2374
J . Phys. Chem. 1988, 92, 2374-2318
also the most extensive desolvation, which virtually dominates the complex stability. This image coincides nicely with the complexation mechanism of a cation with a bis(crown ether), and the present result again supports our proposal that the complexation phenomenon can be understood well in terms of the enthalpyentropy compensation effect.
Acknowledgment. This work was supported by the National Science Fund of China and in part by Grant-in-Aid for Scientific Researches 61550645 from the Ministry of Education, Science, and Culture of Japan, which are gratefully acknowledged. Y.L. is grateful for the generous financial support of Ako Chemical Co. for his stay at Himeji Institute of Technology.
Solvatochromic Behavior of Binary Supercriticai Fluids: The Carbon Dioxlde/S-Propanol System C. R. Yonker* and R. D. Smith Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, Richland, Washington 99352 (Received: September 8, 1987)
Dependence of the solvatochromic shift for binary supercritical fluid mixtures as a function of modifier mole fraction, temperature, and pressure is discussed. Carbon dioxide/2-propanol binary fluid mixtures were studied at both subcritical and supercritical conditions. The local composition of the cybotactic region was seen to be enriched in organic cosolvent at low pressures. As the pressure or temperature increased, the local composition of the organic cosolvent decreased. A simple theory is developed to describe the local composition of the fluid in the cybotactic region about the solvatochromic probe molecule.
Introduction The local composition of bulk solvent modifier or cosolvent about a solute in liquid mixtures is an area of active research.'-' In supercritical fluids the local composition and local density of solvent molecules about the solute at high dilution can be related to the partial molar volume of the solute in the fluid phase.*-I0 The solvent cluster of supercritical C 0 2about naphthalene corresponds to approximately 80 solvent molecules over multiple solvent shells at 35 O C and 80 bar.gJ0 The effect of an organic cosolvent on the local composition surrounding a solute molecule for binary supercritical fluid solutions has only recently been studied.I0 The enrichment of the organic cosolvent by both specific and nonspecific solutesolvent interactions should provide a basis for understanding solvent modifier effects in supercritical fluids as well as the phenomena of cluster formation in pure and binary supercritical fluid solutions. The specific solute-binary fluid interactions in the local environment of the solute molecule have direct relevance to supercritical fluid extractions,I1J2chromatographic ~electivity,'~-'~ and molecular diffusion." The cybotactic region of the solute molecule (the region of solvent molecules whose structure is influenced by the presence of the solute molecule), is best explored by a technique that does (1) Sandler, S . I. Fluid Phase Equilib. 1985, 19, 233. (2) Lee, K. H.; Lombardo, M.; Sandler, S.I. Fluid Phase Equilib. 1985, 21, 177. (3) Lee, K. H.; Sandler, S. I.; Patel, N. C. Fluid Phase Equilib. 1986, 25, 31. (4) Makanish, K.; Tanaka, H . Fluid Phase Equilib. 1983, 13, 371. (5) Hu, Y.; Azevedo, E. G . ;Prausnitz, J. M. Fluid Phase Equilib. 1983, 13, 351. (6) Hy, Y.; Ludecke, D.; Prausnitz, J. Fluid Phase Equilib. 1984, 17, 21 7. (7) Mansoori, G. A,; Ely, J. F. Fluid Phase Equilib. 1985, 22, 253. (8) Eckert, C. A.; Ziger, D. H.; Johnston, K. P.; Kim, S . J . Phys. Chem. 1986, 90, 2138. (9) Ziger, D. H. Ph.D. Thesis, University of Illinois, Urbana, 1983. (IO) Kim, S . ; Johnston, K. P. AIChE J . 1987, 33, 1603. ( 1 1 ) Wong, J. M.; Johnston, K. P. Biotechnol. Prog. 1986, 2, 29. (12) Dobbs, J. M.; Wong, J. M.; Johnston, K. P. J. Chem. Eng. Data 1986, 31, 303. (13) Yonker, C. R.; Smith, R. D. Anal. Chem. 1987, 59, 727. (14) Yonker, C. R.; Smith, R. D. J . Chromatogr. 1986, 361, 25. (15) Blilie, A. L.; Greibrokk, T. J . Chromatogr. 1985, 349, 3 17. (16) Schmitz, F. P.; Hilger, H.; Lorenschat, B.; Klesper, E. J . Chromatogr. 1985. ... .346. 69 (17) Saiiat, R. R.; Mourier, 0.;Caude, M. H.; Rosset, R. H. Anal. Chem. 1987, 59, 1164. ~
not perturb the local equilibrium composition. The spectroscopic study of the local solvent environment has been applied to liquids18 and recently to pure supercritical The solvatochromic behavior of the nearest-neighbor solvent shell about a solute, obtained by monitoring their effect on the stabilization of the excited-state dipole of the solute probe molecule, directly provides information on the local composition for pure and binary supercritical fluid systems. In this paper we report a study of the cybotactic region for a solvatochromic probe molecule and determine the local composition as a function of temperature, pressure, and concentration of organic cosolvent. The specific system chosen for study, C02/2-propanol, is also one of significant practical importance. The understanding of solvent structure and composition of the cybotactic region for supercritical fluids provides a basis for bridging the gap between gas-phase and liquid-phase cluster formation and should provide a greater understanding of organized molecular structures in both supercritical fluids and liquids.
Theory The ground state of a dilute solute probe solvated by a pure supercritical fluid, such as carbon dioxide, will have an equilibrium energy of solvation determined largely by dipole-induced dipole interactions with the solvent molecules. This assumes there are no specific intermolecular interactions such as hydrogen bonding or charge-transfer-complex formation. With the introduction of a polar organic cosolvent (e.g., 2-propanol) to the supercritical fluid, both dipole-dipole and specific intermolecular interactions become possible. Spectroscopic studies of the solvent structure about the solute can be accomplished by the solvatochromic technique. On excitation of the ground state of the solute molecule to the excited state, the dipolar orientation of the solvent molecules remain frozen in their ground-state configuration because the (18) Kamlet, M. J.; Abboud, J.-L. M.; Taft, R. W. Prog. Phys. Org. Chem. 1980, 13, 485. (19) Yonker, C. R.; Frye, S. L.; Kalkwarf, D. R.; Smith, R. D. J . Phys. Chem. 1986, 90, 3022. (20) Frye, S . L.; Yonker, C. R.; Kalkwarf, D. R.; Smith, R. D. Supercritical Fluids: Chemical and Engineering Principles and Applications; Squires, T. G.;Paulaitis, M. E., Eds.; ACS Symposium Series 329; Amercian Chemical Society: Washington, DC, 1987; Chapter 3. (21) Hyatt, J . A . J . Org. Chem. 1984, 49, 5097. (22) Sigman, M. E.; Lindley, S. M.;Leffler, J. E. J . Am. Chem. SOC.1985, 107, 1471.
0022-3654/88/2092-23~4%01.50/0 0 1988 American Chemical Society
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2375
Carbon Dioxide/2-Propanol System solvent dielectric relaxation time is many times greater than the lifetime of the excited The inductive component of solvation is therefore in equilibrium with the excited-state dipole of the solute. For binary fluid mixtures containing a polar organic cosolvent the solvatochromic bevavior of the solute can be described by"
v,,, = vo
+ Sa* + aa
(1)
The maximum frequency of solute absorbance in the supercritical fluid is v,, vo is the absorbance frequency in a reference solvent, S is the susceptibility of the solute molecule to the solvent's polarity/polarizability, A* is a n empirical solvent strength parameter related to the fluid's polarity/polarizability, a is the hydrogen-bond donor capability (acidity) of the fluid, and a is the susceptibility of the solute to hydrogen-bond formation.1s-z2 The a scale for this linear solvation energy relationship is difficult to quantitate because of self-association of weak organic acids such as 2-propan01.l~ Therefore, the net effect of the organic cosolvent on the solvatochromic shift will be included in the a* value and eq 1 reduces to Vmax
= vo
+ Sa*'
here T*' contains the hydrogen-bonding contribution coupled with the solvent polarity/polarizability. From eq 1, the a* value for a pure supercritical fluid, e.g., C 0 2 ( a = 0), can be determined from simple spectroscopic experiment^.^^^^^ Equation 2 can be used to determine the T*' value of a binary supercritical fluid containing a polar organic cosolvent capable of hydrogen bonding with the solute probe. Assuming the cybotactic region of the solute molecule extends over the nearest-neighbor solvent shell at low fluid densities (although at higher fluid densities next-nearest-neighbor interactions may come into play), then the solvatochromic information will directly reflect the local composition of the solvent molecules and be related to the number of organic cosolvent and solvent molecules contained in the cybotactic region. The equilibrium composition of the cybotactic region will be determined by the solute molecules' ground state and remains invariant upon excitation due to the solvents' dielectric relaxation time. On the basis of these assumptions, one can determine the difference in the solvatochromic shift (AT*) between that calculated for the bulk composition and the experimentally measured value (a*m)for the binary supercritical fluid:
The local composition of organic cosolvent (Xlz) and of the dense gas (X3-J in the cybotactic region is approximated by the bulk ~ are determined from composition values. The a*'] and T * values solvatochromic measurements for the pure organic cosolvent and pure supercritical fluid, respectively, at the specified pressure and temperature. Using eq 3, one can determine the relationship between the bulk binary fluid composition and the local composition of the organic cosolvent in the cybotactic region. If AT* is zero, then the bulk and local compositions are the same; if AT* is greater than zero, the local composition of organic cosolvent is greater than the bulk composition; and if AT* is less than zero, the local composition is depleted in organic cosolvent compared to the bulk. The calculation of the local composition of the organic cosolvent in the cybotactic region is obtained, assuming AT* = 0 and XI2 X32 = 1; therefore
+
X12 = T*m
- 7r*3/A*'1
- a*3
(4)
where XI, is the mole fraction of organic cosolvent in the cybotactic region of the solute. Equation 4 is based upon the assumption of a linear relationship between the mole fraction of the organic cosolvent in the cybotactic region and the T* values for the pure cosolvent and pure supercritical fluid. This simple relationship (23) Ehrenson, S. J . Am. Chem. SOC.1981, 103, 6036. (24) McRae, E. G . J . Phys. Chem. 1957, 61, 562.
in eq 4 has been used to calculate local fluid compositions as a function of pressure, temperature, and mole fraction of the organic cosolvent in the binary fluid. The limitation of this methodology entails the grouping of specific and nonspecific intermolecular interactions together into a*']. This does not allow one to determine the extent of hydrogen-bonding interactions in the local composition about the solute. However, an important advantage is the ease with which spectroscopic measurements can be made and the local composition of the cybotactic region calculated. Experimental Section
The experimental procedure and equipment have been discussed in detail e l s e ~ h e r e . The ' ~ ~ spectrophotometer ~~ used was a Varian 2200 operated in the UV region from 335 to 280 nm. The signal was smoothed and the peak maximum was determined by using the midway point of the peak width at 80% of the total peak height. This methodology resulted in greater reproducibility of the maximum absorbance frequency. The pressure in the sample cell was controlled by a high-pressure syringe pump operated under microprocessor control. The solvatochromic probe (2-nitroanisole) was introduced into the magnetically stirred sample cell with a high-pressure H P L C sampling valve. Carbon dioxide was the supercritical fluid solvent used combined with 2-propanol as the organic cosolvent. The binary fluid mixtures were mixed in a lecture bottle and then loaded into the syringe pump. The mole fraction of organic cosolvent was calculated by weighing the amount of organic cosolvent and amount of gas condensed into the lecture bottle. The high-pressure sample cell was constructed from stainless steel (SS 304) having dimensions of 8.25 cm X 14.0 cm X 5.0 cm. The optical path along the axis of the cell was 5.0 cm long X 1.9 cm in diameter with a sapphire window at each end which is 2.5 cm in diameter X 1.3 cm thick. The total volume of the sample cell is approximately 14.5 cm3 and was temperature regulated to f O . l OC. The solvatochromic probe was 2-nitroanisole which has an S value of -2.428. The temperatures studied were 44,62, and 122 O C at binary mole fractions of 2-propanol in COz of 0.051,0.106, and 0.132. These organic cosolvent mole fractions and temperature conditions were chosen on the basis of the vapor-liquid equilibria data of Radosz.25 From the vapor-liquid equilibria data, one is assured of knowing the relevant phase behavior, Le., when the binary system is a supercritical or a subcritical two-phase system. The solvatochromic shifts of 2-nitroanisole in pure C 0 2 and 2-propanol a t 44, 62, and 122 OC as a function of pressure were determined and used in eq 4 for the calculation of the local composition of organic cosolvent about the solute molecule. Results and Discussion
The local composition around a solute molecule can be studied through the solvatochromic behavior of the solute. The solvatochromic behavior of the solute molecule is a direct manifestation of the solvent environment in the cybotactic region. For binary supercritical fluids, where density and/or local composition in the cybotactic region can change as a function of pressure, the solvatochromic behavior should show a marked dependence on pressure. This pressure dependence is also seen in pure supercritical fluid systems and may be attributed solely to changes in fluid d e n ~ i t y . ' ~ ~ ~ ~ Figures 1,2, and 3 show the dependence of the solvatochromic behavior (a*') on pressure for the bulk mole fractional compositions of 2-propanol in the binary supercritical fluid of 0.00,0.051, 0.106, and 0.132 at 44, 62, and 122 OC, respectively. In Figures 1 and 2, the 2-propanol mole fractions of 0.106 and 0.132 are subcritical liquids at 44 and 62 "C; therefore, the a*' values for these concentrations are roughly similar and (since the dependence of density upon pressure is much less than for supercritical fluids) the composition of the cybotactic region of the solute molecule is similar. Under supercritical conditions for 0.05 1 mole fraction of 2-propanol at 44 and 62 "C, increasing pressure increases a*'. (25) Radosz, M. J . Chem. Eng. Data 1986, 31, 43.
The Journal of Physical Chemistry, Vol. 92, No. 8,1988
2376
0.25
.+ .+
Yonker and Smith
.O'O
+ a +
+
A
0
0
4
b
$+
+
0
0
-0.2
A
+ A
b-, +
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a
A
+
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+
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+
+
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+
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A A
*A
* A
+ +
.0.4
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++ A
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.
,
.
,
too
0
.
I
.
I
300
200
***
l
-1 .o 0
L
400
100
200
400
300
PRESSURE (bar)
versus pressure for bulk binary mole fractions of (+) 0.0, (A)0.051, (+) 0.106, and (0) 0.132 2-propanol in carbon dioxide at 44 "C.
Figure 1.
T*'
TABLE I: Solvntochromic Data for 2-Propanol at 44.62. . . and 122
02 0
. .
o+ a&+
+
+
. +
4
OC, Which Includes Hydrogen-Bond Contributions
+
+
press., bar
A
A
A
A
oc
++
A AA
A
A A
A
E
0.515 0.532 0.543
145.2 208.5 276.2
0.493 0.493 0.493
138.4 207.1 275.7
0.466 0.479 0.494
OValues of
4 ' 4
*+
f -0 6
i
100
44 o c 278.7 346.0 62 OC 348.2 402.6 122
k
.o
73.2 142.5 210.2
A
-02
200
A*' ~~
A
+
mess.. bar
T*'
300
400
51
PRESSURE (bar) T*' versus pressure for bulk binary mole fractions of (+) 0.0, (A)0.051, (+) 0.106, and (0) 0.132 2-propanol in carbon dioxide at 62
Figure 2. D/,
- L.
The 2-propanol undergoes hyrogen-bonding interactions with 2-nitroanisole,contributing to the solvatochromic shift of the solute molecule. The density dependence of the binary fluid H*' value is similar to that for pure carbon dioxide. Therefore, the cybotactic region of the solute molecule contains some number of organic cosolvent molecules that is roughly proportional to the initial bulk concentration of the organic cosolvent. As the bulk mole fraction of the organic cosolvent increases, the H*' value increases pro-
0.549 0.560 0.540 f 0.0017" 0.498 0.510 0.497 f 0.007"
oc 344.6 413.6
**' at 44, 62, and 122
O C ,
0.497 0.511 0.489 f 0.017"
respectively.
portionately, approaching the H*' value for 2-propanol (see Table I). The local density in the cybotactic region increases with pressure resulting in an increase in H*'. Remember that, as defined, H*' contains contributions due to not only nonspecific density-dependent intermolecular interactions but also specific hydrogen-bonding interactions which may or may not be density dependent. There are a fixed number of interaction sites between 2-nitroanisole and 2-propanol, involving the nitro group and the benzene ring. Once these sites are saturated, the lifetime of these associations and the local densities of COzand cosolvent molecules determine the solvatochromic shift seen for the binary fluid. In Figure 3, the organic cosolvent concentrations studied at 122 "C are all above their critical temperatures. As the concentration of the organic cosolvent and pressure increase, the T*' value becomes less negative, reflecting the influence of the 2-propanol and the increase in local density of the cybotactic region, respectively. An interesting region at 122 OC is the portion of the H*' curve where the transition occurs from a two-phase, vapor-liquid region to a single-phase, binary supercritical fluid region. This transition occurs at approximately 110 7 bar for the binary fluid mixtures of 0.106 and 0.132 2-propanol/COz. Figure 3 shows a transition
*
The Journal of Physical Chemistry, Vol. 92, No. 8, 1988 2311
Carbon Dioxide/2-Propanol System TABLE I 1 Local Composition Mole Fraction of 2-Propanol in Carbon Dioxide at 44 OC
press., bar AT* XI2 Bulk Mole Fraction of 2-Propanol = 0.051 109.9 0.09 0.19 f 0.02 124.0 0.09 0.19 f 0.02 139.2 0.06 0.16 f 0.02 214.2 0.07 0.18 f 0.02 252.2 0.06 0.17 f 0.03 342.7 0.01 0.06 f 0.03
TABLE I V Local Composition Mole Fraction of 2-Propanol in Carbon Dioxide at 122 OC
press., bar AT* XI 2 Bulk Mole Fraction of 2-Propanol = 0.051 103.0 0.04 0.08 f 0.01 117.5 0.06 0.10 f 0.01 138.9 0.00 0.05 f 0.01 0.07 f 0.01 194.6 0.02 289.5 -0.01 0.04 f 0.02 366.1 -0.04 0.01 f 0.02
Bulk Mole Fraction of 2-Propanol = 0.106; Subcritical Liquid 111.6 0.17 0.37 f 0.02 128.3 0.12 0.30 f 0.02 154.1 0.08 0.25 f 0.02 216.8 0.08 0.25 f 0.03 282.1 0.06 0.24 f 0.03 328.8 0.05 0.21 f 0.03
Bulk Mole Fraction of 2-Propanol = 0.106 134.1 0.02 0.12 f 0.01 169.5 0.00 0.11 f 0.01 246.5 -0.01 0.10 Z!= 0.01 309.8 -0.01 0.10 0.02 387.5 -0.04 0.06 Z!= 0.02 Bulk Mole Fraction of 2-Propanol = 0.132 134.1 0.09 0.21 f 0.01 0.07 0.20 f 0.01 205.8 273.4 0.03 0.16 f 0.02 311.4 0.04 0.18 f 0.02 346.7 0.03 0.16 f 0.02
Bulk Mole Fraction of 2-Propanol = 0.132; Subcritical Liquid
117.0 144.6 205.9 286.1 338.9 393.0
0.14 0.10 0.08 0.03 0.02 0.01
0.35 f 0.02 0.30 f 0.02 0.28 f 0.02 0.20 f 0.03 0.19 f 0.03 0.16.f 0.03
TABLE III: Local Composition Mole Fraction of 2-Propanol in Carbon Dioxide at 62 OC Dress.. bar AT* XI .I Bulk Mole Fraction of 2-Propanol = 0.051
126.1 142.5 207.7 296.5 388.4
0.12 0.09 0.04 0.03 0.02
0.22 f 0.02 0.19 f 0.02 0.12 f 0.02 0.11 f 0.03 0.08 f 0.03
Bulk Mole Fraction of 2-Propanol = 0.106; Subcritical Liquid 112.4 0.26 0.38 f 0.02 126.7 0.24 0.38 f 0.02 144.1 0.21 0.36 f 0.02 179.6 0.14 0.28 f 0.02 261.8 0.10 0.25 f 0.03 320.0 0.10 0.26 f 0.03 368.6 0.08 0.23 f 0.03 Bulk Mole Fraction of 2-Propanol = 0.132; Subcritical Liquid 121.0 0.26 0.40 f 0.02 140.2 0.20 0.35 f 0.02 227.2 0.15 0.33 f 0.02 311.8 0.1 1 0.28 f 0.03 359.1 0.12 0.31 f 0.03 425.9 0.12 0.32 f 0.03 in this pressure region, which is especially prominent for the 0.106 mole fraction mixture. A plausible explanation for the change in solvatochromic behavior of the solute is due to the liquid phase in the two-phase region having a higher mole fraction of 2-propanol than the vapor phase, which contributes to a more positive ?r*’ value.2s As the single-phase supercritical region is entered, the vapor- and liquid-phase compositions converge at a common composition of 2-propanol in carbon dioxide, so that ?r*’ decreases. The local composition of organic cosolvent in the cybotactic region of the solute molecule was calculated with eq 4 by using the ?r*’ values for 2-propanol given in Table I. The spectroscopic shifts for 2-propanol and carbon dioxide were used at the appropriate pressure and temperature to calculate the local composition. These values were fit by a polynomial least-squares method to facilitate comparison of different pressures at the various temperatures. The results from this simple model are given in Tables 11-IV. A general trend is apparent at low pressure in the data for the binary fluid mixtures, indicating the cybotactic region of the solute molecule is enriched in organic cosolvent. As pressure increases, the local composition of cosolvent tends to decrease. The local composition of the organic cosolvent as a function of pressure for a specific mole fraction (0.051) of cosolvent at different temperatures, above the mixture critical temperature
0.0 100
200
300
J
PRESSURE (bar)
Figure 4. Local composition versus pressure for a constant bulk composition of 0.051 mole fraction at (0)44,(X) 62, and (m) 122 OC.
in all cases, is shown in Figure 4. The specific hydrogen-bond interactions between 2-propanol and 2-nitroanisole contribute to the enrichment in the cybotactic region under these conditions. As temperature or pressure increases, the net enrichment of the cybotactic region shows a concomitant decrease.26-2s The work of Weeks et al.27using high-temperature expansion perturbation theory and the Monte Carlo computer simulation data of Lee et a1.,28 predicts the local composition of organic cosolvent decreases as density (pressure) increases. Therefore, the attractive interactions between the solute and organic cosolvent (26) Hu, Y.; Azevedo, E. G.; Prausnitz, J. M. Fluid Phase Equilib. 1983, 13, 351. (27) Weeks, J. D.; Chandler, D.; Anderson, H. C. J . Chem. Phys. 1971, 54, 5237. ( 2 8 ) Lee,K.-H.; Sandler, S.I.; Patel, N. C. Fluid Phase%quilib. 1986, 25, 3 1 .
2378 The Journal of Physical Chemistry, Vol. 92, No. 8, 1988
function of pressure for all the fluid mixtures, approaching the bulk composition at higher pressures (densities) as predicted from t h e ~ r y . ~At ~ ~a ~bulk ' composition mole fraction of 0.051, the binary mixture is supercritical while subcritical liquidsz5exist at mole fractions of 0.106 and 0.132 at 62 O C . However, the general trend in their solvatochromic behavior still indicates a decrease in the local composition of organic cosolvent with pressure as expected. At 122 OC, all mole fractions show an asymptotic approach to the limiting bulk composition within experimental error. The general trends seen in Figure 5 support the statistical-mechanical theory and computer simulations predicting the local composition of an organic cosolvent about a solute molecule to decrease with pressure.
LOCAL COMPOSITION at 62°C
1 -
L0.051
I
t
I
I
I
100
200
300
400
500
0.0
PRESSURE (bar) LOCAL COMPOSlTlON at 122°C 0.3
z 0 L
Xioa=0.132
0.2
0.1
0.0 ! 100
Yonker and Smith
I
I
I
200
300
400
PRESSURE (bar) Figure 5. Local composition versus pressure for constant temperature at 62 and 122 O C at (0)0.051, (X) 0.106, and (m) 0.132 bulkmole fraction
compositions. are dominant where the binary fluid has its greatest compressibility (Le,, low densities). At higher densities the molecular size difference will begin to play a role in local composition effects.% The local composition of organic cosolvent in the cybotactic region should asymptotically approach the limiting value of the bulk composition as pressure or temperature increases. As temperature increases, thermal agitation will randomize the solvent molecules in the cybotactic region approaching the bulk composition level. The local composition values calculated from the spectroscopic data indicated in Figure 4 show a decrease with pressure as predicted from statistical mechanical The error in the experimental measurements have been included in Figure 4. The general trends in the spectroscopic solvatochromic data for binary supercritical fluids follow expectations based upon statistical mechanics, with an asymptotic approach of local composition to the bulk level. The local composition of cosolvent for different concentrations of 2-propanol as a function of pressure at 62 and 122 OC is shown in Figure 5 . At 62 "C the local composition decreases as a
Conclusion The spectroscopicstudy of the solvatochromicbehavior of binary fluids is a sensitive probe of the cybotactic region of a solute molecule and the enrichment of the local composition caused by the specific intermolecular interactions (Le., hydrogen bonding). The local composition about the solute molecule in the cybotactic region can be calculated and for 2-propanol in CO, showed a net enrichment of 2-propanol in the cybotactic region when compared with the bulk composition. The local composition in the cybotactic region was dependent on pressure and temperature. As pressure increased, the local composition of the organic cosolvent in the cybotactic region was seen to decrease for the specific binary fluid mixtures studied. The solvatochromic behavior of the binary subcritical fluids generally showed an enrichment as well as an apparent saturation of the cybotactic region by the organic cosolvent under some conditions (0.106 and 0.132 mole fraction of 2-propanol in COz a t 44 "C). In the latter case the T*' values showed much less of a decrease with increasing pressure over the range studied for these binary mixtures. The limitations of the present approach are primarily due to the need to group the density-dependent solvatochromic effect with the hydrogen-bond formation between the polar cosolvent and solute. In principle, methods can be developed to segregate the contributions due to cosolvent polarity and hydrogen bonding; such studies are currently being attempted. It is also clear that the local composition enrichment may be strongly dependent upon the solute probe. Recent work by Donohue and co-workersz9 suggests spectroscopic methods for selection of cosolvents having specific interactions. Clearly, the present approach can be extended to studies of solutes or cosolvents selected by such methods to explicitly study the role of hydrogen bonding upon local composition in fluid mixtures. The implications of the study of local composition dependence on pressure and temperature for supercritical fluid chromatography and extraction processes are great. The ability to study cluster formation in a low-density fluid solvent and determine the effect of local organic cosolvent composition on chromatographic retention or extraction selectivity could lead to improved separation processes. Supercritical fluids can be used to study cluster formation, bridging the gap between gas-phase clusters and liquidphase solvation and aggregation phenomena. Further experimental studies of the solvatochromic behavior of different organic cosolvents in binary fluid mixtures and their local composition as a function of pressure and temperature are being undertaken. Registry No. C 0 2 , 124-38-9; 2-propanol, 67-63-0; 2-nitroanisole, 91-23-6. (29) Walsh, J. M.; Ikonomou,G. D.; Donohue, M. D. Fluid Phase Equilib. 1987, 33, 295.
