Elucidation of SoluteFluid Interactions in Supercritical CF3

Chapter 4

Elucidation of Solute—Fluid Interactions in Supercritical C F H by Steady-State and Time-Resolved Fluorescence Spectroscopy Downloaded via TUFTS UNIV on July 11, 2018 at 19:06:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Thomas A . Betts, JoAnn Zagrobelny, and Frank V . Bright Department of Chemistry, Acheson Hall, State University of New York at Buffalo, Buffalo, N Y 14214 Steady-state and multifrequency phase and modulation fluorescence spectroscopy are used to study the photophysics of a polar, environmentally-sensitive fluorescent probe in near- and supercritical C F H . The results show strong evidence for local density augmentation and for a distribution of cluster sizes. These results represent the first evidence for lifetime distributions in a "pure" solvent system. 3

Much progress in supercritical fluid science and technology has occurred during the past decade (1-6). Many experimental (7-17) and theoretical advances (18-28) have helped to improve our understanding of solute-fluid interactions in supercritical fluids. However, progress is impeded somewhat because certain chemical information is lacking. Specifically, we lack a detailed understanding of many of the molecular processes occurring in the local solvation shells about the solute (i.e., the cybotactic region). To address this issue, our research group has set its sights on improving our understanding of the kinetics and mechanisms of solvation is supercritical media (29,30). From this new information, we believe chemists and engineers may begin to develop less empirical correlations and thus more accurate theories to predict fluid phase equilibria. This paper reports on our most recent investigations of solvation processes in a pure, polar supercritical fluid, fluoroform (CF H). Fluoroform was chosen because it has a permanent dipole moment (1.60 D), easily accessible critical parameters (T = 26.1 °C; P = 48.6 bar), and dielectric properties are easily adjusted over a broad range with pressure. In this work, we employ the fluorescent probe P R O D A N (Figure 1). P R O D A N has been used previously to study dynamics in proteins (31) and normal liquids (32). P R O D A N was also chosen because it is: 1) available in high purity, 2) effectively excited with our present laser-based systems, 3) strongly fluorescent in C F H , and 4) extremely sensitive to the local physicochemical properties of the solvent (e.g., refractive index and dielectric constant). Thus, as the properties of the fluid vary, P R O D A N can be used to directly monitor and quantify the change. 3

c

c

3

0097-^156/92AM88-0048$06.00A) © 1992 American Chemical Society

Bright and McNally; Supercritical Fluid Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

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Experimental. P R O D A N was purchased from Molecular Probes and the purity checked by reverse phase H P L C . There were no detectable impurities. Stock solutions (1 mM) were prepared in absolute ethanol and stored in the freezer. C F H was purchased from Matheson and passed through a single adsorptive O2 trap (Matheson) prior to entering the pumping system. According to the manufacturer this gives an 0 level < 5 ppm. The high-pressure cells and temperature control units are similar to the ones described by Betts and Bright (29). Samples for analysis were prepared by directly pipetting the appropriate amount of stock solution into the cell. To remove residual alcohol solvent, the optical cell was placed in a heated oven (60 °C) for several hrs. The cell was then removed from the oven, connected to the high-pressure pumping system (29), and a vacuum (50 um Hg) maintained on the entire system for 10-15 minutes. The system was then charged with C F H and pressurized to the desired value with the pump (Isco, model SFC-500). Typically, we performed experiments at 10 uM P R O D A N and there was no evidence for primary or secondary interfilter effects. H P L C analysis of P R O D A N subjected to supercritical solvents showed no evidence of decomposition or additional components. A l l steady-state absorbance measurements were made using a Milton-Roy model 1201 UV-Vis spectrophotometer. The majority of the steady-state and dynamic fluorescence measurements were performed using a modified multifrequency phasemodulation fluorometer (29) with parallel acquisition capabilities (33). One set of experiments was also carried out on a time-correlated single-photon counting instrument (see reference 32 for description). The results recovered by our frequency-domain instrumentation were extremely similar and indicate lack of artifacts or anomalies. For all experiments reported here, the excitation source was an argonion laser (351.1 nm; Coherent, model Innova90-6) and emission was monitored using either long-pass or an ensemble of band-pass filters (Oriel). For all dynamic measurements, magic angle polarization was used to eliminate any polarization biases (34). Dynamic data analysis was performed using a commercially available global analysis software package (Globals Unlimited). The details of this software and its features can be found elsewhere (35-37). Our main criteria for choosing one kinetic model over another are based on chi-squared (x ), randomness of the residual terms, and physical significance of the model. More details on these criteria can be found in recent review articles (38-40). Additional software for determining the emission and absorbance center of gravity, etc. were developed in house and written in BASIC. The density- and temperature-dependent dielectric constant and refractive index for C F H were calculated using the Debye equation and the molar refractivity, respectively (41). The values obtained from this approach agree to within 5% with those determined by interpolation of existing data (42). 3

2

3

2

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Results and Discussion Steady-State Fluorescence. As mentioned in the Introduction section P R O D A N is an environmentally-sensitive fluorescent probe. To illustrate this point, Figure 2


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\x - \i = 20 Debye Figure 1. P R O D A N structure.

'[2

1.0

Emission Wavelength (nm) Figure 2. Normalized steady-state emission spectra for 10 uM P R O D A N in normal liquids. X = 351 nm. cx


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shows the normalized steady-state emission spectra for P R O D A N in a series of liquid solvents at 25 °C. Clearly, as the polarity of the liquid increases the spectrum red shifts appreciably. Figure 3 shows the effect of CO2, N 0 , and C F H fluid density on the emission center of gravity for P R O D A N . Several interesting trends are apparent in these data. First, for N 0 there is little influence of density on the emission spectra. (Experiments below a reduced density of 1.0 are difficult with N 0 because of background from the N 0 itself.) Second, there is a significant change in emission center of gravity with CQ2 density. We have not looked at this particular system in detail, but conclude that there is some degree of solute-solute or solute-fluid interaction (in C 0 ) near a reduced density (p ) of unity. Third, in C F H there is a significant shift in the P R O D A N emission center of gravity with density. This is not too unexpected considering that C F H possesses a significant permanent dipole moment and its dielectric properties vary appreciably with density (42). This densitydependent spectral shift is a result of the significant increase in dielectric constant (e) and the refractive index (n) of the medium. In CO2 and N 0 these changes are modest (above p = 1) compared to C F H . In order to investigate the observed spectral shifts in more detail, it is important to recognize that the solvent and its physicochemical properties influence the energy difference between the ground and excited states (43). These effects are most often described using the Lippert formalism (43): 2

3

2

2

2

2

r

3

3

2

r

3

(1) where h is Planck's constant, c is the speed of light, a is the radius of the cavity occupied by the fluorophore, v and v are the centers of gravity (in cm ) for the absorbance and fluorescence, respectively, and the term (u* - u) is the difference between the excited- and ground-state dipole moments. The bracketed term that depends on the dielectric constant and refractive index is often referred to as the orientational polarizability. For P R O D A N the cavity radius is about 4.2 A and the dipole change is near 20 debye (31). Figure 4 shows a plot of - v as a function of the orientational polarizability for P R O D A N in supercritical C F H (points). In addition to this data, we show also the theoretical results expected, based on solvent dielectric constant, refractive indices, dipole change and cavity radius, for P R O D A N in normal liquid solvents. Clearly, there are significant deviations at low density, but at higher density the two data sets begin to track one another. In addition it appears that the observed shift seen at low density is much greater than predicted by theory. That is, the local density about the probe is far greater that the bulk density of the fluid. This result is shown most clearly in Figure 5 and is a result of C F H molecules preferentially associating about the P R O D A N in the highly compressible region near the critical point. The spectral width of the fluorescence contour also gives insight into the immediate environment about the probe. Figure 6 shows that the spectral full-widthat-half maximum (FWHM) is initially wide at low density, goes through intermediate values as density is increased, and levels off at higher densities. These results are 1

k

f

f

3

3


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440

431 CF H (30 °C) C 0 (35 °C) N 0 (40 °C) 3

422-

2

2

c (j c q E

413--

404--

395 0.5

1.0

2.0

1.5

2.5

Reduced Density

Figure 3. Density-dependent emission centers of gravity (in nm) for 10 uM P R O D A N in C 0 , N 0 , and C F H . 2

2

3

Figure 4. Lippert plot for 10 uM P R O D A N in C F H . The solid points are the experimental data. The dashed line represents the theoretical prediction for PRODAN. 3


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1.0

0.2-I 45

1 55

1 65

1 75

Pressure

1 85

1 95

1 105

(bar)

Figure 5. Local (points) and bulk fluid density versus pressure. 3350

3100-1 0.5

1 0.9

1 1.3

1 1.7

1 2.1

Reduced Density

Figure 6. Influence of density on the spectral full-width-at-half maximum (FWHM) for 10 uM P R O D A N in supercritical C F H . 3


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consistent with P R O D A N being in an ensemble of domains initially (at low density), the number of domains diminishing at intermediate densities, and the local environment about the P R O D A N eventually coalescing into a more homogeneous environment at higher densities. In an effort to learn more about probe-fluid interactions we investigated the effects of temperature on the P R O D A N emission contour. Figure 7 presents the steady-state emission spectra for P R O D A N in C F H as a function of temperature. These spectra are interesting because as one increases temperature one observes a systematic blue shift. In normal liquids, at mild temperatures (< 200 °C), one typically sees the opposite trend. At fairly high temperatures there is often a reversal in peak emission and spectra often begin to blue shift at higher temperatures. The explanation for this effect is that the thermal energy input into the system is large enough such that it precludes the solvent molecules from being able to reorganize about the solute during the excited-state lifetime (44,45). We propose the same mechanism here and attribute the effect to the much weaker interactions in supercritical media compared to normal liquids. To this point it is important to recognize that steady-state fluorescence spectroscopy gives one only an average picture of the system under study. In order to determine the details of the photophysics, it is necessary to employ time-resolved methodologies. 3

Time-Resolved Fluorescence. To improve our understanding of the photophysics of the P R O D A N - C F H system we performed a series of time-resolved fluorescence experiments at p = 1.25 as a function of temperature. Traditionally, one describes the time-resolved fluorescence intensity decay (I(t)) by a sum of (n) discrete exponentials of the form (43,46): 3

r

w

= E «i

e x

( 2 )

P

Here a is the pre-exponential (amplitude) factor corresponding to lifetime Tj. The fractional intensity contribution (fj) from component i is related to the pre-exponential factors by: {

i where E fj = 1. Recent reports from many groups, including our own, have shown (47-55) that systems originally described by discrete decay processes (Eqn. 2) are more accurately described using continuous distributions of decay times. In these situations, the fluorescence intensity decay can be written in integral form as: I(t) = | a ( t )

exp(-r/T>fT.

( ) 4

In the present work, we have found that a continuous Lorentzian lifetime distribution (two floating parameters) described by Eqn. 5, best modelled the experimental data:


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Solute-Fluid Interactions in Supercritical CFJI CC(T) = A/{1+[(T-T )/(H72)] }. C

55 (5)

2

Here A is a constant from the normalization condition: J a(r) = 1, r is the center value of the lifetime distribution, and W is the full-width-at-half maximum for the Lorentzian function. Fits to single (one floating parameter) and double (three floating parameters) exponential decay laws are always poorer as judged by the x and residual traces. In the case where we assume that there is some type of excited-state process (e.g., solvent relaxation) we find that the spectral relaxation time is > 20 ns. This is much, much greater than any reasonable solvent relaxation process in supercritical C F H . For example, in liquid water, the solvent relaxation times are near 1 ps (56). Figure 8 presents the recovered temperature-dependent lifetime distributions for P R O D A N in C F H (p = 1.25). From these results it appears that, in the highly compressible region where these particular experiments were conducted, P R O D A N is subjected to an ensemble of domains. To the best of our knowledge, however, such an observation in a pure "solvent" is unprecedented and merits further discussion. In other media like micelles, cyclodextrin, binary solvent mixtures, and proteins (47-55), lifetime distributions are routinely used to model the decay kinetics. In all of these cases the distribution is a result of the (intrinsic or extrinsic) fluorescent probe distributing simultaneously in an ensemble of different local environments. For example, in the case of the cyclodextrin work from our laboratory (53-55), the observed lifetime distribution is a result of an ensemble of 1:1 inclusion complexes forming and coexisting. These complexes are such that the fluorescent probe is located simultaneously in an array of environments (polarities, etc.) in, near, and within the cyclodextrin cavity, which manifest themselves in a distribution of excited-state lifetimes (53-55). In the present study our experimental results argue for a unimodal lifetime distribution for P R O D A N in pure C F H . The question then becomes, how can a lifetime distribution be manifest in a pure solvent? We propose that the recovered lifetime distribution is a result of the different cluster sizes (i.e., domains) encountered by P R O D A N in C F H . That is, if P R O D A N were simultaneously distributed in clusters or aggregates with different solvation characteristics (sizes) one would anticipate a distribution of decay times. Thus, it appears that the observed lifetime distributions recovered here may in fact be a consequence of the actual distribution of cluster sizes in the highly compressible region of supercritical C F H . c

2

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3

r

3

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Conclusions. The results from the present experiments lead to several conclusions and hints to the dynamics of solvation in supercritical C F H . [1] The local density about the P R O D A N probe is far greater (in some cases 120% greater) than the bulk density. This is most likely a result of fluid aggregation (i.e., clustering, charm, charisma) about the solute in the highly compressible region. [2] Our steady-state fluorescence experiments show that the spectral F W H M is extremely sensitive to density. These particular observations are consistent with an ensemble of local domains about the solute. [3] The excited-state decay kinetics for P R O D A N in C F H 3

3


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/ '

/ / /

v\ /

N

* / \ 7 . v

\

T(°C) — 29.9 - - 36.0 40.8 • - 45.0 51.1 v - — 55.4

\

////A/\\\\ «(r) / V

0

/

/

V

2

\

x

FLUID T E C H N O L O G Y

N

4

1

6

8

Lifetime ( n s )

Figure 8. Temperature-dependent Lorentzian lifetime distributions for 10 uM P R O D A N in C F H . 3


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are not monoexponential. In normal liquids like Q-C5 alcohols, C H C N , and water the fluorescence intensity decays are always well modelled by a nearly monoexponential decay law; only in supercritical C F H is the intensity decay modelled by a distribution of lifetimes. Because of our present time resolution (20 ps), it is possible that there is a much faster component that remains unresolved. The best fit model for describing the intensity decay is a simple unimodal Lorentzian lifetime distribution. The evidence for a lifetime distribution has prompted us to propose that we are observing an ensemble of P R O D A N molecules each being subjected to different local compositions of C F H . That is, there is not a unique solvent cage about P R O D A N ; under our experimental conditions the cybotactic region is nonuniform and heterogeneous. Presently, we are initiating density-dependent experiments on this system, elucidating the origin of the distribution, and determining if the observed results are probe dependent. 3

3

3

Acknowledgements. This work has been generously supported by the United States Department of Energy (DE-FGO2-90ER14143). We also thank the Tennessee Eastman Co. for sponsoring the ACS Analytical Division Fellowship (TAB) and SUNY-Buffalo for the Mark Diamond Research Award (TAB). Special thanks are also extended to Gary Sagerman for his continued help with instrument construction. Finally, we thank Professor Mark Maroncelli for graciously allowing us time on his time-domain spectrometer to confirm our experimental observations. Literature Cited 1. 2.

3. 4.

5.

van Wasen, U.; Swaid, I.; Schneider, G . M . , Angew. Chem. Int. Ed. Engl. 1980, 19, 575. Supercritical Fluid Chromatography; Smith, R.M., Ed.; Royal Society of Chemistry Chromatography Monograph; The Royal Society of Chemistry: London, 1988. McHugh, M.; Krukonis, V. Supercritical Fluid Extraction: Principles and Practice; Butterworths: Boston, M A , 1986. Supercritical Fluid Science and Technology; Johnston, K . P . ; Penninger, J . M . L . , Eds.; American Chemical Society Symposium Series, No. 406; American Chemical Society: Washington, D C , 1989. Reid, R.C.; Prausnitz, J.M.; Poling, B . E . The Properties of Gases and Liquids 4 Ed.; McGraw-Hill: New York, N Y , 1987. Brennecke, J.F.; Eckert, C . A . AIChE J. 1989, 35, 1409. Eckert, C . A . ; Ziger, D . H . ; Johnston, K . P . ; Kim, S. J. Phys. Chem. 1986, 90, 2738. Lemert, R . M . ; Johnston, K . P . Fluid Ph. Equil. 1989, 45, 265. Dobbs, J . M . ; Wong, J.M.; Lahiere, R . J . ; Johnston, K . P . Ind. Eng. Chem. Res. 1987, 26, 56. Kim, S.; Johnston, K . P . AIChE J. 1987, 33, 1603. Yonker, C . R . ; Smith, R . D . J. Phys. Chem. 1988, 92, 235. Okada, T.; Kobayashi, Y.; Yamasa, H.; Mataga, N . Chem. Phys. Lett. 1986, 128, 583. th

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Hrnjez, B.J.; Yazdi, P.T.; Fox, M.A.; Johnston, K . P . J. Am. Chem. Soc. 1989, 111, 1915. Kajimoto, O.; Futakami, M.; Kobayashi, T.; Yamasaki, K . J. Phys. Chem. 1988, 92, 1347. Brennecke, J.F.; Eckert, C . A . In Supercritical Fluid Science and Technology; Johnston, K . P . ; Penninger, J . M . L . , Eds.; American Chemical Society Symposium Series, No. 406; American Chemical Society: Washington, D C , 1989, chapter 2. Brennecke, J.F.; Tomasko, D . L . ; Eckert, C . A . J. Phys. Chem. 1990, 94, 7692. Johnston, K . P . ; Kim, S.; Combs, J. In Supercritical Fluid Science and Technology; Johnston, K . P . ; Penninger, J . M . L . , Eds., American Chemical Society Symposium Series, No. 406; American Chemical Society: Washington, D C , 1989, chapter 5. Cochrun, H . D . ; Lee, L . L . In Supercritical Fluid Science and Technology; Johnston, K . P . ; Penninger, J . M . L . , Eds., American Chemical Society Symposium Series, No. 406; American Chemical Society: Washington, D C , 1989, chapter 3. Panagiotopoulos, A . Z . In Supercritical Fluid Science and Technology; Johnston, K . P . ; Penninger, J . M . L . , Eds., American Chemical Society Symposium Series, No. 406; American Chemical Society: Washington, D C , 1989, chapter 4. Cochran, H . D . ; Pfund, D . M . ; Lee, L.L. Sep. Sci. Tech. 1988, 23, 2031. Petsche, I.B.; Debenedetti, P.G. J. Chem. Phys. 1989, 91, 7075. Debenedetti, P . G . Chem. Eng. Sci. 1987, 42, 2203. Debenedetti, P . G . ; Kumar, S.K. AIChE J. 1988, 34, 645. Debenedetti, P.G.; Mohamed, R.S. J. Chem. Phys. 1989, 90, 4528. Cochran, H . D . ; Lee, L.L.; Pfund, D . M . Fluid Ph. Equil. 1988, 34, 161. Cochran, H . D . ; Lee, L.L.; Pfund, D . M . Fluid Ph. Equil. 1987, 34, 219. Cochran, H . D . ; Lee, L.L. AIChE J. 1987, 33, 1391. Cochran, H . D . ; Lee, L.L. AIChE J. 1988, 34, 170. Betts, T . A . ; Bright, F . V . Appl. Spectrosc. 1990, 44, 1196. Betts, T . A . ; Bright, F . V . Appl. Spectrosc. 1990, 44, 1203. Weber, G . ; Farris, F . J . Biochemistry 1979, 18, 3075. Chapman, C.F.; Fee, R.S.; Maroncelli, M. J. Phys. Chem. 1990, 94, 4929. Fedderson, B . A . ; Piston, D.W.; Gratton, E . Rev. Sci. Instrum. 1989, 60, 2929. Spencer, R . D . ; Weber, G . J. Chem. Phys. 1970, 52, 1654. Beechem, J . M . ; Gratton, E . In Time-Resolved Laser Spectroscopy in Biochemistry; Lakowicz, J.R., Ed.; Proc. SPIE 909: Los Angeles, C A , 1988, pp 70.


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Beechem, J . M . ; Ameloot, M.; Brand, L . Chem. Phys. Lett. 1985, 120, 466. Beechem, J . M . ; Ameloot, M.; Brand, L . Anal. Instrumn. 1985, 14, 379. Bright, F.V.; Betts, T . A . ; Litwiler, K . S . C.R.C. Crit. Rev. Anal. Chem. 1990, 21, 389. Lakowicz, J.R.; Lackzo, G . ; Gryczynski, I.; Szmacinski, H.; Wiczk, W. J. Photochem. Photobiol. B: Biol. 1988, 2, 295. Jameson, D . M . ; Gratton, E . ; Hall, R . D . Appl. Spectrosc. Rev. 1984, 20, 55. Barrow, G . M . Physical Chemistry 4 Ed.; McGraw-Hill: New York, N Y , 1979, chapter 16. Brennecke, J.F., Ph.D. Thesis, University of Illinois 1989. Lakowicz, J.R. Principles of Fluorescence Spectroscopy; Plenum Press: New York, N Y , 1983, chapter 7. Cherkasov, A . S . ; Dragneva, G.I. Opt. Spectrosc. 1961, 10, 283. Piterskaya, I.V.; Bakhshiev, N . G . Bull Acad. Sci. USSR, Phys. Ser. 1963, 27, 625. Demas, J . N . Excited State Lifetime Measurements; Academic Press: New York, N Y , 1983. Alcala, J.R.; Gratton, E . ; Prendergast, F . G . Biophys. J. 1987, 51, 587. James, D . J . ; Ware, W.R. Chem. Phys. Lett. 1985, 120, 455. James, D . J . ; Ware, W . R . Chem. Phys. Lett. 1986, 126, 7. Gryczynski, I.; Wiczk, W . ; Johnson, M.L.; Lakowicz, J.R. Biophys. Chem. 1988, 32, 173. Lakowicz, J.R.; Cherek, H.; Gryczynski, I.; Joshi, N.; Johnson, M . L . Biophys. Chem. 1987, 28, 35. Eftink, M.; Ghiron, C . A . Biophys. J. 1987, 52, 467. Bright, F.V.; Catena, G . C . ; Huang, J. J. Am. Chem. Soc. 1990, 112, 1344. Huang, J . ; Bright, F . V . J. Phys. Chem. 1990, 94, 8457. Catena, G . C . ; Bright, F . V . J. Fluor. 1991, 1, 31. Jarzeba, W . ; Walker, G . C . ; Johnson, A . E . ; Kahlow, M.A.; Barbara, P.F. J. Phys. Chem. 1988, 92, 7039. th

R E C E I V E D November 25, 1991


Elucidation of SoluteFluid Interactions in Supercritical CF3

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