 and a2 = 0.151. The parameter e, is the bulk permittivity of the appropriate cosolvent at the appropriate temperature, which for methanol is 28.1. We can use this function in order to estimate the permittivity vs mole fraction for the ethanol/COz system, by retaining the shape parameters determined from the fit of the methanol/C02 system and using c, = 20.1, the value for the ethanol permittivity at 55 OC. Equation 7, therefore, provides an approximate empirical correlation between the permittivity of the ethanol/COzsystem and the polar mole fraction, based on the measurements of methanol/C02 permittivity. While the Kirkwood-Frohlich equation provides a more rigorous estimate of the solvent permittivity for mixtures of polar modifiers and nonpolar fluids, it requires several empirical input parameters, and the correlation approach using eq 7 should be adequate for the current purpose since the mixed fluid systems are similar. In order to calculate the values for the solvent polarity functions described above, we also need a mixing rule for the estimate of the mixed fluid refractive index, and for this purpose, we use the well-known Lorentz-Lorenz law,
It should suffice to say that the effective local solvent polarity in this solvent system is less than that of decanol.
IV. Discussion It is instructive to compare the solvent polarity values extracted from the spectral shifts and intensities with measurements of the bulk permittivityof themethanol/COzsystem, in order toestimate the enrichment of the local environment in the modifier mole fraction. Roskar et a1.20and Dombro et al.21 measure a sigmoidal dependence of the bulk permittivity on the methanol mole fraction, spanning a range of values from the pure CO2 value ( N 1.3) to the methanol value (-25 at 65 "C). Importantly, the density and temperature dependencies of these results are well described by a modified Kirkwood-Frohlich equation,Z2 which predicts a smooth dependence of permittivity on composition and only a slight dependence on T and p at low modifier mole fractions. We anticipate similar results for the ethanol/C02 system a t low modifier mole fractions, since the bulk permittivity must approach the bulk CO2 values as the ethanol mole fraction is decreased. The measurements of Roskar et al.20 show that up to 20 mol % methanol the bulk permittivity of the solvent system is less than 3, indicating that the bulk solvent polarity is dominated by the COz in this range of modifier concentration. On the basis of the permittivities of the alcohols examined in the present study, the effective local permittivity of the mixed supercritical fluid in the vicinity of the solute is expected to be in the 9-15 range, which is a 4-fold to ?'-fold increase over the anticipated bulk value. From the plots of permittivity vs mole fraction methanol,z0 we note that this corresponds to the bulk permittivity of an approximately 50-65 mol % methanol/COz mixed fluid. If we assume that the local permittivity is the result of local cosolvent enrichment around the APS solute molecules, this suggests that a nearly 10-fold increase in the local ethanol concentrations is responsible for the observed C T spectral component. We have fit the results of Roskar et a1.20 to a sigmoidal function given by
The subscripts m, c, and s refer to the mixed solvent parameters, the cosolvent parameters, and the parameters for the major supercritical fluid solvent component, respectively. The modifier mole fraction is given byf, which is multiplied by an enrichment factor h. The enrichment factor is included phenomenologically to account for the increase in the local modifier concentration al represents the center of the transition region, and a2 is the width
where the enrichment factor has again been included phenomenologically. The pure ethanol refractive index is taken to be 1.359, and the density-dependent refractive index of CO2 is determined by linear interpolation of the results of Yonker et al.23 The two solvent polarity functions examined in this study given by eq 5 and 6 have been calculated using the values for n, and 6, determined by eq 7 and 8 for three values of the enrichment factor and are tabulated in Tables 3 and 4. In the case of both the Rettig solvent polarity function and the Mataga solvent polarity function, the calculated values match the empirically determined values a t high pressure when an enrichment factor of 8-10 is used. We attribute this to an approximately 10-fold increase in the local modifier mole fraction around the solute molecules. Note that a similar 10-fold increase of the local cosolvent concentration in 1 vol % study would cause the permittivity to increase by less than a factor of 2 over the entire pressure range examined here, which would not be enough to support a large C T spectral component. This is consistent with the observations made in this study of APS dissolved in 1 vol % ethanol/COz. It is interesting to note that two empirical measures of solvent polarity which utilize the correlations between the solvent properties and different experimental observables give similar values for the effective local solvent permittivity under a particular set of conditions for the 5 vol % spectra. This is an important result with regard to developing a detailed understanding of the underlying cause of the observed spectral changes. We can postulate two qualitatively different scenarios which might give rise to both spectral shifts and CT band intensity changes as the fluid composition is varied. Case i. The first scenario is that the spectra are composed of only the two components which have already been identified, namely, the L E and C T components. In this case, the observed spectral changes are primarily the result of changes in the position, intensity, and width of the C T band, since the LE state is not expected to be strongly influenced by changes in solvent polarity. The fractional contributions of the L E and C T states are governed in this case by the energetics of the charge-transfer reaction, which areinfluenced by the local solvent permittivity. The barrier between the two isomers determines the rate of the charge-transfer CT) reaction is very fast, reaction. If the forward (i.e., LE then the equilibrium between the isomers is achieved within the radiative lifetime of the fluorescent states, and the relative contributions of each isomer to the overall spectrum are governed
-
8800
Schulte and Kauffman
The Journal of Physical Chemistry, Vol. 98, No. 35, 1994
by that equilibrium. We note that this case represents the assumptions which have been made in previous investigations of APS in liquids, in which reasonable agreement between experiment and theory has been obtained. However, the peculiar nature of supercritical fluid solvents-being compressible fluids susceptible to multibody effects such as density and compositional fluctuations-motivates us to consider a second possible spectral model. Case ii. It is possible in this system that two or more distinct types of environments exist, giving rise to two or more distinct local solvent polarities and, therefore, two or moredistinct excitedstate potential energy surfaces. In this case, the kinetic and equilibrium considerations discussed above must be applied individually to each distinct environment, and the steady-state spectrum will be the sum of contributions from APS solvated in each distinct environment. Bright and coworkers24have suggested such a scenario in their analysis of fluorescence lifetime measurements from prodan in supercritical CF3H. They found that the measured phase and modulation spectra were best fit toa Gaussian distribution of lifetimes and interpreted this as resulting from a distribution of local solvent environments. In the case of APS dissolved in 5 vol % ethanol/C02, such a scenario may be appropriate since the solute is insoluble in pure C02 and since it has several different hydrogen-bonding sites. The strong influence of increasing ethanol composition on the observed spectrum suggests that multiple specific interactions may indeed play a role in determining the APS spectrum. In this case, we might expect poor correlation between the overall C T peak shift and the ratio of C T to LE areas, since each distinct environment would be associated with a peak centered at a particular frequency, while the area ratio would be determined by the partitioning of the APS among these environments, rather than by the stabilization of a single C T molecular state. Reasonable agreement between the permittivities estimated from each of the two measures of solvent polarity is observed, and similar trends are observed by both measures. This may provide supporting evidence that the single environment picture adequately represents the spectral behavior, though this is less clear in the higher pressure regime. It is possible that multiple environments become more likely as the mixture becomes more ethanol lean.
V. Summary We have examined the spectra of APS dissolved in ethanolmodified C 0 2 under a variety of conditions. We have correlated spectral parameters of the C T emission band with solvent polarity in liquids and have used this correlation to estimate the local solvent permittivity in the mixed supercritical fluid environment. By comparison of the extracted permittivities with direct measures of bulk permittivities in mixed supercritical fluids, we estimate that the local solvent in the vicinity of the solute is enriched in the cosolvent by a factor of 10. Our measurements provide important insight into the mechanism of solvation of a solute such as APS which is insoluble in pure C02 but can be easily solubilized in the mixed fluid. In this regard, it will be of interest
to compare the results presented here with similar studies of DMAPS, which is soluble in both fluid components. Such a study is currently under way in our lab. The results presented here are consistent with a simple two-state model of the observed spectra, particularly in the high-density region. Unfortunately, we cannot rule out the possibility of a more complex distribution of environments giving rise to the observed spectra. A detailed study of the charge-transfer kinetics of APS in mixed supercritical fluids via temperature studies and fluorescence lifetime investigations will be essential in establishing the underlying cause of the observed dependence of the spectra on fluid conditions.
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. References and Notes (1) Rettig, W. Angew. Chem. 1986, 25, 971. (2) Rettig, W.; Chandross, E. A. J. A m . Chem. SOC 1985, 107, 5617. (3) (a) Sun, Y. P.; Fox, M. A.; Johnston, K. P. J . Am. Chem. SOC.1992, 114, 1187. (b) Morita, A.; Kajimoto, 0.J . Phys. Chem., 1990,94,6420. (c) Hara, K.; Suzuki, H. J . Phys. Chem. 1990,94,6420. (d) Rettig, W. J. Molec. Struct. 1982,84, 303. (e) Sun, Y. P.; Bennett, G.; Johnston, K.P.; Fox, M. A. Anal. Chem. 1992,64, 1763. (4) Su, S.; Simon, J. D. J . Phys. Chem. 1986, 90, 6475. (5) Su, S.; Simon, J. D. J . Phys. Chem. 1988, 92, 2395. (6) Su, S.; Simon, J. D. J. Phys. Chem. 1990, 94, 3656. (7) Lippert, E. Ber. Bunsenges. Phys. Chem. 1988, 92, 417. (8) Hicks, J.; Vandersall, M.; Babarogic, 2.;Eisenthal, K. B. Chem. Phys. Lett. 1985, 116, 18. (9) Hildebrand, J. H. The Solubility of Nonelectrolytes; Dover: New York, 1964. (10) (a) Monin, J. C.; Barth, D.; Perrut, M.; Espitalie, M.; Durand, B. Adu. Org. Geochem. 1987, 13, 1079. (b) Hyatt, J. A. J . Org. Chem. 1984, 49, 5097. (c) Kurnik, R. T.; Reid, R. C. Fluid Phase Equilib. 1982, 8, 93. (1 1) (a) Kim, S.; Johnston, K. P. AIChE J. 1987,33, 1603. (b) Dobbs, J. M.; Wong, J. M.; Lahiere, R. J.; Johnston, K. P. Ind. Eng. Chem. Res. 1987, 26, 56. (12) Betts, T. A.; Bright, F. V. Appl. Spectrosc. 1990, 44, 1203. (13) Jennings, D. W.; Lee, R.; Teja, A. S. J . Chem. Eng. Data 1991,36, 303. (14) Suzuki, K.; Sue, H.; Itou, M.; Smith, R. L.; Inomata, H.; Arai, K.; Saito, S. J. Chem. Eng. Data 1990, 35, 63. (15) Takishima, S.;Saiki, K.; Arai, K.; Saito, S. J . Chem. Eng. Jpn, 1986, 19, 48. (16) Nagarajan, V.; Brearly, A. M.; Kang, T.; Barbara, P. F. J. Chem. Phys. 1987,86, 3183. (17) Su, S . ; Simon, J. D. J . Chem. Phys. 1987,87, 7016. (1 8) Mataga, N.; Kubota, T. Molecular Interactions and Electronic Suectra; Marcel Dekker: New York. 1970. . (19) Kajimoto, 0.;Futakami, M.;'Kobayashi, T.; Yamasaki, K. J . Phys. Chem. 1988. 92, 1347. (20) Roskar, V.; Dombro, R. A.; McHugh, M. A.; Prentice, G. A,; Westgate, C. R. Fluid Phase Equilib. 1992, 77, 241. (21) Dombro, R. A.; McHugh, M. A,; Prentice, G. A,; Westgate, C. R. Fluid Phase Equilib. 1991, 61, 227. (22) Bottcher, C. J. F. Theory of Electric Polarizability; Elsevier: New York, 1973. (23) Yonker, C. R.; Frye, S. L.; Kalkwarf, D. R.; Smith, R. D. J . Phys. Chem. 1986, 90, 3022. (24) Betts, T. A.; Zagrobelny, J.; Bright, F. V. J. Supercritical Fluids 1992, 5, 48. (25) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids; McGraw-Hill: New York, 1987.
