Molecular Interactions and CO2-Philicity in Supercritical CO2. A High

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J. Phys. Chem. B 2007, 111, 1318-1326

Molecular Interactions and CO2-Philicity in Supercritical CO2. A High-Pressure NMR and Molecular Modeling Study of a Perfluorinated Polymer in scCO2 Ma´ rcio Temtem, Teresa Casimiro, A. Gil Santos,* Anjos L. Macedo, Eurico J. Cabrita,* and Ana Aguiar-Ricardo* REQUIMTE/CQFB, Departamento de Quı´mica, Faculdade de Cieˆ ncias e Tecnologia, UniVersidade NoVa de Lisboa, 2829-516 Caparica, Portugal ReceiVed: September 15, 2006; In Final Form: December 13, 2006

The carbon and fluorine chemical shifts of mixtures of carbon dioxide and Krytox, a carboxylic acid endcapped perfluorinated polyether used as stabilizer for the dispersion polymerization of methyl methacrylate, have been studied using high-pressure, high-resolution nuclear magnetic resonance. 13C and 19F spectra were measured in the density region between 0.54 and 0.73 g.cm-3 at 334 K for different solutions of Krytox in scCO2 (0.22, 1.13 and 1.72 w/w %). An in-house developed high-pressure apparatus with the capability to change in situ the sample composition was used for this purpose using a 10 mm polyether ketone NMR tube. The nature of CO2-Krytox interaction was assessed both by comparing the CO2 δC variation of neat CO2 with that of mixtures with increasing surfactant composition and by the analysis of Krytox 19F corrected chemical shifts in terms of medium magnetic susceptibility. Ab initio calculations, at the second-order Møller-Plesset level of theory to include the effects of electron correlation, were performed to access and compare the nature of the interactions between CO2 and perfluorinated and nonfluorinated analogue model molecules. Both experimental 13C and 19F HP-NMR results and molecular modeling studies support a F...CO2 site-specific Lewis acid-Lewis base interaction model. A positive entropic variation for the formation of CO2-fluorinated solute complex is advanced as an explanation for the higher solubility of perfluorinated molecules when compared to the nonfluorinated analogues.

Introduction Over the last years there has been an extensive research in the use of supercritical carbon dioxide, scCO2, as a polymerization reaction medium,1-3 particularly in dispersion polymerization. As CO2 is a poor solvent for polymeric macromolecules except for silicones and fluoropolymers, a stabilizer is required to prevent the aggregation and precipitation of the growing polymer. An efficient stabilizer must be able to anchor to the growing polymer particles, while exhibiting sufficient solubility in the solvent to form a stable dispersion and prevent precipitation. Successful stabilizers are block polymers which incorporate either a siloxane, e.g., poly(dimethylsiloxane), PDMS, or a fluorinated, e.g., poly(fluorooctyl acrylate), PFOA.4-8 Beckman et al. have shown that fluorinated graft copolymers are also effective stabilizers for dispersion polymerization.9 All these stabilizers have in common both “polymer-philic” and “CO2philic” moieties that stabilize the polymer in the continuous phase and prevent flocculation and precipitation allowing the polymerization process to continue successfully to completion. A major drawback is that their synthesis can be complex and thus may not be economically viable. Moreover such macromonomers have the disadvantage of being incorporated into the final polymer product.10 Christian et al.11,12 and Casimiro et al. 13 reported the use of a commercially available carboxylic acid terminated perfluoropolyether, Krytox 157FSL, as stabilizer for the dispersion polymerization of methyl methacrylate(PMMA) and diethylene glycol dimethacrylate (PDEGDMA) respectively. This stabilizer seems to interact with the growing polymer by reversible hydrogen bonding between the carboxylic * Corresponding authors. E-mails: [email protected], [email protected], and [email protected].

acid termination of the stabilizer and the ester group of the monomer, leading to a pseudograph copolymer11 while the fluorinated tail provides the required solubility in scCO2. The fluorinated backbone of the stabilizer does not have C-H bonds which prevents hydrogen abstraction and chemical incorporation of the stabilizer into the polymer.14 The right anchor-soluble balance of Krytox, which is the ratio of the polymer-philic to the CO2-philic parts of the stabilizer, leads to the successful dispersion polymerizations of both PMMA and PDEGDMA. Despite IR evidence of hydrogen bonding interaction between the polymer and the stabilizer, there are no studies to confirm such interaction at high pressure at the polymerization conditions. Due the importance of knowing the nature of stabilizergrowing polymer interactions for the development of new stabilizers, in this work we apply high-pressure nuclear magnetic resonance, HP-NMR, to investigate the CO2-philicity of Krytox and the molecular interactions between the stabilizer and scCO2. HP-NMR offers unique, highly localized structural molecular information and has shown to be a very powerful tool for the investigation of molecular interactions under the influence of high pressure.15-17 CO2-Fluorocarbon Specific Interactions and CO2Philicity. Substitution of hydrogen atoms by fluorine atoms in organic molecules is a generally accepted method to increase miscibility or solubility with scCO2, i.e., to make otherwise insoluble compounds more CO2-philic. The factors responsible for the high solubility of perfluorocarbons in scCO2 have been the subject of many experimental and theoretical studies but, despite all the research efforts, the mechanism for the enhanced solubility is still an open question. Some authors have advanced the presence of specific CO2-fluorine interactions as an

10.1021/jp0660233 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/24/2007

Molecular Interactions and CO2-Philicity explanation for the observed enhanced miscibility in comparison to the nonfluorinated systems, but this hypothesis is not consensual. Currently there are contradictory experimental and theoretical studies, some suggesting the existence of these specific interactions and other pointing to nonexistence of specific CO2-fluorine interactions in comparison to the non fluorinated systems. In a very recent report about the polar attributes of scCO2, Raveendran et al. have reviewed the most significant studies about CO2-fluorine interactions and have concluded that in spite of the fact that all the work published helps to clarify the differences in hydrocarbon and perfluorocarbon interactions with CO2 it does not allow to establish a mechanism for the enhanced solubility.18 Ab initio calculations and HP-NMR are among the preferred tools to investigate the nature of the interactions of fluorocarbons or hydrocarbons with scCO2. One of the most relevant experimental evidence that supports the existence of specific CO2fluorine interactions comes from density dependent 1H and 19F chemical shift measurements of hydrocarbons and fluorocarbons. Dardin et al. have shown that CO2 density differentially influences 1H and 19F chemical shifts of n-hexane and perfluoron-hexane dissolved in scCO2.15 Excess chemical shift effects have been found in 19F NMR of the fluorinated solute dissolved in scCO2 which were in contrast to the normal bulk susceptibility dominated 1H NMR observations of the hydrocarbon solute. This excess magnetic shielding was attributed to van der Waals interactions between the fluorinated sites in the solute and scCO2. These results are, however, in contrast with the results of Yonker et al., who have studied the nuclear shielding of CH3F and CHF3 in neat and CO2 solutions and found no evidence for distinct or specific interactions between the fluoromethanes and CO2.19 Yonker has also studied the solution dynamics of perfluorobenzene, benzene and perdeuteriobenzene in CO2 using high-pressure NMR.20 The 19F, 1H, and 2H T1 relaxation times of the solute molecules were measured in CO2 as a function of pressure and temperature. Since the NMR relaxation measurements provide information about the rotational reorientation and spatial reorientation of the molecules in solution, it was hypothesized that at high pressure/high density the specific interaction postulated to occur between CO2 and fluorine should be more prevalent and this interaction could alter the relaxation rate/process of perfluorobenzene in solution as compared to benzene. The results from these measurements showed that at high densities the solution viscosity dominates the relaxation process and the relaxation time for both 19F and 1H are the same, therefore no experimental manifestation of a specific intermolecular interaction between CO2 and fluorine was observed. The difference in the nature of the interactions of hydrocarbons and fluorocarbons with CO2 is also controversial from the theoretical point of view to what concerns the results of molecular modeling studies. In 1996 Cece et al. reported Hartree-Fock calculations in which the binding energies for clusters (CO2)n-C2H6 and (CO2)n-C2F6 with n ) 1-4 revealed that the interaction of the fluorocarbon with CO2 is predominantly electrostatic in nature resulting in a favorable interaction energy of 0.75-0.8 kcal/mol for each CO2 molecule in the first solvent shell.21 The calculations revealed minimal interaction between the CO2 and the hydrocarbon molecule and a clustering of CO2 molecules around C2F6 intercalating the positively charged CO2 carbon between two negatively charged fluorine atoms. In a comment to these results, Han and Jeong pointed out that they were erroneous since the calculations did not take into account the basis set superposition errors.22 These authors have demonstrated that HF/6-31G* calculations, as used by Cece

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1319 et al., cannot give any useful information, even qualitatively, on the interaction of hydrocarbon and fluorocarbon with CO2, since the majority of the results on binding energies might have originated from the basis set superposition error, which is an artificial mathematical effect. The ab initio calculation of the binding energies between CO2 and CH4, C2H6, CF4, and C2F6 was revisited by Diep et al. with a curious result;23 while Hartre-Fock calculations with flexible basis set fail to give appreciable binding for any of the four dimers, MP2 calculations using flexible basis set and including corrections for basis set superposition error gave slightly larger binding energies for the CO2-hydrocarbon complexes than for the corresponding CO2perfluorocarbon cluster. The authors conclude that it is not possible to discern the reason for the greater solubility of perfluorocarbons than of hydrocarbons with the small clusters used in the study. More recently Raveendran and Wallen have carried out ab initio calculations at MP2 level in an effort to understand the effect of stepwise fluorination on the CO2philicity of methane.24 By using simple quantum chemical calculations, these authors have shown that even though the interactions of CH4 and CF4 with CO2 are energetically comparable they are of a fundamentally different nature. While CO2 acts as a weak Lewis acid in CO2-fluorocarbon interactions it acts as a Lewis base in CO2-hydrocarbon interactions. The variation of strength of the CO2-philic interactions with a stepwise increase in the number of fluorine atoms for fluoromethane show that the polar fluoromethanes interact more strongly with CO2 than CF4, and that there is a turnover in the interaction energies as a function of the number of substituted fluorine atoms. The C-H bonds in partially fluorinated methane, by virtue of their electron deficient nature, can participate in C-H‚‚‚O bonds with the CO2 oxygen atoms. For the respective CO2 complexes of CH3F, CH2F and CHF3 the interaction between oxygen of the CO2 and the hydrogen of the fluoromethane becomes stronger and the H‚‚‚O distance decreases systematically. Due to the high electronegativity of fluorine, the initial fluorinations result in high polar C-F bonds, and this enables the fluorine atom of the CO2-CH3F complex to act as a potential Lewis base for interaction with the carbon atom of CO2; however, further fluorination results in competition among the individual electron withdrawing fluorine atoms, making them weaker electron donors. As the authors state in their conclusions, as far as the enthalpic contributions to the solute-solvent interaction is concerned, the results indicate that there is a limit to the number of fluorine atoms for maximum CO2-philicity. However the solubility-miscibility of a solute in scCO2 will be decided by a number of parameters, including the enthalpic and entropic contributions from solute-solute, solute-solvent, and solvent-solvent interactions. NMR Background. The magnetic resonance signal of a molecule in a gaseous or liquid medium is displaced from the position it would have if no medium were present. The chemical shift measures only the difference between the mean shielding of the nucleus of interest in one chemical environment and the mean shielding in some reference environment. Therefore, changes in the chemical environment or medium will produce a shift of the chemical shift. A part of these shifts arises from the solvent bulk diamagnetic susceptibility effect but the most interesting part is due to short range interactions between the solute and its nearest solvent neighbors. To understand the nature of these shifts, it is necessary to understand the nature of the nuclear shielding and to relate it to the chemical shift. The nuclear magnetic shielding of a molecule in the dilute gas phase is an average value which is determined by rovibra-

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Temtem et al.

cional averaging in independent molecules as well as collisional averaging for pairs of interacting molecules. Provided that the density is sufficiently low, the observed nuclear resonance frequency is linearly related to density. The shielding of a nucleus in a molecule can be expressed in terms of a virial expansion in the density:25

σ(T,F) ) σ0(T) + σ1(T)F + σ2(T)F2 + σ3(T)F3...

(1)

In this equation σ0 is the rovibrational shielding of an isolated molecule at a temperature T (shielding of the solute in vacuo), σ1 is the second virial coefficient of the shielding resulting from binary collisions, and σ2 and σ3 are due to higher order multibody interactions. The equation reflects the importance of the nonlinear terms corresponding to ternary and higher order interactions at higher densities. If the higher order terms in eq 1 are neglected and the isolated molecule is taken as a reference, then the experimentally measured chemical shift of the nucleus, δ, can be related to the nuclear shielding as follows:

-δ(T) = (σ(T,F) - σ0(T)) ) σ1(T)F

(2)

According to eq 2, at low densities, shielding changes can be related to the pairwise shielding effect σ1. Since the observed shielding can be considered to result from a collection of additive terms, the pairwise shielding effect σ1 can be expanded into four contributions:26

σ1 ) σb + σw + σE + σa

(3)

where σb is the shielding due to the bulk magnetic susceptibility of the solvent molecules (volume susceptibility effects in a nonspherical sample), σw is the contribution to shielding from van der Waals interactions of the solvent and solute molecules, σE is related to shielding effects from the local electric fields generated by neighbor solute molecules near the molecules of interest (dipole-dipole interactions), and σa arises from the neighbor-molecule magnetic anisotropy (magnetic anisotropy in the molecular susceptibility). The bulk susceptibility shielding is due to the magnetic field at the nucleus that results from the diamagnetic polarization of the solvent molecules by the applied external field. It depends upon the shape of the sample and for an infinite cylinder aligned with the magnetic field, σb is27

σb ) -

χm

(4π3 - Rj)χ ) - 4π3 V v

m

)-

4π χm F 3 M

(4)

where χV is the diamagnetic volume susceptibility of the medium, χm is the molar susceptibility of the medium, Vm the molar volume, M the molecular weight, F the density of the medium, and R j is the medium shape factor, which is zero for an infinite cylinder aligned with the magnetic field. Going back to eq 3, for the case of pure carbon dioxide, dipole-dipole interactions can be excluded, the term due to the anisotropy of the molecules is in general very small, and van der Walls interactions are inexistent. Thus, the observed shielding would have only the contribution of the bulk susceptibility, σb. Considering only this contribution and assuming low densities, eq 1 can be written as:

σ ) σ0 -

4πχm 4π χv ) σ0 F 3 3M

SCHEME 1: Molecular Structure of the Surfactant Used in This Work (Krytox 157 FSL Mw 2500)

(5)

which implies a linear relation between σ or δ and density. Experimental Section Materials. Krytox 157 FSL (Mn 2500, Scheme 1) was kindly donated by DuPont and was used without further purification.

A high-resolution 19F NMR spectrum of a solution of krytox in hexafluorobenzene was acquired in order to confirm the purity and average molecular weight in number of the polymer. A glass capillary containing a solution of trichlorofluoromethane in benzene-d6 was placed inside NMR tube for lock and 19F chemical shift referencing purposes. To derive the average molecular weight, Mn, from the NMR spectrum, integrated peak areas for the responding fluorines in the internal unit (-OCF2CF(CF3)O-, δ ) 145 ppm, I ) 14.1 and -OCF2CF(CF3)O-, δ ) 80 ppm, I ) 78.7) and end group (-OCF2CF2CF3, δ ) 130 ppm, I ) 2) were determined. The normalized areas were obtained by dividing the integrals by the number of responding fluorines and the values, n = 15 and Mn = 2820 were determined. These values are in good agreement with the supplier’s description. Carbon dioxide was supplied by Air Liquide with purity better than 99.998%. High-Pressure Apparatus. The mixtures under study were prepared using a new high-pressure apparatus, schematically presented in Figure 1. This apparatus was build over a mobile support specially designed to interface with the NMR facility. The mixtures under study are prepared in a 5.3 mL stainless steel cylindrical cell similar to the one described by Sampaio et al.28 The cell is equipped with two aligned sapphire windows and is stirred by means of a Teflon coated magnetic stir bar. It is coupled to a hand pump (High-pressure Equipment CO., model 87-6-5) that allows operating with variable volume. Both the cell and the hand pump are immersed in a thermostated water bath heated by means of a PID controller (Hart Scientific, model 2100) that maintained the temperature within ( 0.15 °C. The pressure is monitored with a pressure transducer (Setra Systems Inc., model 204) with a precision of (100 Pa. Carbon dioxide is loaded into the system using a high-pressure pump (New Ways of Analytics). The gas passes trough a bed of molecular sieves and a filter in order to eliminate traces of water and impurities. The surfactants and monomer are introduced into the cell trough a calibrated loop connected to a HPLC valve. The HP-NMR PEEK tube is filled with the mixture using another HPLC valve. The HP-NMR cell, their connections and the tube connecting and to the high-pressure equipment are made of nonmagnetic materials (PEEK, Delrin) obtained from Upchurch Scientific (maximum 48.3 MPa). To minimize the temperature oscillations the HP-NMR inlet tube is placed inside a thermostated water pipe. High-Pressure NMR Tube and Experiments. In literature one can find various types of high-pressure cells for in situ NMR investigation. A good review has been recently published.29 A great variety of high-pressure NMR cells made from distinct pressure-bearing materials have been reported in literature.30-36 In this work a new polyether ketone (PEEK) cell was built, with 10 mm o.d., suitable to record NMR spectra on a Bruker system. It was based on a similar design published by Flynn et al.37 A schematic diagram of the HP-NMR tube is shown in Figure S1 (see Supporting Information). The tube was fabricated using standard machining techniques from a standard commercially

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Figure 1. Schematic diagram of the apparatus: (1) Hand pump compressor; (2) molecular sieves; (3) filter; (4) check-valve; (5) syringe; (6) HPLC valve; (7) pressure transducer; (8) temperature controller; (9) cell with sapphire windows; (10) thermostated water bath; (11) HP-NMR tube; (12) spectrometer Bruker, ARX400; (13) recirculating pump; (14) thermostated pipe.

available rod with 1” diameter (McMaster-Carr Supply). It consists of two pieces connected by a 3/4” thread, sealed with a buna o’ring. The cell was tested hydrostatically at temperature for signs of failure. Procedure. In a typical experiment, the apparatus is cleaned, dried, and placed in vacuum for several hours in order to eliminate any remaining residues. The bath is then thermostatized, and CO2 is added to the required pressure. The volume of the apparatus is controlled using the hand pump, and CO2 is quantified using the density of pure carbon dioxide38 at the operating pressure and temperature conditions. After 30 min of continuous stirring, the mixture is expanded to the HP-NMR Peek tube through the HPLC valve and the tube placed inside the spectrometer previously thermostatized. High-pressure high resolution 19F and 13C NMR spectra were recorded on a Bruker ARX400 spectrometer, equipped with a temperature control unit, and operating at a fluorine frequency of 376.90 MHz and a carbon frequency of 100.61 MHz. The length of the 90° pulse was 10.1 µs for fluorine and 5.4 µs for carbon. A recycle delay of 5 s was used for all experiments. A glass capillary of benzene-d6 containing trichlorofluoromethane was placed inside the peek NMR tube and used as internal reference for 13C and the 19F spectra respectively. Pressure-dependent 13C NMR spectra of CO2 were obtained at 334 K and pressures from 15 up to 22 MPa. The corresponding densities were calculated using the new equation of state for carbon dioxide from Span and Wagner.39 After each incremental pressure change (∼1.4 MPa) the system was allowed to equilibrate (approximately 30 min) until the pressure reading was stable ((0.007 MPa on the instrument). Spectra of CO2 and the surfactant under study were performed at the same CO2 densities. Mixtures of CO2 and surfactant are prepared in the stainless steel cell and allowed to flow into the NMR tube by means of the manual compressor. Computational Methods. Ab initio calculations were performed using Gaussian0340 at the second-order Møller-Plesset (MP2)41,42 level of theory to include the effects of electron correlation. Optimizations were carried out at the MP2 level using Dunning’s polarized, correlation consistent aug-cc-pVDZ basis set,43,44 augmented by diffuse functions. Dissociation electronic energies were calculated using the “supermolecule” method45 as the difference between the energy of the complex

and the sum of the isolated energies of the monomers. Basis set superposition errors (BSSE) were applied using the method of Boys and Bernadi.46,47 Single point calculations were performed at the MP2/aug-cc-pVQZ level over the previously optimized structures. Partial atomic charges were calculated by fitting the electrostatic potential using the CHELPG subroutine of Gaussian03. Natural charges were calculated using NBO version 3, as implemented in Gaussian03. NMR calculations were performed using the Gauge-Independent Atomic Orbital (GIAO) as implemented in Gaussian03. Conformational analysis were performed at the MP2/6-31G** level, using Spartan04, Macintosh version.48 Results and Discussion Verification of the High-Pressure Cell. Using the magnetic susceptibility of CO2 given in literature49 (χm ) -21 × 10-6 cm3‚mol-1) and eq 5 the value of -1.98 × 10-6 cm3 g-1 is calculated as the expected slope for the linear relationship between the 13C chemical shift of CO2 and density. Recently, Tsukahara et al.17 have studied the behavior of the 13C chemical shift of CO2 with density and observed a linear dependence with slope -1.93 × 10-6 cm3 g-1 in the density region between 0.6 and 0.9 g.cm-3, which agrees well with the above value expected from a change of only the bulk susceptibility of CO2. We have used this dependency as a test to evaluate the accuracy of the HP-NMR tube. 13C NMR spectra of scCO2 were recorded in the density region between 0.54 and 0.73 g‚cm-3 at 334 K. Figure 2 shows the linear relationship obtained in this work. The experimental slope of -1.88 × 10-6 cm3 g-1 was obtained which is about 5% lower than the expected value. We attribute this small deviation to differences in the shape factor since eq 5, used to calculate the expected slope, assumes an infinite cylindrical shape for the sample. Interaction scCO2-Perfluoropolyether. In order to understand the type of interactions between the surfactant under study and the medium a series of 19F and 13C spectra was measured in the density region between 0.54 and 0.73 g cm-3 at 334 K for different solutions of Krytox in scCO2. The variation of the 13C chemical shift of scCO and of the 19F chemical shifts of 2 the surfactant with density can then be interpreted in terms of solute-solvent interaction. Moreover, since not all of the fluorine atoms in the Krytox polymer chain are chemically

1322 J. Phys. Chem. B, Vol. 111, No. 6, 2007

Temtem et al. TABLE 1: Slopes of the Linear Relationship of Pure scCO2 and scCO2 Solutions of Krytox

Figure 2. 13C chemical shift of CO2/surfactant mixtures as a function of CO2 density at 334 K. ∆, pure CO2; 9, 0.22% (w/w) Krytox; ], 1.13% (w/w) Krytox; [, 1.72% (w/w) Krytox.

equivalent it is possible to evaluate if there is any relation between the fluorine position in the chain and the observed shielding, i.e., if there is any site-specific interaction between solvent and solute. For a solute in scCO2, it is expected that with increasing CO2 density the 13C chemical shift of CO2 will follow the trend of the bulk susceptibility, resulting in a continuous upfield shift. According to our experimental results for pure scCO2, if the bulk susceptibility is the only contribution then, as was mentioned above, a linear relationship with slope -1.88 × 10-6 cm3 g-1 is expected. Deviations to this value or to linearity are an indication that the bulk susceptibility is not the only contribution to the magnetic shielding of the nuclei and may be interpreted in terms of eqs 1 and 3. Analysis of the 13C Chemical Shift. Figure 2 shows the variation of the 13C chemical shift (δc) of CO2 with density for pure CO2 and for three solutions with increasing concentration of Krytox, at 334 K. The solid lines represent the least-squares fit of the data. As it is seen from the Figure there is a linear dependence of the chemical shift with density, with the slopes of the linear plots for scCO2 containing Krytox moving to higher field in parallel with the δc values of neat scCO2. This parallel shift of δc was previously described by Tsukahara et al.,17 who have demonstrated that the comparison of the linear density dependence of δc values of scCO2 and scCO2 containing solute molecules provide a very useful and reliable criterion for evaluating the CO2-philicity of the solutes in scCO2. These authors studied the behavior of δc of CO2 in the presence of different carbonyl compounds and found a correlation between the parallel shifts, the magnitude of the parallel shifts (∆δc) of scCO2 and the structures of the solute molecules, which could be explained in terms of Lewis acid-Lewis base interactions and/or hydrogen bonding between the solute and CO2 molecules, since local CO2-solute interactions influence a large number of CO2 molecules under supercritical conditions. In their study, the authors have related the direction of the observed parallel shift (to lower field) to the generation of an enhanced electron deficiency at the carbon atom of CO2, probably due to O-H‚‚‚O hydrogen bonding, and have shown that the magnitude of the shift, ∆δc, increases with the enhancement of the CO2-solute interactions through Lewis acid-Lewis base (LALB) interactions and/or hydrogen bonding. Remarkably different from the results of Tsukuhara et al. is the direction of the parallel shift observed in this work. As can be seen in Figure 2, for the fluorinated solutes studied, the δc values of CO2 are shifted to higher field, in contrast to the lowerfield parallel shift found in the already cited work. The

solute

σ (cm3‚g-1)

δC’ (at 0.71 g‚cm-3)

pure scCO2 Krytox (0.22%) Krytox (1.13%) Krytox (1.72%)

-1.88 × 10-6 -1.77 × 10-6 -1.61 × 10-6 -1.54 × 10-6

127.140 126.956 126.926 126.889

generation of an enhanced electron density at the carbon atom of CO2, as a consequence of a LA-LB weak interaction between the fluorine atoms of the solutes and the electronically positive carbon atom of the CO2 molecule, is the most plausible explanation for the higher field shift of δc. This type of interaction, where CO2 acts as a weak Lewis acid, is well predicted by molecular modeling studies.25 Nevertheless, our theoretical predictions on chemical shits do not fully agree with our experimental observations. The chemical shift of the carbon atom in a carbon dioxide molecule complexed with a molecule of tetrafluoromethane is predicted to be 0.19 ppm low field, relatively to the carbon atom in a free CO2 molecule. On the other hand, the theoretical results agree with the observations from Tsukuhara et al., predicting a shift of the CO2 carbon atom of 0.17 ppm to higher field, when complexed with a molecule of methane. These results were obtained with MP2/aug-ccpVDZ. Smaller basis sets predict opposite shift for the carbon atom of CO2 complexed with methane and a shift of 0.30 ppm to low field, in the case of CF4 complexes (still opposite to experiment). Probably this means that larger basis sets, or higher-level approximations, are needed to get results in fully agreement with the experimental observations. Nevertheless, at present time this is not feasible. As is shown in Figure 2 and Table 1, when comparing the shifts among solutions of Krytox the magnitude of the parallel shift, ∆δc is markedly higher for the more concentrated Krytox solution. Assuming that ∆δc increases with the enhancement of CO2-solute interactions, this result indicates an enhancement of the total number of LA-LB C-F interactions, meaning that a larger number of CO2 molecules are influenced by these local CO2-Krytox interactions when the concentration of Krytox increases. Data from Table 1 shows that the slopes of the linear plots of δC values against density decrease slightly with increasing Krytox concentration. In Figure 2, it can also be observed that when the concentration of Krytox increases the lines corresponding to the solutions of CO2-Krytox become less parallel to the one corresponding to net CO2, in the lower-density region. This behavior can also be detected in the work of Tsukuhara et al. for the measurements with increasing concentrations of 1,1,1,5,5,5-hexafluoroacetylacetone. We believe that this is probably related with the fact that as the concentration increases the critical density of the solution will increase when compared to net CO2 (Fc of CO2 ) 0.469 g cm-3). Therefore, the deviations of δC observed at the lower density measurements for these mixtures evidence the vicinity of their increasing critical mixture densities. Analysis of the 19F Chemical Shift. As before, the 19F chemical shifts of Krytox were recorded as a function of CO2 density in a range from 0.54 to 0.73 g cm-3 at 334 K. 19F spectral assignments of the polymer resonances were based on literature values.50,51 Figures 3 and 4 show, respectively, the assignment and 19F chemical shift of CO2 + Krytox (1.72% w/w) as a function of CO2 density at 334 K. As the density increases, the general trend in the variation of the chemical shift is an upfield shift of the fluorine resonances, following the trend of the contribution of the bulk susceptibility.

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Figure 3. Experimental high-pressure 19F NMR spectrum and assignment of Krytox in CO2 at T ) 334 K and CO2 density of 0.73 g.cm-3. The perfluorinated solution has a composition of 1.72% w/w of Krytox.

As was mentioned before, the chemical shift can be directly related to the pairwise shielding constant (δ ) -106σ1), which in turn can be split into the contributions from the bulk susceptibility (σb), van der Walls forces (σw), dipole-dipole interactions (σE), and anisotropy effects (σa) - eqs 2 and 3. Since in this study, due to CO2 structure, the contribution from the magnetic anisotropy of the solvent and from dipole-dipole interactions are expected to be very small, the chemical shift can be interpreted in terms of the contribution of the bulk susceptibility and intermolecular interactions by wan der Walls forces between the solute and the CO2 solvent molecules: σ1 ) σb + σw. To determine the contribution of σw a correction was applied to the observed 19F chemical shift to account for the bulk magnetic susceptibility contribution (σb):

δcorr ) δobs -

4π χ × 106 3 v

(6)

For this correction it was assumed that the infinite dilute condition is satisfied, χv (sample) ≈ χv (CO2), and therefore

δcorr ) δobs -

4π χm(CO2) F × 106 3 M

(7)

In Figure 5 the plots of the relative changes of the corrected chemical shifts (∆δcorr) of Krytox against density of CO2 are presented for the different Krytox mixtures. Although the linear bulk susceptibility (σb) contribution is removed an almost linear relationship with a positive slope between the chemical shift and density persists which can be attributed to van der Walls type interactions (σw) between the surfactant and the CO2 molecules. As the density increases the van der Walls contribution is more evident. The plots comparison of Figure 5a-c, shows three different groups of linear trends that can be associated to the type of fluorine unit in the polymer chain, thus suggesting a site-specific interaction with CO2. The three different groups of slopes become more clear in the mixture with the higher Krytox composition (Figure 5c). The first one, which includes peaks e-g, evidence a larger variation with 19F

Figure 4. Absolute 19F chemical shift of Krytox in scCO2 (1.72% w/w) versus CO2 density. Peaks d-i are specified in Figure 3.

density. The second, with an intermediate variation incorporates peaks i and d. And finally the third, which include peaks a-c, with the smallest relative variation. Due to the lower signalto-noise ratio in the 19F NMR spectra for the 0.22 and 1.13%

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Temtem et al.

Figure 6. Structures of CO2 complexes with methane (A) and tetrafluoromethane (B) with contact distances in Å.

Figure 5. Relative change in corrected 19F chemical shift (∆δcorr ) δcorr - δinit corr) of Krytox dissolved in scCO2 vs CO2 density for different mixtures. (a) 0.22% w/w; (b) 1.13% w/w; (c) 1.72% w/w.

w/w mixtures the chemical shift dependence of peak h could not be determined accurately for these concentrations. However, this was possible for the 1.72% w/w mixture and the data obtained (Figure 5c) shows that this fluorine atom experiences a relative high variation with density, thus belonging to the first group. The assignment of the Krytox 19F spectrum (Figure 3) allow us to link the first group, which has the larger slope, to the terminal CF3 and CF2 (CF3CF2CF2O-) peaks e and g, respectively, from one end of the polymer chain and to the carboxylic R-fluorine and R-CF3 (-OCF(CF3)COOH) peaks h and f, respectively, from the opposite end of the chain. All these, due to their position in the polymer chain are in principle the most CO2 exposed fluorine atoms. The intermediate variation corresponds to the resonances of the CF3, CF2, and CF groups of the inner polymer chain (-OCF2CF(CF3)O-) peaks d and i. In the third group, associated with the smaller density dependence we find peaks a-c, these where not assigned in Figure 3, but due to their chemical shift, we believe they can be assigned to CF3 and CF2 groups of the larger polymer chains

which are the less CO2 accessible fluorines and therefore experience the smaller shift variation. As was said before, the most obvious explanation for the observed site-specificity of the F...CO2 interaction seems to be the difference in the accessibility of CO2 to the different fluorine groups that compose the chain of the polymer. However, differences in the electronic density of the fluorine atoms cannot be excluded. As stated before, the trend observed is also in agreement with our theoretical predictions for the calculated chemical shifts of the fluorine atoms in a complex of a CO2 and CF4. The coordinated fluorine atoms are predicted to be 0.52 ppm shifted to lower field, indicating an electronic donation to the carbon atom of the CO2 moiety. The fluorine atom in anti position relatively to the CO2 molecule is 0.26 ppm shifted to lower field, while the fluorine atom near the oxygen atom of the CO2 molecule remains in the same position (0.02 ppm to higher field). In average, the fluorine signal is predicted to be 0.32 ppm shifted to lower field. Molecular Modeling. Hopping to add some new insights to this discussion, we have undertaken a molecular modeling study, based on the recent work of Wallen,24 where it was made evident the need of correlated methods and large basis sets for the proper description of the complexation energies of carbon dioxide with methane or fluorinated derivatives. The conclusions from these studies were also extrapolated to larger systems but no computational verification has been made until now. According to Wallen’s, the complex configurations of CO2 with CH4 (A) and CF4 (B) are the ones shown in Figure 6. When complexing with methane, carbon dioxide behaves as a Lewis base, with each one of its oxygen atoms establishing a hydrogen bond with one of the hydrogen atoms in the methane molecule. On the other hand, with tetrafluoromethane CO2 behaves as a Lewis acid, establishing two simultaneous connections with a pair of fluorine atoms, these ones acting as Lewis bases. Other fluorinated methane derivatives form similar complexes with CO2. In these cases the carbon atom of CO2 binds to the fluorine atom(s) while one of the oxygen atoms binds to one of the remaining hydrogen atoms in the methane derivative. The calculation of the dissociation energies of all CO2‚CHnF4-n (0 < n < 4) complexes allowed for a first rationalization of the relative solubilities of methane and its fluorinated derivatives in CO2. One of the main conclusions was that CF4 and CH4 should have similar solubilities (similar dissociation energies of their CO2 complexes) and that the maximum solubility should be reached for CH2F2. In this study we first compared the relative dissociation energies of several complex configurations of CO2 with npentane or perfluoro-n-pentane (Figure 7), calculated with the same theoretical model as used by Wallen. As it can be seen, while for the fluorinated structure (A) the complex configuration is similar to the one of CO2 with CF4, for n-pentane the lower energy configuration (D) (Dec ) 1.46 kcal/mol) is the one were the CO2 molecule lays side by side with the n-pentane structure. This configuration allows for a contact of the oxygen atoms with several hydrogen atoms of the pentane moiety, maximizing the interaction. A similar configuration for the perfluoro derivative is not expected, since in this situation the contact

Molecular Interactions and CO2-Philicity

J. Phys. Chem. B, Vol. 111, No. 6, 2007 1325

Figure 9. CHELPG partial atomic charges for a simplified Krytox monomer. Natural partial charges are given in brackets.

Figure 7. Structures of CO2 complexes with perfluoropentane (A) and pentane (B-D). Contact distances in Å and dissociation energies in kcal/mol. To simplify the picture, in symmetrical structures B and D, the contacts to one of the oxygen atoms of CO2 were omitted.

Figure 8. Rotational profiles for n-butane (b) and pefluoro-n-butane (O). Conformational energies in kcal/mol and angles in degrees.

should be established with the carbon atom of the CO2 molecule, which is only efficient in a configuration similar to A. The comparison of the energetic values of complexes A and D (Figure 7) suggests a higher solubility of n-pentane relatively to the perfluorinated derivative. Nevertheless, while Wallen compared enthalpic values, we are comparing only electronic ones, since we were not able to perform a frequency calculation, due to the extremely expensive theoretical model in use. This means that no thermodynamic correction could be included, even thinking that entropic factors can be of major importance, as already suggested by Wallen (he also did not include them). Comparing complexes A and D (Figure 7), the importance of entropic factors resulting from conformational molecular changes can be more evident. While in complex A the CO2 moiety should not difficult the rotation around any of the C-C bonds, in structure D the complex formation would lock the rotation around two (or even four) C-C bonds, which would be translated in a negative entropic variation. The other n-pentane complexes show similar restrictions. On the other hand, the comparison of the energetic rotational profiles for alkanes and perfluroalkanes, as exemplified in Figure 8 for n-butane and perfluoro-n-butane, suggests that alkanes have more conformational freedom. If so, any complexation that restricts the

conformational motion in alkanes would result in a greater entropic variation than a similar restriction in perfluorinated derivatives. A similar interpretation can be made for a recent result obtained by Eastoe and Steyler, indicating that branched alkanes are considerable more soluble in scCO2 than linear ones.52 Since the interaction between CO2 and terminal methyl groups shall contribute less to a negative entropic change than a similar interaction with methylene groups in a long alkylic chain, branched alkanes should interact more effectively with scCO2, rendering them more soluble. The charts obtained for the chemical shift variations of different fluorine atoms in Krytox, can be a result of several factors. The most evident one is due to different steric effects at different points of the molecular chain. A second factor can be attributed to a pure statistical phenomenon. While a difluoromethylene group has only one possible orientation to interact with CO2 a trifluoromethyl group has three equivalent orientations, making it more efficient in the step of complex formation. Since the complexes are very weak, this statistical contribution can be of major importance. Fluoromethyne groups can have lower interaction with scCO2 due to their inability to establish an optimal connection, as shown by Wallen for the case of fluoromethane. This is due to the preference of CO2 to form a bridge between two geminal fluorine atoms, not possible in CH3F or fluorine atoms in tertiary carbons. A last factor contributing to the fluorine differentiation can be due to different electronic densities in different fluorine atoms. Wallen suggested that a good correlation could be established between the CHELPG partial atomic charges on the fluorine atoms in CF4 and derivatives with their ability to bind to the CO2 carbon atom. We thereby calculated, using the same theoretical model, the CHELPG charges on a simplified Krytox monomer and their averaged values are shown in Figure 9, together with the averaged natural partial charge values. From these results we can suggest that electrostatic factors can also contribute to the differentiation of the fluorine interactions with scCO2, but a clear tendency is not evident. Conclusions Our results show the suitability of the HP-NMR technique to investigate the solubility of a high molecular weight perfluorinated surfactant in scCO2 by combining CO2 13C chemical shift and Krytox 19F chemical shift variations with medium density. For this purpose a new high-pressure mobile and versatile apparatus was assembled with the capability to change in situ the sample composition and all the relevant operational conditions (p, F, T) for the NMR measurements. The nature of CO2-Krytox interaction was assessed both by comparing the CO2 δC variation of neat CO2 with that of mixtures with increasing surfactant composition and by the analysis of Krytox 19F corrected chemical shifts in terms of medium magnetic susceptibility. Evidence for a weak LA-LB interaction between the Krytox fluorine atoms and the electroni-

1326 J. Phys. Chem. B, Vol. 111, No. 6, 2007 cally positive carbon atom of CO2 was found. The Lewis acid character of CO2 carbon atom is translated in the observed parallel shift toward higher field of the δC values as a function of density when compared with neat CO2 The relative magnitude of the LA-LB interaction between CO2 and specific fluorine atoms of the surfactant chain was indicated by the observed different dependences of ∆δF with density. ∆δF values can be used as a criterion to evaluate the strength of the fluorine Lewis base interaction with CO2, higher ∆δF values meaning stronger interactions. Molecular modeling studies of the interaction of a simplified perfluorinated molecule with CO2 support the LA-LB nature and the site-specificity of the F...CO2 interaction. Our overall results allowed us to propose a theoretical model to describe the interaction between a perfluorinated molecule and CO2 which accounts for the higher solubility of these molecules in scCO2 when compared to the nonfluorinated analogues. According to this model, the higher solubility of perfluorinated molecules might be related to a fundamental difference found in the nature of their interaction with CO2 when compared with the analogue nonfluorinated ones. While in perfluorinated molecules the carbon atom of CO2 acts as a Lewis acid and the fluorines as Lewis bases, in nonfluorinated molecules the oxygen atoms of CO2 act as a Lewis base and the protons of the hydrocarbon chain act as acid. These differences lead to the formation of different CO2-solute complexes with higher restrictions in C-C bond rotation for the nonfluorinated CO2 solvated molecules; thus, a larger negative entropic variation is expected when compared to the perfluorinated analogues where these restrictions are not predicted. The work in progress will extend these studies to other perfluorinated surfactant molecules with different structure and CO2 solubility in order to clarify the relation between the surfactant structure and entropic contributions for the solubility. Acknowledgment. The authors thank financial support from Fundac¸ a˜o para a Cieˆncia e Tecnologia (FCT) through the contract POCTI/42313/QUI/2001 and the grant SFRH/BD/ 16908/2004 and by FEDER. We thank M.R. Caras Altas for the technical support. Supporting Information Available: Figure showing a schematic diagram of the HP-NMR tube. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Cooper, A. I. J. Mater. Chem. 2000, 10, 207. (2) Kazarian, S. G. Polym. Sci. Ser C 2000, 42, 78. (3) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. ReV. 1999, 99, 543. (4) DeSimone, J. M.; Maury, E. E.; Menceloglu, Y. Z.; McClain, J. B.; Romack, T. J.; Combes, J. R. Science 1994, 265, 356. (5) Canelas, D. A.; DeSimone, J. M. Macromolecules 1997, 30, 5673. (6) Hsiao, Y. L.; Maury, E. E.; Desimone, J. M.; Mawson, S.; Johnston, K. P. Macromolecules 1995, 28, 8159. (7) Hems, W. P.; Yong, T. M.; Van Nunen, J. L. M.; Cooper, A. I.; Holmes, A. B.; Griffin, D. A. J. Mater. Chem. 1999, 9, 1403. (8) Kendall, J. L.; Canelas, D. A.; Young, J. L.; DeSimone, J. M. Chem. ReV. 1999, 99, 543. (9) Lepilleur, C.; Beckman, E. J. Macromolecules 1997, 30, 745. (10) Giles, M. R.; Griffiths, R. M. T; Aguiar-Ricardo, A.; Silva, M. M. C. G.; Howdle, S. M. Macromolecules 2001, 34, 20. (11) Christian, P.; and Howdle, S. M. Macromolecules 2000, 33, 237. (12) Christian, P.; Giles, M. R.; Griffiths, R. M. T; Irvine, D. J.; Major R. C., Howdle, S. M. Macromolecules 2000, 33, 9222. (13) Casimiro, T; Banet-Osuna, A. M.; Ramos, A. M.; Nunes da Ponte, M.; Aguiar-Ricardo, A. Eur. Polym. J. 2005, 41, 1947.

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