J. Phys. Chem. 1993,97, 11823-1 1834
11823
Spectroscopic Study of Structure and Interactions in Cosolvent-ModifiedSupercritical Fluids David L. Tomasko,' Barbara L. Knutson, Frederic Pouillot, Charles L. Liotta, and Charles A. Eckert' School of Chemical Engineering, School of Chemistry, and Specialty Separation Center, Georgia Institute of Technology, Atlanta, Georgia 30332-0100 Received: February 1 , 1993; In Final Form: May 10, 1993'
Cosolvents can be used to tailor supercritical fluid solvents for increased selectivity by increasing the solvent polarity or polarizability and participating in specific interactions with solutes. Specific interactions in these fluids are often inferred from thermodynamic measurements although independent confirmation of the structure of cosolvent/solute interactions is lacking. The solutes 2-naphthol, S-cyano-2-naphthol, and 7-azaindole exhibit well-characterized spectral responses to an organized, hydrogen-bonded solvent environment in liquids. We describe fluorescence and UV absorption experiments to probe the structure and interaction between polar cosolvents and these solutes in a variety of supercritical fluids. In addition, new solubility data are reported for 7-azaindole in supercritical COz with and without methanol and in supercritical ethane. The results suggest that the interaction between cosolvent and solute can be strong yet exhibit no evidence for actual complexes in solution.
Introduction Further developments in supercritical fluid (SCF) processing are becoming increasingly reliant on tailored intermolecular interactions to effect a separation. Such modifications of phase behavior require a fundamental understanding of specific interactions and solution structure on a molecular scale. While thermodynamic measurements of solubilities are necessary for design, they do not yield much detail regarding the intermolecular interactions in solution. Other independent methods such as spectroscopy or computer simulation must be used to look at the solution on a microscopicscale and provide such detail. Enhanced solubilities caused by the addition of cosolvents have often been considered to result from a general increase in the polarizability of the solvent environment and from specific, structured interactions such as hydrogen bonding. It is these latter interactions in which we are particularly interested, as they provide the basis for improving the selectivity of the solvent. Specifically, we use spectroscopic techniques to probe the structure and strength of cosolvent/soluteinteractionstodiscem the mechanism of solubility enhancement. In this paper we present results from ultraviolet absorption and fluorescence spectroscopy using probes that interact specifically with water and alcohols. Our aim is to study directly the interaction between a solute and a cosolvent in a supercritical fluid with emphasis on local compositions of cosolvent and structure of the solvent about a solute as compared with results from liquids. Solubilitydata in pure and cosolvent-modified SCF's are presented, and the relation of spectroscopic studies to thermodynamic measurements is discussed in terms of the cosolvent effect on solubility.
Background Supercritical fluids are becoming viable processing solvents for specialty chemical and environmental separations.I4 In addition, recent work on micelle formation and particle nucleation suggests completely new technologies arising from the unique properties of SCF's.5.6 Due to the high compressibility and high mass-transfer rates achieved in these fluids,extraction and reaction processes are significantly altered and, in several cases, vastly t
Department of Chemical Engineering, University of Illinois, Urbana, IL
61801.
* Corresponding author. *Abstract published in Aduance ACS Abstracts, August 15, 1993.
improved. They are potential processing solvents for high-value, low-volume products or processes with stringent purity or waste stream requirements-applications which are typically batchoriented with varying feedstocks. Designers can carefully influence the solvent environment through operating conditions or the addition of cosolvents, which affect both equilibria and rates. Many of the potential applications rely on SCF's to separate components which are not amenable to conventional distillation or liquid extraction, due to either the thermal lability of the compounds or the sensitivity to solvent contamination. In many cases, a pure SCF will not give the selectivity to perform a desired separation efficiently, and it is necessary to alter the selectivity of the solvent chemically while maintaining the desired physical characteristics of the SCF state. A common method of tailoring these solvents is the addition of 1-5 mol % cosolvent, which may be chosen to improve selectivities or kinetic processes through a variety of chemical interactions. This addition brings two challenges for design: first, the added complexity of describing ternary phase equilibria, and second, the inadequacy of using bulk properties to model solutions containing local, specific chemical interactions. These challenges point to the need for reliable experimental data using cosolvents as well as an accurate description of the solution behavior on the microscopic scale. Current models of phase equilibria are mildly predictive for solid solutes in pure and mixed SCF's.' Some models have been expanded to incorporate cosolvent effects following the method of Heidemann and Prausnitzs using chemical equilibria coupled with physical equationsof state?JO A chemical-physicalapproach has the advantage of characterizing detailed molecular level interactions coupled with macroscopic phase behavior, which is a more robust method of handling specific interactions in many types of systems." The limitation of these models is the increased number of adjustable parameters which must be determined from experimental data, making prediction difficult. One way of ameliorating this difficulty is measuring equilibrium constants independently through spectroscopic or calorimetric techniques.12J3 More important, these techniques can give us a molecular level understanding of the strength and number of interactions. Developing models of the solution behavior in these highly compressiblefluids will rely on understanding the solvent structure about the solute as it relates to the mechanism of cosolvent enhancement. Several investigatorshave used nonthermodynamic techniques (both spectroscopic and computational) showing the
0022-3654/93/2097-11823$04.00/0 0 1993 American Chemical Society
Tomasko et al.
11824 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
H
’ 0 ‘
H
A
I I
NORMAL (MONOMER)
HpH ’
HpH ’ ‘OH
I
A
’ 0 ‘
k
I
k
+ O ‘H
TAUTOMER
I
A
H Figure 1. Proton transfer from 2-naphtholin the presence of a hydrogenbonded network of water.
‘‘0;
H
I
R
Figure 2. Double proton transfer between alcohol and 7-azaindole to local solvent density to be significantly larger than the bulk.I”l* form the azaindole tautomer. This density enhancement has been called “clustering” or “condensation” of solvent molecules about the solute. Unforprobe could give an almost quantitative indication of local tunately, these terms already mean specific things in certain composition about the solute. research areas so we propose to use the term‘molecular charisma” Recent work on naphthol derivatives shows that the addition to describe this phenomenon. of an electron-withdrawingsubtituent in the 5- and/or 8-position The combination of molecular charisma and addition of a on the naphthalene backbone increases the photoacidity signifcosolvent brings up the possibility of local composition enhanceicantly.27 5-Cyano-2-naphthol (5CN) dissociates with smaller ments. This adds some complexity to the modeling of the concentrations of water and will dissociate in alcohol solution as interactions and reinforces the need for molecular level experwell, presumably through the same solvent network mechanism iments. Substantial evidence exists for local composition endescribed above. hancement in liquid solutions.1g-22 Several studies in SCF’S,*~,~~ As probes for SCF solutions, these molecules have several most of which use data over the entire range of solvent/cosolvent advantages. Unlike several probe molecules or dye indicators, composition, are consistent with local compositionenhancements the physical properties of 2-naphthol are known, as is its solubility of cosolvents in the vicinity of solute molecules. In the range of in several SCF’s. These data, coupled with local composition cosolvent composition of interest to SCF technology, primarily data, could prove quite valuable for developing phase equilibria 0-10% cosolvent, composition gradients may be significant. models. The cyanonaphthol has the advantage of sensitivity to However, the effect of cosolvent composition must be decoupled alcohols which are common SCF cosolvents. Alcohols are from the effects of proximity to the critical point. In other words, typically more soluble in SCF’s than water, giving a more flexible changing the solvent composition also changes the critical point range of experimental conditions. and therefore the region of high compressibility(where molecular We have used 2-naphthol and 5CN in order to probe the local charisma would be expected) is changed as well. Molecular solvent environment and more specificallyto note the conditions, charisma in solutions containing a cosolvent has not yet been if any, under which the cosolvent (water or alcohol) is structured investigated in great detail. Generally speaking, the interactions sufficiently to support proton transfer. between cosolvent and solute are expected to be greater than A second type of probe, 7-azaindole, is more sensitive in the between solvent and solute, making it difficult to separate respect that one can monitor two independent interactions with molecular charisma from a local composition effect. In our work, alcohol cosolvents. First, the tautomer of azaindole resulting we have chosen to work at similar reduced temperatures and from double proton transfer can be observed as a broad reduced densities as we believe these two properties will be more structureless emission red-shifted from the monomer by 120effectivein defining a constant environment for studyingcosolvent 160 nm. Second, an exciplex emission arising from an excitedcomposition effects. state alcohol-azaindole complex can be observed as a distinct red-shifted peak (20-60 nm) similar in shape and overlapping Spectroscopy is a powerful technique for studying these local with the monomer fluorescence band. interactions resulting from specific, structured environments. Charge-transfer complexation and excited-state reactions such The tautomer species is a well-documented result of double as proton transfer are two examplesof spectroscopicevents which proton transfer in the excited state of doubly-hydrogen-bonded azaindole dimers or a cyclic hydrogen-bonded azaindole/alcohol occur in specific solvent environments and are sensitive to small complex.2b33 In non-hydrogen-bonding solvents, the relative concentrationsof cosolvent. Fluorescencespectroscpy is sensitive intensity of tautomer fluorescence is sensitive to temperature, enough to study dilute solutions of solutes (1W mole fraction) excitation wavelength, and concentration. In this case, it has so that the cosolvent/solute interaction may be isolated from been shown that the tautomer results from double proton transfer solute/solute interactions. in an excited-statedimerof twoazaindolemolecules. In hydrogenbonding solvents (e.g., alcohols), the relative intensity of the Fluorescent Solvent Robes tautomer does not depend on concentration and is therefore not the result of dimer formation. Instead, the tautomer results from Proton transfer from excited-state naphthols has been shown a cyclic hydrogen-bonded complex in the ground state which to be sensitive to solvent structure in liquids. The hydroxylic undergoesdouble proton transfer upon excitation. This is depicted proton on the naphthols undergoes a large change in acidity upon schematically in Figure 2. excitation and, in the presence of a hydrogen-bonded solvent The formation of the hydrogen-bonding species of azaindole network, will dissociate,leaving the naphtholate anion which emits dimers or azaindole/alcohol complexes, precursorsto the excitedat a different characteristic wavelength than the neutral species state species which undergo double proton transfer to form the (see Figure 1). Studies of 2-naphthol in aqueous solution show tautomer, occurs in the ground state and can be observed through that the proton transfer is dependent upon specific solvent absorption measurements. The dimer or complex is manifested organization around the proton, and 2-naphthol will dissociate in a red-edge shoulder to the azaindole spectrum. Absorption only in the presence of water but not in the presence of alcohols studies give the equilibrium constant for the formation of dimer or solvents with lower dielectric constants.25 A random-walk in liquid 3-methylpentane as 1.8 X IO3M-1 with Ah = -9.6 kcal/ solvation model of the quantum yields in aqueous solutionindicates mol. For the ethanol/azaindole complex in liquid hydrogen a minimum cluster size of four water molecules necessary to solvent, a Bensesi-Hildebrand analysis confirmed a 1:1 stoichifacilitate proton transfer.26 Therefore, using 2-naphthol as a
Cosolvent-Modified Supercritical Fluids
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11825
QUENCH BY EJECTION TO SOLVENT \
COSOLVENT VESSEL COMPRESSOR
Figure 3. Exciplex formation between alcohol and 7-azaindole.
ometry with an equilibrium constant of 49 M-I, much smaller than that observed for the d i m e r i ~ a t i o n . ~ ~ ~ ~ ' The exciplex between alcohols and 7-azaindole has been less well characterized than the tautomeric equilibria. The work of Collind4 draws on previous work carried out on indole and demonstrates very nicely a similar mechanism for exciplex fluorescence in the case of azaindole. The azaindole emission maximum shifts sharply with the addition of small amounts of alcohol or water cosolvents to liquid hydrocarbon solvent and begins to level out at higher concentrations (>0.1 M). The concentrationscausingthisinitial shift are far below that necessary to alter the dielectric properties of the solvent. With the addition of alcohol, an initial broadening and then narrowing of the overall bandwidth was indicative of two emitting species at intermediate alcohol concentrations with complete conversion to exciplex fluorescenceat concentrationsgreater than 0.1 M. Figure 3 shows the proposed mechanism of exciplex formation wherein the nonbonded electrons on the hydroxylic oxygen interact with the vacant A orbital of the excited azaindole. The azaindole emission appears to go through a minimum in intensity at 0.5 M alcohol, and the greater intensity at higher concentrations is attributed toexciplexformation with aggregated alcohol. Through a kinetic analysis, the exciplex was found to result from a single alcohol complexing with a single azaindole molecule. It has been shown that the mechanism of exciplex formation in indoles is not due to hydrogen bonding with the >N-H moiety. Walker et al.35 observed similar strong shifts in the emission behavior with 1-methylindole/ethanol and indole/ethanol. This result led them to the conclusion that the interaction causing the exciplex occurs above the plane of the ring. In view of the electronic similarities with indole, there is no reason to believe that the exciplex mechanism for azaindole would be different.
Experimental Section Fluorescence Spectroscopy. The apparatus and technique for carrying out fluorescencemeasurements on supercriticalsolutions have been described previously.*s Experiments were carried out for a series of densities in the same range for several cosolvent concentrations in each particular fluid. Temperature was measured by recording the resistance of a thermistor probe (Omega type 44032) calibrated to fO.l OC. Pressure was monitored with a Texas Instruments quartz tube pressure gauge (Model 140) accurate to *0.2 bar. Temperature was controlled with a custom-built three-mode controller constant to f0.02 OC. For low-density conditions at high cosolvent concentrations, the onset of phase separation was readily indicated by a severe loss of fluorescence intensity, so all data reported are in a singlephase fluid. Mixture densities were determined by multiplying the pure SCF density by a ratio of mixture to pure density calculated from a Soave-Redlich-Kwong equation, using a binary interaction parameter (k12) which was set to 0.1 or fit to VLE (vapor-liquid equilibrium) data, if available. Pure C02 and C2H4 density data were interpolated from published tables (C02,36C2H437)while pure C2& and CHF3 density data were calculated by modified BWR equations (C2H6,38 CHF339). The binary interaction
MIXING VESSEL
Figure 4. Schematic of high-pressure system for preparing cosolvent modified SCF's.
W ABSORPTION APPARATUS
u
I
Piston Pressure Generator Figure 5. Schematic of apparatus for recording UV absorption spectra. parameter was fit to VLE data for the COz/methanol mixtures: k12 = 0.12 (35 "C), 0.14 (50 "C), and 0.16 (70 "C). A schematicof the high-pressuresystem for preparing cosolvent solutions is shown in Figure 4. A small filter paper was coated with a desired amount of solute in a liquid solution, and thevolatile solvent was allowed to evaporate. The filter paper was attached to the top plug of a 1.6-L pressure vessel, which was evacuated for 30 min to D 0.003 in C2H6 and X M ~ D > 0.005 in COz. NMR chemical shift data for dilute alc0hols5*-~~ appear to indicate that it is necessary to go to concentrations below 10-3mole fraction to have unassociated alcohol in deuterated cyclohexane at 34 OC.
11832 The Journal of Physical Chemistry, Vol. 97, No. 45, 1993
Tomasko et al.
TABLE V: I-Azrindole Fluorescence Data in Fluorofom P
Pmir P [EtOH] a ,, (mol/L) (cm-1) (mol/L) (mol/L) 7-Azaindole 0, = 1 X lW):CHF,, T = 30.1 OC (Tr = 1.01) 139.4 14.28 14.28 O.Oo0 29 856 O.Oo0 29 997 91.7 13.03 13.03 O.Oo0 30 079 80.2 12.52 12.52 O.Oo0 30 085 71.0 11.97 11.97 O.Oo0 30 171 65.6 11.52 11.52 61.6 11.03 11-03 0.000 30 252 O.Oo0 30 339 58.5 10.47 10.47 O.Oo0 30 414 56.9 10.04 10.04 O.Oo0 30 643 55.5 9.44 9.44 O.Oo0 30 631 54.2 8.92 8.92 O.OO0 30 931 54.2 8.32 8.32 O.Oo0 31 636 53.7 7.78 7.78 O.Oo0 31 951 53.3 7.49 7.49 53.0 7.13 7.13 0.000 31 908
(bar)
7-Azaindole (y = 1 X 10-6): EtOH (y = 3 X 1P3):CHF3, T = 30.1 OC (Tr = 1.01) 91.0 12.98 13.04 0.039 29 558 91.2 12.98 13.05 0.039 29 683 71.2 11.96 12.06 0.036 29 708 61.2 10.95 11.12 0.033 29 733 56.8 9.94 10.21 0.03 1 29 777 56.0 9.54 9.85 0.030 29 684 55.1 8.96 9.93 0.030 29 661 54.8 8.54 9.38 0.028 29 706 55.0 8.58 8.97 0.027 29 821 54.8 7.91 8.27 0.025 29 983 54.4 7.86 8.29 0.025 29 912 54.2 7.80 8.27 0.025 30 187 53.6 7.67 8.14 0.024 30 397 53.2 7.16 7.66 0.023 30 499 52.8 6.88 7.44 0.022 30 615 7-Azaindole (y = 1 X 10-6): EtOH (y = 1.2 X 1p2):CHF3, T = 30.1 OC-(Tr = 1.01) 97.7 13.22 13.48 0.162 30 117 84.4 12.70 13.02 0.156 30 149 73.6 12.12 12.54 0.150 30 175 66.2 11.51 12.07 0.145 30 182 61.1 10.83 11.57 0.139 30 176 57.4 9.90 10.94 0.131 30 175 57.6 9.88 10.91 0.131 30 157 55.8 8.94 10.27 0.123 30 146 54.6 7.75 9.44 0.113 30 086 54.3 7.27 9.11 0.109 30 131 8.62 0.103 30 146 563.7 6.57 0.097 5.95 8.1 1 53.1 30 124 0.095 5.72 7.93 52.8 30 162 7-Azaindole 0, = 1 X 10-6): EtOH (y = 3.8 X 10-2):CHF3, T = 30.1 OC (Tr = 1.01) 134.2 14.25 14.78 0.562 28 465 95.0 13.07 13.91 0.529 28 454 78.2 12.42 13.53 0.514 28x590 63.2 11.25 13.01 0.494 28 580 56.8 10.02 12.62 0.480 29 783 54.8 9.08 12.23 0.465 29 848 53.9 8.22 11.76 0.447 29 864 53.1 7.16 11.20 0.426 29 825 52.6 6.39 10.68 0.406 29 822 51.9 5.67 10.45 0.397 29 806 7-Azaindole 0,= 1 X 10-6):CHF,, T = 44.8 OC (Tr = 1.06) 143.6 13.06 13.06 O.Oo0 30 225 125.4 12.51 12.51 0.000 30 324 O.Oo0 30 267 112.6 12.02 12.02 102.8 11.54 11.54 O.Oo0 30 268 O.Oo0 30 364 94.8 11.04 11.04 O.Oo0 30 339 88.7 10.54 10.54 84.2 10.04 10.04 0.000 30 348 9.51 O.Oo0 30 458 80.4 9.5 1 O.Oo0 30 475 77.6 9.03 9.03 75.3 8.56 8.56 O.Oo0 30 496 0.000 30 632 73.0 8.04 8.04 71.3 7.59 7.59 O.Oo0 30 687 0.0oO 30 953 69.2 6.99 6.99 85.6 6.16 6.21 0.0 19 30 073 82.9 5.80 5.85 0.018 30 086
P
(bar)
P
Pmir
(mol/L)
(mol/L)
[EtOH] (mol/L)
7-Azaindole 0, = 1 X 10-6): EtOH (y = 3 X lO-’):CHF,, T - 44.8 OC (Tr 155.4 13.31 13.35 0.040 141.1 12.97 13.02 0.039 112.3 11.98 12.04 0.036 94.7 11.00 11-08 0.033 84.3 10.00 10.10 0.030 80.8 9.33 9.44 0.028 77.8 8.88 9.00 0.027 75.6 8.45 8.58 0.026 73.6 7.94 8.07 0.024 73.5 7.95 8.08 0.024 71.7 7.51 7.64 0.023 70.8 7.25 7.37 0.022
a ,,
-
(cm-1) 1.06) 29 607 29 596 29 601 29 657 29 700 29 727 29 822 29 786 29 787 29 925 30 029 30 036
7-Azaindole (y = 1 X 10-6): EtOH 0, = 1.2 X 1k2):CHF3, T = 44.8 OC (Tr = 1.06) 136.2 12.84 13.04 0.156 29 633 107.5 11.78 12.06 0.145 29 653 90.9 10.73 11.11 0.133 29 726 81.6 9.70 10.18 0.122 29 703 78.4 7.19 9.70 0.116 29 725 75.8 8.68 9.23 0.1 11 29 743 75.9 8.68 9.23 0.111 29 743 73.7 8.19 8.76 0.105 29 719 0.100 29 747 71.8 7.72 8.3 1 70.1 7.24 7.83 0.094 29 776 68.5 6.78 7.36 0.088 29 753 65.9 6.01 6.52 0.078 29 841 61.2 4.84 5.16 0.062 29 998 57.5 4.14 4.35 0.052 30 188 7-Azaindole 0, = 1 X 10-6): EtOH 0, = 3.8 X lO-*):CHF3, T - 44.8 O C (Tr = 1.06) 137.2 12.88 13.50 0.513 28 889 137.2 12.88 13.49 0.513 28 988 131.0 12.69 13.35 0.507 28 997 115.0 12.1 1 12.91 0.491 28 857 100.5 11.40 12.39 0.471 28 815 90.4 10.68 11.88 0.451 28 893 83.4 9.93 1 1.34 0.431 28 811 83.6 9.93 11-34 0.431 28 794 78.7 8.57 10.06 0.382 28 802 75.5 8.56 10.32 0.392 28 758 72.7 7.90 9.80 0.372 28 771 72.6 7.86 9.77 0.371 28 778 70.2 7.22 9.24 0.351 28 724 68.5 6.75 8.82 0.335 28 737 7-Azaindole 0, = 1 X 10-6):CHF3, T = 64.4 OC (Tr = 1.13) O.OO0 30 120 176.6 12.25 12.25 O.OO0 30 058 153.8 11.47 11.47 O.Oo0 30 073 140.8 10.98 10.98 0.000 30 128 130.4 10.48 10.48 O.Oo0 30 114 122.1 10.00 10.00 O.Oo0 30 188 114.8 9.48 9.48 O.OO0 30 151 109.2 9.06 9.06 O.Oo0 30 235 103.8 8.56 8.56 99.4 8.10 8.10 0.000 30 283 95.7 7.66 7.66 O.Oo0 30 324 61.5 7.15 7.15 O.OO0 30 581 87.9 6.65 6.65 0.000 30 484 84.1 6.12 6.12 O.Oo0 30 591 7-Azaindole (y = 1 X 10-6): EtOH 0, = 3 X 10-3):CHF3, T = 64.4 OC (Tr 1.13) 171.5 12.04 12.07 0.036 29 567 1 1.06 11.11 0.033 29 657 143.0 124.0 10.09 10.14 0.030 29 679 116.9 9.60 9.66 0.029 29 646 110.9 9.10 9.16 0.027 29 669 105.7 8.62 8.68 0.026 29 530 105.8 8.63 8.69 0.026 29 657 101.0 8.11 8.17 0.025 29 682 97.0 7.65 7.71 0.023 29 783 93.1 7.14 7.20 0.022 29 848 89.2 6.64 6.70 0.020 29 857
-
Cosolvent-Modified Supercritical Fluids
The Journal of Physical Chemistry, Vol. 97, No. 45, 1993 11833
TABLE V (Continued) P P Pm* [EtOH] UrmX (bar) (mol/L) (mol/L) (mol/L) (cm-1) 7-Azaindole (y = 1 X 10-6): EtOH 0, 1.2 X lW2):CHF3, T = 64.4 'C (Tr = 1.13) 174.8 146.5 126.9 120.5 113.0 113.0 107.8 102.7 98.5 94.5 90.7 87.2 82.1 77.4 72.2 67.6
12.13 11.20 10.25 9.85 9.28 9.28 8.81 8.30 7.83 7.33 6.84 5.98 5.69 6.08 4.41 3.97
12.28 11.40 10.48 10.09 9.54 9.54 9.08 8.58 8.10 7.60 7.10 6.19 5.90 5.25 4.60 4.07
0.147 0.137 0.126 0.121 0.114 0.114 0.109 0.103 0.097 0.091 0.085 0.074 0.07 1 0.063 0.055 0.049
29 538 29 520 29 516 29 525 29 550 29 512 29 532 29 553 29 592 29 605 29 647 29 853 29 891 29 998 30 244 30 406
2.0
W 0
f 1.5 m
CY
---
0.5
PURE ETHYLENE
- --- 0.5% ACETIC ACID
\
- 3.4% ACETIC ACID
0.01
250
"
280
"
270
"
280
"
290
"
300
"
310
320
330
WAVELENGTH (nm) Figure 13. UV absorption spectra of 7-azaindole in supercritical C2H4 with 0,0.5, and 3.4% acetic acid cosolvent (13 "C). Spectra have been normalized to the maximum intensity.
TABLE VI: Solubility Data for 7-Azaindole temp t 0.05 press 0.3 solubility f 5% (mole fraction) solvent ("C) (bar)
*
co2 COz/methanol (3.25%)
ethane
35.0 35.0 35.0 35.0 35.0 35.0 35.0 35.0 36.3 36.3 36.3 36.3
90 100 120 1so 90 100 120 150 57.7 65.2 78.2 100
,
0.OOO 86 0.001 34 0.001 65 0.001 92 0.005 32 0.005 44 0.006 57 0.007 28 0 . m 37 0.000 46 0.OOO 56 0.000 77
This observation along with the results presented above seems to indicate a dynamic solution having a much greater free volume than a liquid at a comparable density. In this environment, the molecules are close enough to interact strongly,giving macroscopic results such as enhanced solubilities, but there appears to be insufficient structure to restrict the intermolecular interactions to a particular conformation. While it is certainly enticing to model SCF solutions using chemical equilibria for cosolvent/ solute interactions,perhaps this shouldbedonewithsomecaution.
P (bar)
PmiX P [EtOH] (mol/L) (mol/L) (mol/L) 7-Azaindole 0, = 1 X 10-6): EtOH 0, 3.8 X 1P2):CFH~,T = 64.4 "C (Tr
169.6 151.8 137.8 126.9 118.5 118.5 111.8 105.9 101.2 95.6 90.3 85.5 79.7
11.98 11.41 10.83 10.26 9.71 9.71 9.18 8.63 8.12 7.47 6.78 6.13 5.36
12.50 12.00 11.51 11.01 10.53 10.53 10.05 9.54 9.05 8.40 7.69 6.98 6.02
0.475 0.456 0.437 0.418 0.400 0.400 0.382 0.363 0.344 0.319 0.292 0.265 0.231
UrmX
(cm-1)
1.13) 28 686 28 685 28 743 28 775 28 771 28 778 38 793 28 767 28 749 28 773 38 772 28 741 28 784
ecules, and some kinetic data.s2 All the results available are consistent in that it seems that hydrogen bonding is less influential in SCF's than in liquids, either because it occurs less or because it is of a different nature. A conclusion that is not inconsistent with these observations is that the structure and especially molecular mobility in the fluid play a large role in determining the nature of the hydrogen bonding. This mobility can be more conveniently expressed in terms of free volume than in terms of density. The molecular dynamic resultsL6show the tremendous mobility of the solvent shell around solutes in SCF's, and this is a consequence of the large free volume. Apparently structuring simply does not occur. It has been tempting to explain many solubility enhancements by cosolvents in terms of supposed hydrogen bonding. Many cases exist of basic solutes and protic solvents, and the hydrogen bond has frequently been observed in liquid solvents. But apparently the governing phenomena may be different in SCF solutions, and there are extensive new data on cosolvent effects which support such a c o n c l ~ s i o n It . ~appears ~ that many cosolvent effects may be due to density enhancement by cosolvent addition, dielectric enhancement by cosolvent addition, dipole-dipole coupling, or charge-transfer complexes. One must explore more subtle interactions to explain specific cosolvent effects (Le., the observed solubility enhancement over that due to the increase in density). It is possible, for example, to have cosolvents and solutes interact by aligning bond dipoles rather than by forming molecular dimers. This type of interaction may result in cosolvent effects between basic cosolventsand basic soluteswhich otherwisewould not be expected. Electroniccharge transfer is particularly efficientwith T systemsin aromatics which comprise the majority of solutes studied in SCF's. This type of interaction could give strong cosolvent effects without structured molecular configurations, and exploratory studies have been reported.$'
Acknowledgment. The authors acknowledgefinancial support from the Department of Energy through Grants DE-FG2288PC8922 and DE-FG22-91PC91287 and the E.I. Du Pont Nemours Company. The able assistanceofJoseph Coppom, Barry West, and Wendy Windsor in the laboratory is sincerely appreciated. We thank Dr. L. M. Tolbert for helpful discussions regarding the naphthol systems and for providing the 5-cyano2-naphthol.
Conclusions
References and Notes
There now appears to be a growing body of evidence that indicates that SCF solvents do not support highly-organized hydrogen bonding in the same manner as do liquid solvents. These data include FTIR measurements Of hydrogen bonding,s3Spectroscopic data measured here on structure-specific probe mol-
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