A Spectroscopic and Computational Exploration of the Cybotactic

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J. Phys. Chem. B 2008, 112, 4666-4673

A Spectroscopic and Computational Exploration of the Cybotactic Region of Gas-Expanded Liquids: Methanol and Acetone John L. Gohres,†,§,| Christopher L. Kitchens,†,§ Jason P. Hallett,†,§ Alexander V. Popov,‡,| Rigoberto Hernandez,*,‡,§,| Charles L. Liotta,†,‡,§ and Charles A. Eckert*,†,‡,§ School of Chemical & Biomolecular Engineering, School of Chemistry and Biochemistry, Specialty Separations Center, and Center for Computational and Molecular Science and Technology, Georgia Institute of Technology, Atlanta, Georgia 30332 ReceiVed: September 19, 2007; In Final Form: NoVember 30, 2007

Local compositions in supercritical and near-critial fluids may differ substantially from bulk compositions, and such differences have important effects on spectroscopic observations, phase equilibria, and chemical kinetics. Here, we compare such determinations around a solute probe dissolved in CO2-expanded methanol and acetone at 25 °C from solvatochromic experiments with molecular dynamics simulations. UV/vis and steady-state fluorescence measurements of the dye Coumarin 153 in the expanded liquid phase indicate preferential solvation in both the S0 and S1 states by the organic species. Simple dielectric continuum models are used to estimate local compositions from the spectroscopic data and are compared to molecular dynamics simulations of a single C153 molecule dissolved in the liquid phase at bubble point conditions. The simulations provide information about the local solvent structure around C153. They suggest the presence of large solvent clustering near the electron-withdrawing side of the probe. Preferential solvation exists in both the S0 and S1 states, but a large disagreement between simulation and experiment exists in the S1 state. Potential reasons for this disparity are discussed.

I. Introduction Gas-expanded liquids (GXLs) are new and exciting mixed solvent systems that offer pressure-tunable properties, facile separation mechanisms, and environmental benefits. They are a liquid phase formed by the dissolution of a gas, typically CO2, in an organic liquid. GXLs have some similarity to cosolventmodified supercritical fluids (SCFs) but are usually formed under more benign conditions and offer greater solvating power. Further, they also exhibit pressure-controllable properties and somewhat better transport abilities than traditional organic liquids. The dissolution of a solute molecule into a solvent medium disrupts the structure of the solvent shell as a result of differences in the potential between solvent-solute and solvent-solvent intermolecular interactions. The presence of a solute molecule gives rise to the cybotactic region, which IUPAC defines as the region of the solvent affected by the solute. The cybotactic region can give rise to compositions or densities that deviate significantly from the bulk value.1 In mixed solvents, one solvent may preferentially solvate the solute and thereby affect solubility, phase behavior, or reaction rates.2-4 UV/vis and fluorescence spectroscopy are noninvasive methods to probe local solvation of a dye molecule in the ground and excited states because of solvent-solute interactions that impact electronic energy of transition. Accordingly, the absorption and emission spectra contain information about the cybotactic region including local polarity, local density augmentation, and local solvent composi* Corresponding authors. E-mail: [email protected] and [email protected]. † School of Chemical & Biomolecular Engineering. ‡ School of Chemistry and Biochemistry. § Specialty Separations Center. | Center for Computational and Molecular Science and Technology.

tion. Many studies have used solvatochromic dyes to probe local solvent behavior in binary organic liquids,5-7 ionic liquids,8-10 SFCs,11-16 cosolvent-modified SFCs,17-20 and GXLs.21,22 Generally, solvatochromic probes are solvated by the more polar species unless a strong specific interaction with the less polar species exists.8 Many of the spectroscopic investigations have applied analytical techniques to ascertain local composition or density information from spectroscopic data. These techniques typically employ expressions of the bulk solvent properties derived from Onsager reaction field theory (ORFT),23,24 Kamlet-Taft solvatochromic properties,20,25 or a linear combination of pure-component properties.5,15,22 Other techniques such as Langmuir equilibrium models have also been applied to describe solvation within the first solvent shell of a solute.20,26,27 Johnston et al. showed very good agreement between simulation and Langmuir modeling of the local density around alkaline ions in supercritical water.28 Computer simulations can be used by comparison to rationalize solvatochromic measurements and provide additional information that cannot be directly measured in an experiment. GXLs have been a focus of several recent Monte Carlo (MC) and molecular dynamics (MD) simulations: (i) MC simulations have been used to study phase behavior of CO2-alcohol systems29-31 as well as a variety of other polar protic, polar aprotic, and nonpolar liquids.31 Thermodynamic and structural data were obtained in CO2-methanol (MeOH) mixtures with MC simulations to study the self-hydrogen bonding of MeOH.29,30 Stubbs and Siepmann found that tetramers and pentamers are the most stable MeOH aggregates in GXLs.29 (ii) MD simulations have been performed in a variety of GXLs, including MeOH, acetone, acetonitrile (ACN), and cyclohexane. These simulations reveal structural information

10.1021/jp077552p CCC: $40.75 © 2008 American Chemical Society Published on Web 03/22/2008

Exploration of the Cybotactic Region of GXLs as well as transport and fluid properties such as diffusion coefficients, dielectric constants, and viscosities.32-37 MeOH exhibits a large degree of self-aggregation that is consistent with MC simulations results. Shukla et al.32 found a high degree of spatial ordering between MeOH molecules; she also found that acetone molecules neither cluster nor form spatial order as CO2 concentration is increased. Li and Maroncelli explored MeOH, ACN, and cyclohexane mixtures and found that ACN clustering is CO2 concentration dependent with a majority of enrichment occurring at high CO2 concentrations and cyclohexane exhibiting random mixing at all simulated CO2 concentrations.33 A better understanding of the GXL structure is necessary to explore its use as a chemical processing medium. The next step in uncovering this structure requires the presence of a probe, and this has been found to be accessible by way of MD simulations and spectroscopy. Solutes disrupt the fluid continuum and give rise to the cybotactic region and a host of local phenomena. For example, Li et al.21 recently used a combination of spectroscopy and MD simulations to study local solvation around C153 and anthracene-based probe molecules in MeOH, ACN, and cyclohexane GXLs. The experimental portion of their article seeks to quantify local organic enhancements with spectroscopy using the linear assumption of Kim and Johnston.15 MD simulations were used to model the spectral shifts with a one-parameter model of electrostatic and dispersion energies and calculate first solvation shell coordination numbers around the solute probes. Their findings indicate an overestimation of local organic enrichment 2-5 times that of the simulation results. While the authors suggest dielectric modeling may lead to better agreement between computational and spectroscopic techniques, they were pessimistic about the use of dielectric models because of insufficient polar solute-quadrupolar CO2 interaction modeling. Our results agree with their conclusion that the linear assumption overestimates local composition (or enrichment); however, our simulations support reaction field modeling as a viable prediction tool. In this work, we used UV/vis and fluorescence spectroscopy to explore local solvation of the probe molecule Coumarin 153 (C153) in both the ground (S0) and first excited state (S1) electronic structures in two GXLs: acetone and MeOH. Reaction field modeling with dielectric properties was used to extract local compositions as an alternative to the commonly used linear local composition approximation. MD simulations provided a basis to compare the reaction field models and showed that good agreement exists between simulation and experiment in the S0 case while a difference exists in the S1 case. Similar to the findings of Li et al.,21 the linear approximation overestimates the local compositions around the S0 state and predicts an even more unrealistic composition in the S1 state. Solvent structures in the cybotactic region were determined from MD simulations and provide a detailed look at local solvent clustering. II. Experimental Procedures A. Materials. Methanol and acetone (HPLC) and Coumarin 153 were purchased from Sigma-Aldrich and used as received. Carbon dioxide was purchased from Airgas and dried over molecular sieves before use. B. Equipment. Absorption and emission spectra were recorded with a UV detector (Hewlett-Packard 1050 Series) and an Ocean Optics USB200 fiber optic detection system. The light source for fluorescence spectroscopy was a Kratos LH151 N/2 short arc lamp with 1000-W power. Incident wavelength was controlled with a monochromator. Spectra were analyzed via software from Origin Labs (v7.5). All solvatochromic experi-

J. Phys. Chem. B, Vol. 112, No. 15, 2008 4667 ments were performed in a high-pressure optical cell with sapphire windows and cooling jacket, as described by Lu et al.9 Temperature was measured with an Omega J-type thermocouple in contact with the liquid phase. Temperature was controlled by pumping an externally coupled ethylene glycol/ water solution through the cooling jacket. Pressure was measured with a Druck pressure transducer. Carbon dioxide was added to the cell with an Isco syringe pump. C. Procedure. A liquid solution of C153 in MeOH or acetone was added to the pressure cell and sealed. Once the solution reached and was maintained at 298 K, the spectra were recorded. CO2 was slowly pumped into the cell and allowed to reach equilibrium at 298 K and the desired pressure before recording spectra. During the entire experiment, the system was rapidly stirred with a magnetic stir bar to maintain uniform conditions within the cell. D. Solvatochromism Background. The absorption of ultraviolet and/or visible radiation causes electronic excitation of a chromophore. The wavelength of maximum absorption for the chromophore is related to the energy required for excitation. The solvent choice can impact the energy of transition by interacting with the chromophore, and solvent molecules nearest the chromophore will have a much larger impact. The solvatochromatic measurement consequently probes only the subset of the cybotactic region that includes those molecules that are close enough to the chromophore to affect the spectral response. As a result, solvatochromic shifts in λmax are an indicator of the local composition within the cybotactic region. In steadystate fluorescence spectroscopy, the chromophore is excited and solvent molecules reorganize around the excited-state dipole moment. As the chromophore decays back to ground state, it emits light with a λmax affected by the arrangement of solvent molecules in the cybotactic region. Excited-state solvatochromism probes the solvent composition in the cybotactic region of the excited dipole. III. Simulation Methods and Models MD simulations provide a direct comparison to the solvatochromic experiments and the added ability to examine the cybotactic region on a molecular scale. This section provides the details of the model potentials, the parameters, and conditions used in the performed simulations and the observables used to measure the solvent structure of the cybotactic region. A. Models. All molecules were modeled as rigid bodies interacting through a combined Lennard-Jones (LJ) (6-12) plus Coulombic potential

{( ) ( ) }

uij ) 4ij

σij rij

12

-

σij rij

6

+

qiqj rij

(1)

where i and j are the interaction sites on two separate molecules, rij is the distance between two sites, and qi are the site charges. The Lennard-Jones parameters ij and σij are the site-site interactions between atoms obtained by the Lorentz-Berthelot combining rules ij ) (ii jj)1/2 and σij ) 0.5(σii + σjj). Carbon dioxide pair interactions were modeled with the TrAPPE potential.38 MeOH and acetone pair interactions were modeled with the J239 and OPLS-derived40 potentials, respectively. LJ interaction parameters for C153 were obtained from the OPLS model41 and were assumed to be the same for ground and excited electronic states. Partial charges for the S0 and S1 excited states were taken from the literature.42 B. Methods. All simulations were performed at 298 K using the DL_POLY package.43 Simulations consisted of one C153

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Figure 1. Solvent effect on absorption and emission energy. Solute (large oval) dipole moment (arrow) changes upon excitation; energy of transition (Eabs) is altered by polarity of the solvent (circles). Solvent dipole moments align with the new solute dipole moment and decrease S1 energy according to solvent polarity.

molecule in a box of 500 solvent and cosolvent molecules for the cases of 20% and 5% organic or 1000 solvent and cosolvent molecules for 2% organic. Periodic boundary conditions were imposed in all simulations, and the potential interactions were consequently “cut off” to zero at distances beyond half of the box length. Constrained geometries were treated with the SHAKE algorithm, and Coulombic interactions were treated through the Ewald summation method with the “automatic parameter optimization” option with a specified tolerance of 1.0 × 10-5 in DL_POLY. The system box consisted of a single liquid phase whose cubic volume was adjusted to match the experimental densities of GXL systems.44 Equilibrium solvent structures were obtained using the canonical (NVT) ensemble sampled with a Nose-Hoover thermostat. The relaxation time was 5 ps. The equations of motion were integrated using the Verlet leapfrog algorithm using a time step of 3 fs. Initial configurations were generated by randomly orienting solvent molecules around a centered C153 molecule. Thirty initial configurations were equilibrated for 400 ps under NVT conditions before a 500-ps NVT simulation. Statistics were taken during this simulation to determine the structural function g(x,y,z), and configurations were saved every 1.5 ps to compute the average local composition in the cybotactic region. IV. Experimental Results The solvatochromic behavior of ground and excited state C153 in CO2-expanded MeOH and acetone was determined at various CO2 concentrations. A solvatochromic experiment probes the solvation effect on the electronic transition energy of a photochromic molecule, denoted by the wavelength or wavenumber at maximum absorption (λmax or νmax). Information about the local composition can be obtained because λmax is affected predominantly by solvent-solute interactions within the cybotactic region. Absorption spectroscopy probes the ground state of C153 because electronic excitation is on the order of 10-15 s, which is too short for the solvent molecules to reorganize. In emission spectroscopy, C153 is photoexcited by monochromatic light; therefore, solvent molecules can reorient around the excited-state dipole (Figure 1) and alter the energy of the emitted light (denoted by λmax). UV/Vis Spectroscopy. Normalized absorption spectra for C153 in neat organic solvents, CO2-expanded organic solvents, and liquid CO2 are shown in Figures 2 and 3. The absorption spectra for both MeOH and acetone consist of one broad peak in the visible region that keeps the same shape over the entire range of CO2 concentration. As CO2 is added, the peak

Gohres et al.

Figure 2. C153 absorption spectra in CO2-expanded MeOH. Leftmost spectrum is absorption in neat MeOH, CO2 concentration increases as the pure CO2 value is approached (circles).

Figure 3. C153 absorption spectra in CO2-expanded acetone.

Figure 4. Positions of C153 absorption maxima at varying CO2 concentrations. Wavenumbers have been subtracted from gas-phase absorption values. Circles are CO2-expanded MeOH maxima; triangles are CO2-expanded acetone maxima. Error bars are within the size of the data point.

undergoes a blue (hypsochromic) shift indicating positive solvatochromism, which is expected because the polarity of the GXL decreases with increased CO2 concentration. Also, the absorption maxima are in close proximity to those of the neat solvents, which indicates preferential solvation by the organic species. The wavenumbers at maximum absorption (νmax), which are proportional to energy of transition, are shown in Figure 4. In this figure, νmax values have been subtracted from νmax for gas-phase C15345 so that the solvent effects on transition energy are seen. A larger difference represents less transition energy. It is apparent that only minor transition energy changes occur up to 80% CO2. After this concentration, there is a steady increase in transition energy which supports preferential solvation by the organic. Also, the transition energy in GX MeOH and GX acetone are comparable. Fluorescence Spectroscopy. The normalized emission spectra (Figures 5 and 6) of GX acetone and GX MeOH show a large difference between the νmax in GXLs and pure CO2. This

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Figure 5. C153 emission spectra in CO2-expanded MeOH. Dashed line is emission in neat CO2.

Figure 6. Normalized C153 emission spectra in CO2-expanded acetone.

large increase in νmax between 95% and pure CO2 indicates solvation dominated by the organic species. Such preferential solvation is expected because the large dipole moment of the excited state is better stabilized by polar solvents, but total solvation at 5% organic is surprising. Also, there is a relatively small change in νmax over the range of organic mole fractions studied (270 cm-1 for MeOH and 400 cm-1 for acetone) compared to that of absorption which changes over 500 cm-1 up to 80% organic before rapidly declining to the pure CO2 value. In addition to complete organic solvation, excited C153 in GXLs is solvated by a similar mechanism over the entire organic composition range. Calculation of Local Compositions. The difference in transition energy between the vapor and solution phase for a system behaving as a dielectric medium can be modeled with ORFT given in eq 2. In ORFT, the solute is modeled as a spherical dipole subject to dipolar and inductive interactions from the surrounding dielectric medium

∆ν ) A

(

) (

0 - 1 n2 - 1 n2 - 1 +B 2 - 2 0 + 2 n + 2 2n + 1

)

(2)

where ∆ν is the wavenumber difference between gas-phase C153 and C153 in solution, ν0 - νmax, and 0 and n are the static dielectric constant and the refractive index of the solvent. A and B are solute-dependent constants. The first term is the orientation polarization, or dipole interaction term, which reflects the solvent’s ability to orient around the solute. For solvents without a permanent dipole,  ≈ n2, and this term is negligible. The second term, the electronic polarizability, accounts for induced dipole-dipole interactions. The parameters A and B are functions of the cavity radius and ground- and excited-state dipole moments of C153. The parameters, A, B, and ν0, for C153 have each been specified in ref 45 only within a range of values. In the present work, slight adjustments were made to

all three parameters to match the experimental maxima observed for pure CO2 and organic species. In all of the solvent systems studied herein, the adjusted parameters were within the literature range or slightly outside. Parameters A and B varied between acetone and MeOH GXLs, while ν0 remained the same in both solvents. Values for these parameters used in this study and those from ref 45 are provided in Table 1. The ORFT requires dielectric constants and refractive indices of the GXL, which can be obtained in three different ways: experimental data, a linear relationship with volume fraction, or mixing rules applied to pure species values. Dielectric constants for GX MeOH have been measured;46,47 however, the data were taken at temperature and compositions not used in this study and data for GX acetone do not exist. Refractive indices for both GXLs are also unavailable. Mixing rules were used in favor of the linear assumption to obtain better estimations for dielectric constants and refractive indices of the mixtures. A comparison to the existing dielectric constant data for GX MeOH validates the mixing rules as reasonable estimates for the actual values. Mixing Rules. Refractive indices for the GXLs were calculated from pure component values with the LorentzLorenz mixing rule (eq 3), which has been successfully applied to liquid mixtures48 and supercritical fluid mixtures.49,50 The refractive indices of pure CO2 at the operating densities were obtained from the Buckingham and Pople formulation51 (eq 4) with virial refractivity coefficients from Besserer and Robinson52 (eq 5). Refractive indices of the pure organic species were obtained from the literature.53

(nmix - 1)2

)

(nmix + 2)2

∑ φi

(ni - 1)2 (ni + 2)2

RLL ) RLL0 + BF + CF2 RLL )

1 n2 - 1 F n2 + 2

(3)

(4) (5)

where F is the density, RLL0 is the limit of the molar refractivity at zero density, and B and C are the second and third virial coefficients, respectively. A variety of mixing rules exist for dielectric constants because heterogeneous anomalies within the medium can greatly impact the effective dielectric constant. This is especially true in self-associating systems such as CO2expanded MeOH, where hydrogen bonding can cause inhomogeneities.54 The Bruggeman equation (eq 6) is an asymmetric formula derived by differentially increasing the concentration of a dispersed component (the organic species), which results in a gradual change of the dielectric constant.55

( )

s,mix - s,d s,m s,m - s,d s,mix

1/3

) 1 - φd

(6)

The variables  and φ are the static dielectric constant and volume fraction; the subscript d refers to the dispersed solvent component, m denotes the continuous solvent component, and mix is that of the mixture. As a result, this formula accounts for inhomogeneities that have serious implications in the low organic concentration regime. The organic component was chosen as the dispersed species in each case because doing so led to better fits of the available dielectric constant data. Local Composition. Calculated ν0 - νmax values (from eq 2) for gas-expanded MeOH absorption spectra are shown with experimental data in Figure 7. The calculated spectra were

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TABLE 1: Parameters A, B, and ν0 Used in Eq 2 for Absorption and Emission Studies in the Two Types of GXLsa

a

cosolvent

spectroscopy

A (103 cm-1)

B (103 cm-1)

ν0 (103 cm-1)

acetone methanol ref 45 acetone methanol ref 45

absorption absorption absorption emission emission emission

-1.6 -1.9 -2.1 to -2.8 -1.0 -2.2 -3.8 to -5.2

-11.8 -11.8 -8.0 to -10.0 -8.1 -7.9 -6.0 to -10.0

27.1 27.1 25.3 to 27.9 21.6 21.6 20.8 to 23.3

Absorption and emission parameters from ref 45 are provided for comparison.

TABLE 3: Local Compositions Determined from Spectroscopic Experiments and MD Simulations at Each Simulated Bulk-Fluid Acetone Concentrationa percent acetone 20 5 2 20 5 2

state S0 S0 S0 S1 S1 S1

experiment

simulation

40.3 10.9 4.4 71.3 63.0 20.0

36.0 5.6 2.6 26.0 10.0 8.3

a Experimental values were interpolated to match the simulated counterpart.

Figure 7. Experimental solvatochromic shifts for C153 absorption in CO2-expanded MeOH (triangles) and calculated solvatochromic shifts from ORFT (circles). Linear local composition approximation (dashed line) is shown for comparison.

TABLE 2: Local Compositions Determined from Spectroscopic Experiments and MD Simulations at Each Simulated Bulk-Fluid Methanol Concentrationa percent methanol 20 5 2 20 5 2

state S0 S0 S0 S1 S1 S1

experiment

simulation

59.5 20.4 12.0 73.7 69.8 20.0

57.6 30.5 18.0 32.9 13.6 11.0

a Experimental values were interpolated to match the simulated counterpart.

determined at the same CO2 mole fractions that the experimental spectra were measured. An estimation assuming a linear contribution to the local composition from each solvent is provided for comparison. Figure 7 reveals two interesting features. The first is the larger wavenumbers (energy of transition) calculated by ORFT than the experimental values at the same CO2 concentration. Second is the even larger wavenumber prediction by the linear assumption. Both overpredictions are consistent with an enhanced organic environment around C153. The compositions at the calculated wavenumbers are taken to be the local composition corresponding to a bulk or experimental composition at an equivalent wavenumber. In other words, the environment that the probe “senses” from organic enrichment of the cybotactic region disrupts the expected spectral behavior. A graphical representation of local organic enrichment is illustrated in Figure 7. Local compositions were back-calculated from the ORFT at each experimental maximum. Tables 2 and 3show local compositions determined from the spectroscopic experiments and MD simulations. The local compositions in the cybotactic region have been determined using the statistics recorded during the MD simulations. A detailed discussion of the cybotactic region used in these calculations and the interpretation of the experimentally and computationally derived local compositions are provided in section V. The S0 simulation results are in good agreement with the experimental results, particularly at 20% bulk organic

concentration. Emission spectra of GX MeOH and GX acetone show similar local behavior up to 80% bulk organic. At this composition, both solvents have a modest organic enrichment factor of ∼3.5. Beyond this point, the enrichment factor rapidly approaches the mid-teens, indicating total solvation by the organic species despite cosolvent-like bulk compositions. V. Computational Results Local Solvent Structure. MD simulations provide both a direct comparison to solvatochromic experiments and a more detailed understanding of the local solvent structure. The solvent structure was modeled using a 3D axial distribution function g(x,y,z), which provides more detail than a 1D radial distribution function g(r). This is especially important because C153 is an asymmetric probe whose different functionalities are expected to alter the local environment. To perform a precise axial distribution function (ADF) calculation, a new coordinate system was constructed with C153’s center of mass as the origin and x, y, and z axes consistently directed relative to all C153 atoms. Coordinates of every atom were transformed relative to the new coordinate system, and periodic boundary conditions were enforced relative to the transformed simulation cell. The atomic coordinates were made discrete relative to the C153 center of mass and recorded as a histogram every 12 fs. The ADF was determined from these statistics according to eq 7

gi(x,y,z) )

〈ni(x,y,z;dV)〉 Fi‚dV

(7)

where 〈ni(x,y,z;dV)〉 is the mean number of atom i in a finite element of volume dV at a position relative to the C153 center of mass and Fi is the bulk number density of type i in the system. C153’s planar structure facilitates 2D analysis at different planes that are parallel to the coordinate system. Figure 8 is the axial distribution function of MeOH (a, c) and CO2 (b, d) around a ground-state probe as viewed from two different perspectives: a bird’s eye view of the C153 molecular plane and a sample cross section that captures accumulation above and below the C153 plane. The solvent is a 5% acetone GXL. Acetone forms an unevenly dispersed solvent cage around the C153 molecule with enhanced accumulation near the oxygen atoms and trifluoro group. An enhancement of CO2 in the

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Figure 8. Axial distribution functions at two different vantage points around ground-state C153 in a 5% acetone GXL. Scale represents values of the ADF for (a, b) acetone and (c, d) CO2 clustering around C153. Depicted plane is coplanar to the C153 molecule (a, c). Vertical line through the probe molecule represents the sample plane used to show solvent accumulation above and below the C153 plane (b, d). The solid and dotted parts of this line are used to differentiate between the two poles of the C153 molecule as the vantage point is rotated. Tick marks are spaced ∼3 Å.

cybotactic region exists as an evenly distributed cage around C153; however, the small ADF compared to acetone indicates preferential solvation by acetone. Solvent distribution maps for the other acetone GXLs and the MeOH GXLs are provided in the Supporting Information. Generally, the organic species or CO2 molecules form solvent structures that are independent of the probe’s electronic state. In all GXLs studied, the magnitude of the organic ADFs increased with bulk CO2 concentration; however, the solvent structure in the cybotactic region remained the same. CO2 displays this same concentration-independent partitioning behavior but undergoes only a slight increase in ADF as the bulk CO2 concentration is raised. The organic ADFs are amplified in high-CO2 GXLs because of preferential solvation arising from favorable interactions with the probe, while CO2 ADFs increased from the compressibility of CO2 around the solute at higher pressures. CO2 ADFs were much lower than the organic, which further indicates preferential solvation. CO2 molecules were more evenly distributed around the solvent than the organic

species, although heterogeneities can be seen near the nitrogen, trifluoro, and carbonyl groups. The axial distribution functions about the probe in 5% acetone (Figure S4) show very similar solvation structures about both the ground and excited states; the same is true for the 5% MeOH case (Figure S10); however, the magnitudes are much greater for the excited state. The increased magnitude is localized to small areas near the ester functionality of C153, while the majority of the cybotactic region is solvated similar to the ground state. This occurs because the excited-state dipole moment is directed in a similar direction to the ground state, but the large increase in magnitude results in a large attraction for the organic species. Local Composition. In practice, it is difficult to determine which region is affected by the solute and it is even more difficult to calculate the average composition in such an indistinct area. In earlier work,32 we found that the structure in the first solvation shell around a solute provides an indicator of the structure of the cybotactic region. It also has the advantage

4672 J. Phys. Chem. B, Vol. 112, No. 15, 2008 of being a clearly defined region where properties can be easily and reliably measured. This assumption can be somewhat skewed if the solute is nonspherical (because the curvature of the surface can lead to varying effects on the topology of the neighboring structure) but serves as a simplistic model of a complex region. Although a spherical model may not match the exact definition of the cybotactic region, it can encompass both the first solvation shell and additional regions of the solvent that are affected by the solute. Consequently, it is an approximation that can be made to explore a region that is difficult to define and quantify. On the basis of this assumption, we estimated the cybotactic region from the ADF plots as 7 Å from the C153 center of mass. All local organic compositions are shown with their experimental counterparts in Tables 2 and 3. Local compositions around ground-state C153 agree reasonably well with spectroscopic measurements, particularly at high bulk organic composition. The agreement is less favorable as the CO2 composition is increased. Simulation results for the excitedstate C153 underpredict the experimental data, particularly for GX acetone, which is off by a factor of 6 for the 5% acetone case. All excited state results show preferential organic solvation, but in some cases the organic enhancement is less than the ground-state enhancement. The disparity between fluorescence experiments and excitedstate MD simulations could result from a number of causes, including inadequate models of the excited state or increased specific interactions, both of which are discussed in the literature. Cichos et al.56 attempted to reproduce emission spectroscopy of C153 in MeOH and ACN with MD simulations and found that their emission shifts disagreed with experimental shifts. The authors mention that only a few C153 atoms contribute to the charge-separated S1 state, which leads to a nonhomogeneous S1 state. However, the S1 charges effectively reproduced the S1 dipole moment so the error from the partial charges is likely kept to a minimum. Cichos et al.56 suggest incorrect Lennard-Jones parameters as the most probable error source. A common assumption is to use the same dispersion-repulsion parameters for the S0 and S1 states. It is possible that the increased charge density on certain atoms, specifically the carbonyl oxygen, in the S1 state could increase the collision diameter (σi) and could seriously affect solvent-solute interactions and ultimately the calculated spectra.56 Strong specific interactions are generally unaccounted for in MD simulations but can affect emission spectra. Solvation studies of C153 in MeOH nonpolar mixtures found that nonlinear solvatochromic behavior was the result of strong hydrogen bonding between MeOH and the C153 carbonyl oxygen.57,58 The amount of MeOH hydrogen bonds at the carbonyl oxygen greatly increases upon excitation because the oxygen basicity increases upon excitation. Infrared spectroscopy confirms increased hydrogen bonding at the carbonyl oxygen as methanol is added to hexane.58 Reaction field models do not describe the solvatochromic shifts in solvents that hydrogen bond; however, our local composition model provides the equivalent local environment felt by C153. If hydrogen bonds exist, the reaction field model would predict a larger amount of organic solvent to account for the hydrogen bond stabilization. This could overestimate the local composition and further offset the difference between the experiment and simulation results. While strong specific interactions might explain why emission shifts resemble pure MeOH at very high CO2 concentrations, it does not explain the similar behavior in GX acetone, which is less polar and also aprotic. For this reason, our reaction field model provides a

Gohres et al. reasonable estimation of local composition, but inadequate force fields used in the MD simulations resulted in underestimated local compositions around the S1 state. VI. Conclusion The cybotactic regions around solutes in CO2-expanded MeOH and CO2-expanded acetone have been investigated using a combination of simulations and experiments. This synergistic approach allows the exploration of the relationship between local solvent structure and spectroscopic properties and enables the prediction of local solvent composition from spectral data and simple reaction field models. Absorption solvatochromic shifts are in good agreement with ground-state MD simulations, predicting preferential solvation by the organic component of the GXL. Fluorescence solvatochromic shifts indicate a larger degree of organic solvation in the S1 state. Unfortunately, MD simulations underestimate the experimental local compositions. Two possibilities exist for this disagreement: 1. Underestimation by the simulations because of unrealistic dispersion and repulsion terms in the C153 force field. 2. Overestimation of local composition due to specific interactions. The fact that MeOH can form hydrogen bonds with C153, but acetone does not, suggests that the latter case is not likely because otherwise each would have led to a different observation. The similar trend in the emission spectra in methanol and acetone suggests that dielectric enrichment contributes much more to fluorescence shifts than do specific interactions. MD simulations show that the S1 and S0 states of C153 have similar solvent shells with most of the organic solvation around the acceptor side of the molecule. The S1 state is solvated by more organic molecules than the S0 state, although the increased enrichment is localized in small pockets near the functional groups. This finding links different solvation patterns to observable absorption and emission spectra. The molecular level details provided by MD simulations can play a role in understanding more complex chemistries and designing solvents that interact favorably with reactants, catalysts, or other important solutes. The emission and absorption solvatochromic shifts are in good agreement with the recent results of Li et al.21 Our MD simulation results are presented in a different form, yet the data follow a similar trend. Both studies found that preferential solvation exists at each simulated composition and that the local organic enrichment increases with CO2 concentration. We support their conclusion that the linear relationship between solvatochromic shift and local composition overestimates the local composition. However, we conclude that the reaction field model presented in this work provides reasonable approximations of the local composition over the entire range of CO2 concentration, especially in the ground state. MD simulation results of the S0 state agree with the reaction field models in both GXLs, but the C153 S1 force field needs reparametrization before it can model the fluorescence spectroscopy adequately. This study of the cybotactic region provides important information that opens the door to exciting research opportunities in GXLs such as solvent design, catalysis and reactions, nanoparticle synthesis, and gas-antisolvent crystallization. Acknowledgment. The authors gratefully acknowledge the financial support from the Department of Energy, Office of Basic Energy Sciences. Also the computational facilities at the CCMST have been supported under NSF grant CHE 0443564.

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