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The Nature of the Binary Solvent N-Methylpyrrolidone/ Carbon Disulfide Gary Dyrkacz Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439 Received January 3, 2001. Revised Manuscript Received April 11, 2001

The solvent properties of the binary solvent carbon disulfide/N-methylpyrrolidone (NMP) have been investigated using density, viscosity, solvatochromic, and FTIR measurements. The excess molar volumes were negative throughout the entire range of CS2/NMP ratios suggesting complex formation or preferred solvent structures at intermediate ratios of the solvents. The viscosity measurements suggested either complex formation between CS2 and NMP or very complex selfassociation equilibria. FTIR data did not support the idea of specific interactions between CS2 and NMP, but did show that complex changes in self-association were occurring. The solvatochromic studies indicated that at approximately a 1:1 volume ratio, the dipolar state of the binary solvents was greater than for either solvent separately. All the information together suggested that some type of NMP chain oligomer may be an important factor in understanding the synergistic extraction of coal in CS2/NMP solvent.

Introduction Recent interest in coal extraction has been directed to the use of binary1,2 or tertiary solvents.3-5 One of the best binary combinations is N-methylpyrrolidone (NMP) and carbon disulfide in a 1:1 volume ratio. For a variety of coals, this mixture provides the highest extractability. Despite the steady stream of investigations using this solvent system, there is little known about why this combination is particularly effective in extraction. The present work examines the properties of CS2/NMP (CSNP) solutions to see if answers can be found in the solvent itself. The issues in dissolving coals in binary solvents can be divided into two broad questions. (1) Is there any special interaction between solvent components that can lead to a synergistic effect on extraction? (2) How does the solvent system interact with the coal macromolecules to promote dissolution? The latter problem of coal-solution interactions is particularly difficult to address. First, just what is the structure of a coal macromolecule? Since there are most likely a myriad of such species, how shall we decide on an average structure to represent the unit in coal and in solution? Moreover, the types of macromolecules that could act to help “glue” the coal solid together do not have to look like the average macromolecule at all. Nor do they even have to represent a large fraction of the (1) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639-1647. (2) Dyrkacz, G. R.; Bloomquist, C. A. A. Binary solvent extraction of Upper Freeport Coal. To be published. (3) Ishizuka, T.; Takanohashi, T.; Ito, O.; Iino, M. Fuel 1993, 72, 579-580. (4) Liu, H.-T.; Ishizuka, T.; Takanohashi, T.; Iino, M. Energy Fuels 1993, 7, 1108-1111. (5) Dyrkacz, G. R.; Bloomquist, C. A. A. Energy Fuels 2000, 14, 513514.

types of coal macromolecules. In addition, dissolution involves an intimate knowledge of the solution structure, the physical arrangements and interactions of the macromolecules in the coal solid, and how the coal macromolecules, both in the solid and as dissolved species, interact with the solvent. The first broad question, that of the impact of the nature of the binary solution itself on coal extraction, is also complex. The long and continuing efforts to investigate and model solution structures are a testimony to the complexity.6 However, at least the solvent components have a well-known structure and the components are well characterized chemically. Thus, this aspect of the coal-solvent problem is inherently easier to answer, and can be attacked with a variety of methods. This is the avenue that this paper begins to examine. The often clearly superior extraction solvent, CSNP, makes it an attractive target for investigation. There may be better binary solvent combinations for some coals,1,2 but since CSNP has received the bulk of attention, this initial work focuses on its behavior. Binary solvents for these investigations are both a curse and a blessing. A disadvantage is the potential complication that solvent structure in the reaction field around the coal macromolecules will not necessarily mirror the bulk solution composition. The advantage of twocomponent solvents is that the extra degree of freedom permits us to control and observe some of the subtle coal-solvent interactions without resorting to changing the entire solvent by addition of a component with quite different molecular and solution structure. The nature of the non bonding interactions that assist in holding coal macromolecules together defines some (6) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Verlagsgesellschaft mbH: Weinheim, 1988.

10.1021/ef010003c CCC: $20.00 © 2001 American Chemical Society Published on Web 05/25/2001

Binary Solvent N-Methylpyrrolidone/Carbon Disulfide

of the limits on solvent choice in coal extraction. Solvents capable of breaking hydrogen bonds are critical. Thus, solvents or compounds with high base strength or donor number are very important, and this issue has been extensively addressed.7-10 However, hydrogen bonding/basicity arguments alone cannot explain why neat NMP is less effective than neat pyridine in extractions of some coals.1,2 NMP has a higher base strength.11 So both compounds should be nearly equally effective at breaking the hydrogen bonds which hold coal molecules together. Even their solubility parameters are similar. Of course, the shape of the molecules is different. NMP is a somewhat bulkier molecule. This may reduce the rate of diffusion into coal pores and be a partial reason for extraction differences.12,13 When either pyridine or NMP is mixed with CS2, NMP is generally more effective than pyridine as cosolvent, and in some cases much better.14 For APCS 1 coal, pyridine and CS2/pyridine produced the same yield of extract, which was lower than the CSNP yield.2 Why this reversal occurs compared to the pure solvent extractions is not clear. From only the perspective of similar basicity, enhancement in extractability is not expected in either the pure solvent or binary solvent.9,15 If the molecular size argument invoked above was a major controlling factor, it is not clear why CS2, which compared to NMP is not good swelling solvent for coal, should make a difference.1 There are other possibilities of course. That hydrogen bonds are not the only non covalent interactions holding coal molecules together is becoming more clear. van der Waals forces, charge transfer, π-π, and physical entanglements all must be involved in holding the coal macromolecules together.9,15-17 What is unclear is the relative involvement of each of these interactions in coal structure or coal dissolution. But even so, these type of interactions will be mediated by how the solvent components interact with each other. An equally interesting question is what is so special about carbon disulfide, and why does a 1:1 vol ratio of CS2 with either pyridine or NMP often provide the highest extract yield? Iino et al.1 and Dyrkacz and Bloomquist2 have reported that other cosolvents do not enhance extraction as effectively as CS2. The list includes fairly small molecular volume solvents that are also quite polar, such as acetonitrile or nitromethane. Iino et al. found that both the rate of extraction and the yield of extract were higher for both pyridine and NMP in the mixed solvent.1 The reasons for this behavior were attributed to several factors. One was the lower viscosity of the binary solvent which could pro(7) Marzec, A.; Juzwa, M.; Betlej, K.; Sobkowiak, M. Fuel Process. Technol. 1979, 2, 35-44. (8) Larsen, J. W.; Green, T. K.; Kovac, J. J. Org. Chem. 1985, 50, 4729-4735. (9) Larsen, J. W.; Mohammadi, M. Energy Fuels 1990, 4, 107-110. (10) Suuberg, E. M.; Otake, Y.; Langner, M. J.; Leung, K. T.; Milosavljevic, I. Energy Fuels 1994, 8, 1247-1262. (11) Arnett, E. M.; Mitchell, E. J.; Murty, T. S. S. R. J. Am. Chem. Soc. 1974, 96, 3875-3890. (12) Aida, T.; Fuku, K.; Fujii, M. Energy Fuels 1991, 5, 79-85. (13) Ndaji, F. E.; Thomas, K. M. Fuel 1995, 74, 842-845. (14) Takanohashi, T.; Iino, M. Energy Fuels 1990, 4, 333-335. (15) Nishioka, M.; Larsen, J. W. Energy Fuels 1990, 4, 100-106. (16) Larsen, J. W.; Gurevich, I.; Glass, A. S.; Stevenson, D. S. Energy Fuels 1996, 10, 1269-1272. (17) Quinga, E. M. Y.; Larsen, J. W. Energy Fuels 1987, 1, 300304.

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mote diffusion of the macromolecules into solution, and thereby reduce the residence time of the soluble macromolecules in the coal matrix. A second factor was the increase in coal swelling, which was found to follow the same type of curve as the extract yield when the ratio of NMP/CS2 ratio was varied. Painter and co-workers have suggested that the reason for extraction enhancement in mixed solvents is due to a balance of breaking the dipole self-associations of NMP, and a high enough NMP concentration to still interact strongly with the coal macromolecules.18 This led to the conclusion that other solvents should also be effective. Using several other solvents, but increasing the amount of solvent exposure time, did lead to increases in the extract yield. Since the viscosity of a solution depends on the nature and strength of intermolecular forces between molecules, which can lead to the formation of higher molecular weight associates, the views of Painter and co-workers and Iino and co-workers are in concert. Although not directly suggested as an extraction mechanism, Aida et al. in discussing swelling in binary solvents suggested that a cosolvent “wedge effect” may explain some of the swelling behavior of coals in binary solvents. In this case, a small cosolvent molecule may penetrate more rapidly into the structure and overcome some of the coal-coal interactions, allowing the larger solvent partner to enter and break the stronger coalcoal interactions.19 The high polarizability and small size of CS2 would be expected to favor such a mechanism and should also assist in coal extraction. However, there still is the problem of explaining why other small cosolvents do not provide a similar increase in swelling or extraction. There can be other reasons why a binary solvent may exhibit enhanced extraction and maxima or minima in solvationsfor instance, if a special complex is formed between solvent components, and this complex then interacts more strongly with the coal macromolecules than either pure solvent. Or if the polarity of the solvent goes through a maximum or minimum, this could enhance or reduce the solvation of the coal macromolecules. That the solvent polarity can be greater or less than either of the pure solvents is known.20-23 Clearly, the nature of the solvent in extraction and understanding coal structure is important not only from the perspective of its single isolated molecule properties, but also from the perspective of its aggregate solution structure. The approach used here is to explore the nature of the CSNP binary solvent using a variety of different techniques. No one type of measurement unambiguously provided sufficient answers to the questions posed in the previous discussions. Indeed, in some cases, a wrong (18) Painter, P. C.; Sobkowiak, M.; Gamble, V. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem., Aug 22-27, 1998, 43 (4), 913-916. (19) Aida, T.; Suzuki, E.; Yammanishi, I.; Sakai, M. Prepr. Pap.s Am. Chem. Soc., Div. Fuel Chem. Aug 22-26 1999, 44 (3), 623-628. (20) Langhals, H. Nouv. J. Chim. 1981, 511-514. (21) Langhals, H. In Similarity Models in Organic Chemistry, Biochemistry and Related Fields; Zalewski, R. I., Krygowski, T. M., Shorter, J., Eds.; Studies in Organic Chemistry, 42, Elsevier: New York, 1991; Chapter 6. (22) Maksimovic´, Z. B.; Reichardt, C.; Spiric´, A. Z. Anal. Chem. 1974, 270, 100-104. (23) Fayed, T.; Etaiw, S. E. H. Spectrochim. Acta 1998, 54A, 19091918.

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conclusion might be garnered from a single type of experiment. As will become apparent, even with a variety of different experiments, the picture of the solvent dynamics in CSNP cannot be declared as in sharp focus. Nevertheless, the data may begin to resolve some of the issues that need to be addressed in understanding coal extraction in CSNP. Experimental Section Viscosity. A falling ball viscometer (Gilmont) was used for the viscosity measurements. The viscometer was placed vertically in a temperature bath maintained at 25.00 ( 0.05 °C. The standard error in the time measurements was 2.3%. For calibration, the known viscosities of NMP and CS2 were used.24 However, the calibrations constants were not the same for each solvent. This is because of their very different viscosities. To compensate for this problem, a linear relationship between NMP molarity and the viscometer constant was assumed with the pure NMP and CS2 as endpoints. Although this affects the absolute numbers, the relative changes will remain the same. Density. All densities were measured with a Paar DMA 45 vibrating tube densitometer, equipped with an external microcell. Temperature of the cell was maintained at 25.0 ( 0.05 °C. Solutions were injected into the microcell and readings were taken when the density had stabilized. All density measurements were repeated at least twice on freshly prepared solutions. FTIR. A Nicolet 510P spectrometer was used for the FTIR measurements. This instrument did not have a far-IR detector, but sufficient energy and detector sensitivity were available in the 450 cm-1 to 350 cm-1 region to obtain good spectra without an excessive number of scans; 800 scans at 2 cm-1 resolution were sufficient. Two liquid cells were used; nominally 0.010 mm and 0.050 mm path length AgCl microcells (GC-1; RIIK). The actual path lengths found from the interference fringes were 0.0159 mm and 0.0468 mm, respectively. All spectra were obtained at room temperature of ∼22 °C. The spectra shown here were all corrected using linear baselines. For display purposes, the broad frequency range spectra also have been corrected for the interference fringes using many short linear baseline segments. Band modeling was done using the PeakFit software (SPSS). Lorentzian-Gaussian bands were used to fit the bands. The Pearson 7 Limit Minimization, Σ(ln(1+ABS(residual))∧2)), method was used as the fitting minimization function. In the ν2 region of CS2, two bands at ∼402 cm-1 and ∼392 cm-1 were used to fit the spectra with all parameters allowed to vary. For the carbonyl band, the starting positions of the bands making up the carbonyl band region between 1620 and 1780 cm-1 were chosen on the basis of the second derivative spectra. Fitting was done with and without baseline corrections for the interference fringes. The results were not significantly different.

Results As will become clear, self-association is an important mechanism in understanding the CS2/NMP solvent system. However, there is surprising little information on the possible structures of NMP oligomers in nonpolar solvents. Indeed, there is little experimental or theoretical information on the possible structures of any tertiary amide self-association species in solution.25 (24) Daubert, T. E.; Danner, R. P.; Sibul, H. M.; Stebbins, C. C. Physical and Chemical Properties of Pure Chemicals; Taylor and Francis: Washington, DC, 1997. (25) Bushuev, Y. G.; Zaichikov, A. M. Russ. Chem. Bull. 1998, 47, 17-24.

Figure 1. Representative structures for cyclic and chain O- - -C and O- - -N associated NMP dimers.

There are several possible dipole-dipole associated oligomers possible, even for the simple case of an NMP dimer. These can be divided into cyclic and chain structures. Within each structure group there are several possible association configurations for the two NMP molecules. Even within the cyclic dimer case there are several possible configurations. Two configurations that have been considered are the following. (1) The oxygen atoms of each NMP interact with the corresponding carbonyl carbon atom of the sister molecule (Figure 1A) in a four center interaction. There are two further conformations for this configuration. All the carbonyl atoms may be in roughly the same plane as the ring, but staggered, or they may be placed one ring above the other. These configuration interactions were initially proposed for acetone dimers, but calculations showed that a C-H- -O type interaction is favored, especially as a six-member symmetrical ring structure.26,27 A more complicated case of a C-H- -O type of interaction has been reported for NMP in the solid phase. The crystal structure and ab initio calculations suggested the involvement of the methyl hydrogens between NMP molecules.28 Of course, crystal packing effects can have a profound effect on molecular arrangement and do not necessarily represent the case in solution. (2) The oxygen of one NMP molecule may interact with the nitrogen of the sister molecule in a configuration that resembles a six-membered chair configuration (Figure 1B). Such an oxygen-nitrogen mutual interaction through a six-member ring structure has been proposed for the DMF dimer.29-31 However, unlike DMF, models of both NMP cyclic configurations (26) Turi, L. Chem. Phys. Lett. 1997, 275, 35-39. (27) Frurip, D. J.; Curtiss, L. A.; Blander, M. J. Phys. Chem. 1978, 82, 2555-2561. (28) Mu¨ller, G.; Lutz, M.; Harder, S. Acta Crystallogr. 1996, B52, 1014-1022. (29) Yonezawa, T.; Morishima, I. Bull. Chem. Soc. Jpn. 1966, 39, 2346-2351


suggest that there will be steric interactions between the methyl groups and the methylene hydrogens on the carbon adjacent to the carbonyl group. Just what effect this would have on the structures is unclear. In the case of chain dimers, there would be similar configuration cases except association would be through single headto-tail dipole interactions between the NMP molecules. Figure 1, structures C and D, suggest these possibilities. Despite the lack of knowledge of the structure of NMP associates, there is little doubt they exist. As will be discussed further on, this is clear not only from the data reported here, but from other studies as well. A. Physical Properties Measurements. Nonideal changes in such physical properties such as density or viscosity can indicate how the components of a solution interact. Although such information can rarely establish the chemical nature of the changes, it does provide insight into what solvation mechanisms are most likely. We will treat each case separately. Density. The density of a series of CSNP solutions as a function mole fraction was determined at 25.0 °C. A common way of analyzing such data is in terms of the excess thermodynamic molar volume.32 It is based on the relationship that the molar volume of a solution composed of ideal solvents should simply be the sum of the respective molar volumes. The excess volume is a measure of the deviation from this ideal case. Many factors such as molecular packing, which is related to size and shape of the solvent components, and specific and non specific electronic interactions between like and unlike molecules contribute negative or positive deviations to the excess molar volumes.32-35 The values can be calculated from density data according to the following relationship:36

VE ) [xMNMP + (1 - x)MCS2]/F - xMNMP/FNMP (1 - x)MCS2/FCS2 (1) where, VE is the excess molar volume, M is molecular weight, F is density, and x is the NMP mole fraction. The excess volume data is plotted against mole fraction of NMP in Figure 2. Although there is scatter in the data, it is clear that the excess volume is negative throughout the entire concentration range. There are several interpretations of the sign of the excess volume. In binary solutions, where one component is an alkanol and the other is a fairly large n-alkane solvent, the oligomers occupy a smaller volume than separate monomers because the hydrogen bonds holding the oligomers together are strong and directed. The same is true for carboxylic acids.37 Therefore VE is (30) Rabinovitz, M.; Pines, A. J. Am. Chem. Soc. 1969, 91, 15851589. (31) Woodbrey, J. C.; Rogers, M. T. J. Am. Chem. Soc. 1962, 84, 13-16. (32) Prigogine, I. The Molecular Theory of Solutions; Interscience: New York, 1957. (33) Patterson, D. Pure Appl. Chem. 1976, 47, 305-314. (34) Hill, R. J.; O’Kane, E.; Swinton, F. L. J. Chem. Thermodyn. 1978, 10, 1153-1159. (35) Treszczanowicz, A. J.; Kiyohara, O.; Benson, G. C. J. Chem. Thermodyn. 1981, 13, 253-260. (36) Zhao, Y.; Wang, J.; Xuan, X.; Lu, J. J. Chem. Eng. Data 2000, 45, 440-444. (37) Lark, B. S.; Banipal, T. S. Thermochim. Acta 1998, 91, 141149.


Figure 2. Variation in excess molar volume for CS2/NMP solutions as a function of the mole fraction of NMP in CS2. T ) 25 °C.

positive when these hydrogen bonders are diluted with fairly large n-alkanes. However, when the components differ in their physical makeup, or small inert diluents are used, more complex behavior in VE is often observed.35,38 For example, cyclohexanone or cylcopentanone have positive VE throughout the entire concentration range in long chain alkane solvents.39 However, low carbon number alkane solvents, such as hexane, either show negative VE or change sign as the mole fraction changes. This is due to different packing arrangements in the solvent or solvent reorganization. Another good reason for a negative excess volume is the formation of solution associates or complexes. Just as in the case of cyclohexanone or cyclopentanone, in NMP there will be dipolar interactions. These interactions will compete with thermal motion of the molecules and the oligomers therefore will occupy a reduced volume. Thus, for the binary solvent NMP/ cyclohexane, VE is positive throughout the entire concentration range.40 The predominant mechanism in this case is the breaking of dipolar interactions by dilution with cyclohexane. The similarity of the two solvents, means that the shape and packing factors are about the same. For CS2 and NMP, the negative excess molar volumes throughout the entire concentration range suggests that dilution of NMP results in a more compact solution structure. From the previous discussion, this information suggests three cases: (1) An association complex is formed between CS2 and NMP; (2) The CS2 molecule can easily fit within the existing cavities within the NMP solvent structure so that the effective molar volume of CS2 is accommodated without a linear expansion of the solution volume; and (3) NMP associated structures change to a more compact structure with dilution by CS2. Case 3 would appear to be the least likely. As described above, associated species in inert solvents show positive or sigmoidal negative to positive curves in excess volume. NMP in cyclohexane shows positive excess volumes. Thus, it is unlikely that a more compact NMP associated structure forms with dilution. Distinguishing between Case 2 and Case 1 is not possible. Part of the problem is what to expect for the CS2 excess volume contribution. With n-alkanes, cyclo(38) Benson, G. C.; Kiyohara, O. J. Chem. Eng. Data 1976, 21, 362365. (39) Mahl, B. S.; Kaur, H. Thermochim. Acta 1987, 112, 351-364. (40) Lichter, T. M.; Domanska, U.; Mwenesongole, E. Fluid Phase Equilib. 1998, 149, 323-337.

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Figure 3. Variation in viscosity for CS2/NMP solutions as a function of mole fraction of NMP. T ) 25 °C.

hexane, alkylbenzenes, dioxane, and acetone, the excess volumes with CS2 are all positive.34,41,42 This is consistent with solution volume changes being dominated by the breaking of some level of ordering in these cosolvents. In light of this information, the negative excess volumes found for NMP were rather unexpected. It is not possible then to satisfactorily distinguish between complex formation and more efficient molecular packing with only these results. The minimum in the excess volume data for the CSNP system is found at a mole fraction of ∼0.46. In cyclohexane, the maximum excess volume is found at a mole fraction of 0.36.40 The difference between the minima must be due to some change in the solvent The CSNP mole fraction value corresponds to a volume fraction of NMP of ∼0.55. It is tempting to relate this minimum to some unusual solvent change that may also be important for coal extraction synergy. However, the complex molecular interactions that may lead to a negative excess molar volume suggest that, standing alone, the data is best interpreted only in a qualitative manner. Viscosity Measurements. Viscosity measurements can also provide information on the interaction of solvent components. Figure 3 shows how the viscosity changes as a function of the mole fraction of NMP. If the two solvents did not interact, nor was there any concentration dependent self-association as in a Newtonion fluid, the curve would be linear. This is not the case. The curve is sigmoidal with a sharp change in viscosity at roughly a mole ratio of 0.45. This is a volume fraction of 0.57. Clearly, there is a large viscosity change at a volume fraction close to that of maximum extraction efficiency. Again, as with the density measurements, a system of ideal solvents would produce a linear plot of viscosity as a function of mole fraction. According to Liler and Kasanovicˇ, a concave, negative viscosity plot is indicative of a system with at least one associating component.43 As the self-associating component is diluted, the size of the oligomers decreases and the interactions of the self-associating component are reduced, which reduces the viscosity. However, a concave curve does not rule out the existence of very weak solution complexes. Sigmoidal viscosity plots are usually ascribed to a solvent system where a complex is formed, and one of the components can self-associate.44,45 If a strong com(41) Adya, A. K.; Mahl, B. S.; Singh, P. P. Thermochim. Acta 1974, 9, 318-322. (42) Tomaszkiewicz, I.; Ter-Minassian, L. J. Phys. Chem. 1988, 92, 6824-6827. (43) Liler, M.; Kasanovicˇ, D. J. In Hydrogen Bonding; Hadzˇ i, D., Ed.; Pergamon: New York, 1959; pp 529-543.

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plex is formed in sufficient amounts, then a maximum may even be observed in the viscosity curves. Thus, the sigmoidal plot for CS2 and NMP suggests weak complex formation between the two solvent components. However, other mechanisms that could cause this behavior can be envisioned. If the character of the NMP associates changed dramatically in form or electronic structure, then it is feasible that a sigmoidal viscosity curve could result. Such changes are observed in micelle systems where transitions from one type of micelle to another occur or some change in the ionic structure of the micelle takes place.46-48 The rather abrupt shift in the excess viscosity at ∼0.5 mole fraction in Figure 3 suggests that probably just above this concentration there is a major reorganization of the solvent structure. Because this change is near the same point of maximum extraction efficiency, there may be a strong relationship between this change and the extraction mechanism. Solvatochromic Changes. Solvatochromism has been very useful in probing solution structure and obtaining information on solvent polarity, polarizability, and basicity.49,50 Typically, some absorbance or emission property of a dye probe molecule, especially the shift of the spectral band, is generally measured. A variety of probe molecules are available.51 Solvatochromic changes in binary solutions can be more complicated to interpret.21 Solvatochromism is keenly dependent on the polar characteristics of the molecules in the reaction field. However, the concentration of each species in the reaction field does not always mirror the bulk solution, and depends on the nature of the solvents and the probe molecule itself. One commonly used probe is Reichardt’s dye, which is a substituted betaine [2,6-diphenyl-4-(2,4,6-triphenyl1-pyridinio)phenoxide]. This dye undergoes a large hypsochromic shift with increasing polarity and polarizability of the solvent. However several problems were encountered with this dye: it is not very stable in CSNP; within minutes, the color changed; and second, it was only sparingly soluble at high CS2 concentrations. Another dye, Nile Red (9-diethylamino-5H-benzo[a][phenoxazin-5-one] proved to be completely stable in CSNP solutions and soluble for all ratios of CS2 and NMP.52,53 This dye undergoes a bathochromic shift with increasing solvent polarity and polarizability. The solvatochromic changes for Nile Red are displayed in Figure 4. ENR is the transition energy derived from the shift of the adsorption band at ∼547 nm. A limited number of measurements was also done in cyclohexane/ NMP and are also displayed in Figure 4. The peak shifts (i.e., transition energies) observed for all three pure solvents are consistent with published (44) Kurnakow, N. S. Z. Anorg. Allg. Chem. 1924, 135, 81-117. (45) Fort, R. J.; Moore, W. R. Trans. Faraday Soc. 1966, 62, 11121119. (46) Din, K.; Bansal, D.; Kumar, S. Langmuir 1997, 13, 5071-5075. (47) In, M.; Bec, V. Langmuir 2000, 16, 141-148. (48) Feng, K.-I.; Schelly, Z. A. J. Phys. Chem. 1995, 99, 1720717211. (49) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH Verlagsgesellschaft mbH: Weinheim, 1988; Chapter 6. (50) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. Prog.Phys. Org. Chem. 1981, 13, 485-630. (51) Reichardt, C. Chem. Rev. 1994, 94, 2319-2358. (52) Davis, M. M.; Hetzer, H. B. Anal. Chem. 1966, 38, 451-461. (53) Deye, J. F.; Berger, T. A. Anal. Chem. 1990, 62, 615-622.


Figure 4. Relationship between Nile Red transition energies and volume fraction of NMP with either CS2 or with cyclohexane as cosolvent. Transition energies in kcal/mol were calculated from equation ENR ) 2.85 × 10-3 ν, where ν is in wavenumbers. The Nile Red peak at ∼540 nm was used for all measurements. The measurements were made at room temperature, ∼ 22 °C.

values.53 The effect of the “inert” solvents, cyclohexane or carbon disulfide on the dye peak shifts are quite different, even though both solvents are often considered as nonpolar. The absorption peak in carbon disulfide is more red shifted. The difference is due to the higher polarizability of CS2 compared to cyclohexane.24 What is most interesting in Figure 4 is how the transition energies change as a function of volume fraction for the CS2/NMP system compared to the cyclohexane/NMP system. Both curves exhibit a concave pattern. Such deviation from linear behavior is often attributed to differences in the immediate solvation shell around the dye molecule compared to the bulk.20 The concave pattern suggests that a higher portion of more polar molecules, in this case NMP molecules, are in the solvation shell. The same applies to the CSNP case, but with one very important additional feature. The lowest energy (largest bathochromic shift) is below the energy for either carbon disulfide or NMP. This behavior is usually attributed to a synergistic relationship between the solvents leading to a species that may have a higher polarizability or polarity than either of the solvents separately. In the case here, this could be either the formation of a solution complex between the solvent components or may be due to higher associates of NMP. For instance, a chain dimer should have a larger dipole moment than a cyclic dimer. Although there may be some free rotation of the dipoles in the chain species, on the average the dipole will be larger than the more highly symmetrical cyclic case, where the dipoles will nearly cancel. Note that the minimum in transition energy for the CS2/NMP solvent occurs at a volume fraction of approximately 0.55. This is close to the concentration where most coals seem to have their maximum extractability. Thus, at least part of the reason for enhanced coal extraction in CSNP is due to a change in the character of the solvent itself, and not simply a matching of solubility parameters. The solvatochromic results suggest that the nature of that change involves changes in the polarity or the ionic character of the solvent system. B. FTIR Results. The preceding data so far implicates the formation of some type of complex as one explanation for the changes in properties. The question still remains as to what is the nature of the complex. Is


it a CS2/NMP complex, or is it solely related to NMP self-association phenomena? To provide more direct information on this question, the infrared spectra were recorded for a series of binary solutions. The information from this analysis was expected to be complicated. First, solvent shifts were expected as we vary the concentration of CS2/NMP ratio. These solvent shifts arise for two reasons: One is the ubiquitous effect of nonspecific van der Waals interactions between the target or absorbing molecule and molecules within the reaction field. The second reason for frequency shifts involves specific interactions leading to a complex on the spectral measurement time scale. In the CSNP case, the specific interactions could be donor-acceptor type interactions. If such a complex occurred at all, the most likely case would be an interaction of the NMP carbonyl oxygen with the carbon of the CS2. The problem is disentangling the two effects on the frequency shifts. If the partners in the complex are strongly associated, then large shifts on the order of 10-20 cm-1 might be expected.54,55 However, solving the problem is aided by the possibility that in carbon disulfide there is an additional spectral change that may occur if a strong donor-acceptor complex is formed. Changes in the ν2 Fundamental Bending Band of CS2. It is known that for another simple linear triatomic molecule, CO2, strong bases such as amines, aromatic amines, or carbonyl-containing molecules, will specifically interact with the C atom. The FTIR ν2 band of CO2 is the doubly degenerate bending mode of the CO2 molecule. Strong interactions with bases split the degeneracy and result in out-of-plane and in-plane bending modes. These two new bands will be shifted to lower frequencies, with in-plane mode exhibiting the largest shift.56-58 For the analogous band in CS2, we have been unable to find any references where this band was observed to be split as in the CO2 case. This is not surprising, as it is a relatively recent observation that the ν2 band in CO2 is split in the presence of bases, and this information can even be used to obtain equilibrium constants for base complexes. Furthermore, the ν2 band of CS2 is just above the typical mid-IR range and not usually measured in routine analysis.59 Figure 5 shows the spectra of NMP and CS2 which have been baseline corrected for scattering and for fringes. There are three prominent bands in the CS2 spectra. The most intense band at 1520 cm-1 is the ν3 asymmetric stretching mode. The 2295 cm-1 band is the ν3 + 2ν2 overtone, and the 2155 cm-1 band is the ν1 + ν3 combination band. The ν2 bending vibration for liquid CS2 is at 392.5 cm-1. This agrees well with the literature value of 392 cm-1.60 One bit of good fortune is that there are no interfering NMP bands in the region of the ν2 band. (54) Irusta, L.; Iruin, J. J.; Ferna´ndez-Berridi, M. J.; Sobkowiak, M.; Painter, P. C.; Coleman, M. M. Vib. Spectrosc. 2000, 23, 187-197. (55) Fawcett, W. R.; Kloss, A. A. J. Chem. Soc., Faraday Trans. 1996, 92, 3333-3337. (56) Dobrowski, J. C.; Jamroz, M. H. J. Mol. Struct. 1992, 275, 211219. (57) Nelson, M. R.; Borkman, R. F. J. Phys. Chem. 1998, 102, 78607863. (58) Jamroz, M. H.; Dobrowski, J. C.; Bajdor, K.; Borowiak, M. A. J. Mol. Struct. 1995, 349, 9-12. (59) Meredith, J. C.; Johnston, K. P.; Seminario, J. M.; Kazarian, S. G.; Eckert, C. A. J. Phys. Chem. 1996, 100, 10837-10848. (60) Ribnikar, S. V.; Puzic, O. S. Spectrochim. Acta 1973, 29A, 307317.

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Figure 5. Full mid-infrared absorbance spectra of CS2 and NMP. Spectra were taken in a AgCl microcell with a 0.0159 mm path length, and have been baseline corrected for both scattering and for fringes. The small band at 1698 cm-1 is a slight amount of NMP contamination in the microcell.

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Figure 7. Variation of total peak area for the combination of the CS2 bending vibration ν2, and CS2 ν3-2ν2 combination band as a function of the molarity of CS2.

Figure 8. Relative peak shifts of the ν2 bending vibration and the ν1-ν3 stretching combination vibration bands of CS2. Positive peak shift represents a blue shift. Figure 6. Expanded range infrared spectra of the CS2 ν2 bending vibration region for various concentrations of CS2 in NMP. Numbers beside each curve are the molarities. The volume ratios of CS2/NMP are in parentheses. All measurements at room temperature of ∼22 °C. The spectra have all been baseline corrected.

A series of CS2/NMP solutions of varying composition were analyzed by FTIR using a AgCl microcell. The spectra around the ν2 band region of CS2 are displayed in Figure 6. The very small band at 402 cm-1, observed in all the spectra, is the ν3-2ν2 mixed mode vibration for CS2.60 A definite hypsochromic shift is observed as the NMP concentration is increased. In addition, there appears to be band broadening and different intensity changes for the fundamental and combination bands. The most important missing feature in the spectra in Figure 6 is any obvious indication of a second peak to the low-frequency side of the ν2 band. As an analogy, with CO2, the in-plane bending mode is approximately 6-9 cm-1 lower than the out-of-plane bending mode. The out-of-plane frequency is also very near that for pure CO2.56,59,61 The absence of a new band representing the in-plane bending mode does not completely negate the possibility of complex formation. It may be that the frequency difference between the two bending modes is not sufficiently large to resolve them. If this was true, we would still expect to see band broadening and possibly band distortion. Of course, as already mentioned, there might will be non specific interactions that can produce the same results. (61) Experimentally, the expected split in ν2 degeneracy for CO2 in the presence of NMP was also observed, although there were severe complications with band overlaps; unpublished data.

Attempts to use peak fitting methods did not resolve the issue of complex formation. This was expected. The lack of any specific indication of a third peak meant that the frequency difference must be small and thus a single peak may adequately fit the data if relative peak shifts at different concentrations were similar. Indeed, the data could be fit to two peaks or three peaks with high correlation coefficients. Worse, for the low CS2 concentrations, wide initial variations in width, intensity, and position for all three bands produced very similar correlation coefficients with very different final peak areas and positions. Essentially, peak fitting produced meaningless data. Figure 7 is a plot of the total area of the ν2 and ν32ν2 band region. The data follow a Beer’s Law relationship throughout the entire concentration range. It is highly unlikely that if there was a specific interaction between CS2 and NMP, the free CS2 bending vibration, the ν3-2ν2 combination vibration, the in-plane bending vibration, and the out-of-plane bending vibration bands for the hypothetical complex would all coincidently sum to a constant absorption coefficient. Thus, the area versus concentration plot is good evidence that there is no CS2-NMP complex formed that is stable on the order of the measurement time. In Figure 6, it is clear that there is a progressive hypsochromic shift in the ν2 band as the concentration of NMP increases. This shift is most likely due to nonspecific reaction field effects due the increasingly polarity of the solutions. Figure 8 is a plot of the peak shifts of the maxima in the ν2 and ν3-2ν2 band group and the ν1 + ν3 band. The data for both bands are similar, and nonlinear. There is scatter in the data, which may obscure fine details, but it is apparent that


Figure 9. Wavenumber expanded scale of the infrared spectra of the NMP carbonyl region showing the variation in the carbonyl band as a function of NMP molarity. All spectra were baseline corrected. The small peak at 1690 cm-1 for pure CS2 (0 M) is due to a slight amount of NMP contamination in the microcell.

Figure 10. Peak area changes of the NMP carbonyl band and C-H stretching band in CS2 and the carbonyl band of NMP in cyclohexane. All values based on baseline corrected data.

there is a break in both curves near 8 M CS2. This is very near the 1:1 CS2/NMP volume ratio (8.34 M NMP ) 1:1 volume ratio). Thus, even though there does not appear to be a complex formed, it is clear that there is some major change occurring the CS2/NMP solvent system at a 1:1 volume. Changes in the NMP Carbonyl Stretching Band. Changes were also noted in the NMP carbonyl band as the ratio of the component solvents changed. As with most dipolar compounds in low dielectric media, NMP is known to be self-associated in solution.40,61-64 This will lead to complex changes in the carbonyl band as the NMP concentration varies.54 Figure 9 shows how the spectrum changes in this region as the concentration of NMP changes. At low NMP concentrations only a single narrow symmetrical band is observed. At higher concentrations, new bands emerge, the entire band shape broadens, and band intensity is visibly not linearly related to NMP concentration. At a minimum, four bands appear to constitute the carbonyl absorption region at high NMP concentrations. The effect of NMP concentration on the carbonyl band from the aspect of total band area changes can be seen in Figure 10. Unlike the CS2 bending vibration band, the carbonyl band does not follow a Beer’s Law relation(62) Dumas J-M.; Guerin, M.; Kribii, A. J. Chim. Phys. 1984, 81, 333-341. (63) Bittrich, H.-J.; Kietz, E. Z. Phys. Chem. (Liepzig) 1980, 261, 17-23. (64) Grant, D. J. W.; Higuchi, T.; Hwang, Y. T.; Rytting, J. H. J. Solution Chem. 1984, 13, 297-311.


Figure 11. Changes in the NMP carbonyl band under dilute conditions in the presence of CS2 or ethanol in cyclohexane. Each spectra was corrected for sloping baselines and fringes. [NMP] ) 0.0304 M.

ship in either CS2 or cyclohexane. For contrast, the areas of the NMP C-H stretching band in CS2 are also plotted in Figure 10. This band also does not follow a Beer’s Law relationship, but the deviation is not nearly as large as with the carbonyl band. This is to be expected because it is the carbonyl that is directly involved in self-association. Not only are the activity coefficients changing, but the absorption coefficients of the interacting carbonyls may also be different. The cyclohexane data in Figure 10 show the same general trend as the CS2 case. The carbonyl peak area changes rapidly up to about 2 M NMP, then the rate of change decreases markedly. Below 2 M NMP the carbonyl absorption bands also tend to be relatively sharp and symmetrical. This suggests that the equilibrium constants for self-association presumably to the dimer are not very large. What is surprising is that both the cyclohexane and CS2 exhibit very nearly the same type of curves. The higher polarizability of CS2 might be expected to lead to better solvation of the NMP molecules, and thereby reduce the association constants. This rationale does not appear to be true, or the dynamics are more complex than implied by the qualitative changes in peak area. For reference, a 1:1 volume ratio of CSNP is equal to 5.16 M NMP. Unlike the CS2 FTIR data in Figure 10, no sudden changes are observed in the carbonyl data in this concentration region. One possible reason for this observation is that the distinctive changes observed for the CS2 bands, which may also be associated with the enhanced extraction, may involve oligomer interactions rather than interactions with only NMP monomer. In the previous section, the lack of a second peak and the good Beer’s Law relationship for the CS2 ν2 bending vibration appeared to rule out the notion of a specific interaction between CS2 and NMP. However, these experiments were at concentrations where NMP selfassociation dominates. To provide further evidence whether any CS2-NMP specific interactions are present, several solutions were prepared at low NMP concentration. In the first three cases, the inert dilution solvent was cyclohexane. The NMP concentration was maintained at 0.0304 M. To this, either CS2 or ethanol was added. Figure 11 shows the changes in the carbonyl region of NMP for NMP in cyclohexane with and without the additional components. With the addition of nearly 20-fold amount of CS2 (relative to nmp), there is negligible change in the band shape or intensity

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compared to NMP in pure cyclohexane. In striking contrast, the addition of a 10-fold amount of ethanol results in two bands. One band at 1712 cm-1 is at the same frequency as the original NMP carbonyl. The second, larger band at1687 cm-1, we attribute to the NMP carbonyl strongly associating with ethanol through hydrogen bonding. NMP at very low concentration in CS2 is also displayed in Figure 11. This sample also exhibits a symmetrical carbonyl absorption as with cyclohexane, but the band is red shifted to1698.4 cm-1. Although both solvents may be considered as inert as far as specific interactions, CS2 has a higher polarizability than cyclohexane. Thus, CS2 will interact more strongly with the NMP carbonyl in the reaction field, which is reflected in a bathochromic shift relative to cyclohexane as solvent. Combining all the infrared information so far, the FTIR data indicates that there is no strong association complex formed between CS2 and NMP. Any donoracceptor type interactions are too weak to overcome thermal motion. However, the nonlinear peak shifts observed for the CS2 bands do suggest that there is a rather large change in the solution structure near a 1:1 volume composition. Self-Association of NMPsThe Carbonyl Band. The observation that the NMP carbonyl band undergoes complex changes with concentration suggested additional information on the solution dynamics might be extracted from this band. A quick review of what might be expected in NMP self-association may help in focusing the discussion. The self-association equilibrium constants for NMP in CS2 are unknown, but a rough idea can be implied from the constants known for NMP with other solvents. The dimer self-association constants for NMP in inert solvents all appear to be relatively small. This is true of tertiary amides in general.62 Even the existence of higher order oligomers beyond the dimer for NMP is not clear. Dielectric polarization measurements indicate that up to a concentration of approximately 0.3 M NMP in either cyclohexane or heptane only the dimer was present. The dimer equilibrium constant was 0.21 L mol-1 at 25 °C.62 Vapor phase composition studies up to 0.7 M NMP in isooctane suggested much higher oligomers were present, as high as the hexamer.64 Measurements of shift differences up to a mole fraction of 0.9 were fit to a simple dimer equilibrium model, with an association constant of 0.5 at 28.5 °C.63 From vapor phase osmommetry measurements, Montaudo et al. found a dimer constant of 2.8 L mol-1 in benzene.65 However, VPO measurements on volatile materials can be in error.66,67 On the basis of this limited information, and by analogy with other self-associated carbonyl species,54,55,68 such as ethylurethane, a possible picture of the state of NMP in solution can be surmised. At low NMP concentration there will only be unassociated monomer species. As the NMP concentration increases, (65) Mantaudo, G.; Caccamese, S.; Recca, A. J. Phys. Chem. 1975, 79, 1554-1557. (66) Higuchi, W. I.; Schwartz, M. A.; Rippie; E. G.; Higuchi, T. J. Phys. Chem. 1959, 63, 996-999. (67) Christian, S. D.; Tucker, E. E.; Brandt, D. R. J. Phys. Chem. 1978, 82, 1707-1709. (68) Perelygin, I. S.; Itkulov, I. G.; Krauze, A. S. Russ. J. Phys. Chem. 1991, 65, 410-414.

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Figure 12. Selection of second derivative FTIR spectra of the NMP carbonyl region for various molar concentrations of NMP in CS2. All spectra recorded at room temperature, ∼22 °C. The difference spectra all have the same scaling factor so they are directly comparable.

dimers form. As already discussed, there can be both cyclic and chain dimers. In cyclic dimers, both carbonyls would be nearly equivalent and should produce a single infrared band at lower frequency than the monomer, but with similar peak width. Likewise, in a dimer chain species, no matter what the dipolar interaction is, the carbonyls will not be equivalent. Two bands may be observed. The bands should be at an intermediate position between that of the monomer and cyclic dimer. In addition, this structure would have many similar conformers, and likely produce a broad infrared band. At even higher NMP concentrations, chain oligomers will form. The interior members of the chain will be different than either end of the chains, and a separate band should be observed for this interior species. This band may also be broad, and would be expected to be close to the frequency of the chain dimer. The end members of the oligomer chains should be similar to the species in the chain dimer and separate bands may not be observed. Instead, they would contribute to the absorbance of the chain dimer bands, if it exists. If only a single type of dipole interaction is assumed for the cyclic and chain species, then there are a total of seven distinct absorbing species that may be observed: monomer, cyclic dimer, chain dimer(ends), multimer ends, and multimer interior. But it is likely that only five bands will be observed. Also, two of these five bands, due to end groups in chains, might show a complex pattern of changes with increasing NMP concentration, since they are combinations of different species. Figure 12 displays a set of selected second derivative spectra of just the carbonyl band region as the NMP concentration in CS2 changes. Four bands are clearly present at high NMP concentration. Three other features are of note: one is that the bands all exhibit solvent frequency shifts, but different bands are either blue or red shifted. Second, in the original spectra, Figure 9, there is a rather sharp rise at either side of the carbonyl band envelope and a corresponding increase in absorbance at the middle of the carbonyl band. The second derivative spectra clearly indicate two relatively sharp sidebands at 1713 and 1669 cm-1 (based on the pure NMP spectrum). These bands also undergo opposite frequency shifts with NMP concentration. The corresponding increase in absorption in the middle of


the carbonyl band, despite the separation of the two near center bands, suggests that there must be at least one other band present near the center of the carbonyl band envelope. The lack of any definite indication of this band in the second derivative spectra suggests that it must be broad. Third, the bands do not all grow in at the same rate as the NMP concentration changes. This suggests that a variety of oligomers are present. The behavior of the original spectra in Figure 9, and especially the second derivative spectra in Figure 12, offer clues to the identity of at least three of the bands. At very low NMP concentrations, a single band is observed at 1689.5 cm-1. As the NMP concentration increases, this band progressively blue shifts to ∼1713 cm-1 in pure NMP. On the basis of the smooth nearly linear band shift progression with increasing NMP concentration, this band is assigned to the monomer. A second peak begins to grow in as a shoulder at low NMP concentrations at ∼1693 cm-1. The early appearance of this peak and its red shift relative to the monomer suggests that this band is due to an NMP dimer. From the second derivative spectra, this band appears to be nearly as sharp as the monomer and remains so at all NMP concentrations. Coupling this with the fact that only a single band is observed. this band is most likely due to the cyclic dimer. As the NMP concentration increases, this peak progressively red shifts to1669 cm-1 in pure NMP. Assigning the remaining three bands is difficult. There is a small band at 1731 cm-1 in pure NMP, which becomes prominent only at much higher concentrations than when the dimer becomes apparent. This band follows a similar blue solvent shift to the monomer band. However, the position of this band to higher frequency is not what would be expected for an associating species. Association should weaken the carbonyl bond resulting in a lower frequency as observed for the other bands. The band does seem to be related to the concentration of NMP in a nonlinear relationship, suggesting it is not an impurity, but is related to an oligomer. An intriguing, but untested possibility, is that this high-frequency band is due to transition dipole-dipole coupling. This coupling is very dependent on the orientation of the dipoles. A blue shift is expected if the dipoles where arranged in a parallel fashion.69 A second, small, but clearly identifiable band is found at a very low frequency of 1645 cm-1. This band undergoes a red shift similar to the dimer peak, but again only emerges at relatively high NMP concentrations. The position of this band suggests it is from a very highly associated species; it may represent an interior molecule in a chain oligomer. Or as in the suggestion above, it may be due to a transition dipole-dipole coupling mechanism, but here the dipoles are arranged in an antiparallel fashion. The fifth band at ∼1689 cm-1 must be close to the center of the carbonyl band envelope. Because this central peak cannot be directly observed in the low NMP concentration spectra, it appears to represent a lateforming species, most likely a higher order oligomer. Of course, there is no evidence whether this band is indeed a single broad peak or multiple peaks from many (69) Painter, P. C.; Pehlert, G. J.; Hu, Y.; Coleman, M. Macromolecules 1999, 32, 2055-2057.


different types of oligomers whose bands overlap. The relative frequency assignments of this band as a higher oligomer and its position between the monomer and dimer bands would be consistent with the assignments of the parent compound of NMP, 2-pyrrolidone.70 The earlier prediction that there could be as many as seven bands or more likely five, and observing five bands would seem to support the model. However, because some of the bands cannot be unequivocally assigned to particular carbonyl species, such an assumption is not justified from the present data. As already mentioned in a previous section a smaller set of data with cyclohexane as cosolvent instead of CS2 were also examined (spectra not shown). Not only were the spectra found to be nearly identical to the CS2 cases of the same volume ratios, but even the derivative spectra followed the same pattern as described for CS2. A note about the band shifts is in order. Tertiary amide carbonyls typically show a bathochromic shift with increasing solvent polarity.71,72 In contrast, the 1713 cm-1 band assigned to the monomer undergoes a hypsochromic shift. However, the carbonyl band for dilute NMP (∼0.03 M) in cyclohexane and carbon disulfide are found at 1712 and 1698 cm-1, respectively. Since the polarizability of CS2 is higher than cyclohexane, the solvent shift between these two solvents is in the expected direction. In contrast, the band assigned to the dimer shows the expected bathochromic shift. The reason for the opposite behavior of monomer and dimer is unknown. Attempts were made to model this system in the hope of breaking out the relative changes in the bands as a function of NMP concentration. The carbonyl band envelope was easily modeled with five bands if no constraints were assumed for the band characteristics. Unfortunately, both the number of bands and lack of resolution between them, meant there was a large number of very different, but equally good fits to the data, that depended only on the initial conditions set for the bands. Some spectroscopic basis or prior knowledge for limiting the band parameters is critical in such cases. One constraint was to consider all the bands to be Lorentzian-Gaussian in shape. In addition, the monomer full width at half-height could be assigned from the very low NMP concentration data. Because of the similarity of the peak width in the second derivative minima, the dimer band at 1669 cm-1 was also assumed to be approximately the same bandwidth (∼14 cm-1 fwhm) as the monomer. Using only these constraints, there were still too many very different fits with high correlation coefficients (r2 > 0.999) with different initial band parameters. If the bandwidth of the outermost bands were also constrained to approximately the same width as the monomer, it quickly became clear that a single central band did not provide reasonable fits to the data. Adding more than five bands was necessary. However, adding more uncharacterized bands to the model becomes equivalent to the random fitting men(70) Walmsley, J. A.; Jacob, E. L.; Thompson, H. B. J. Phys. Chem. 1976, 80, 2745-2753. (71) Wohar, M. M.; Seehra, J. K.; Jagodzinski, P. W. Spectrochim. Acta 1988, 44A, 999-1006. (72) Garcia, M. V.; Redondo, M. I. Spectrochim. Acta 1987, 43A, 879-885.

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tioned above. Thus, without further independent details on the nature of the bands underlying the carbonyl band, there was little hope of obtaining an unbiased representation of the dynamics between the various species. Discussion The goal of this work was to examine the CS2/NMP system to see whether there were any recognizable differences in solution structure that may have an impact on explaining why it is a good extraction solvent for some coals, and second, why a 1:1 mixture is the most effective in many cases. The approach was to use a variety of tests to probe whether there is any evidence from the binary solution properties whether there are any special interactions between the solvent components that can account for the enhanced extraction In the Results Section, some interesting information came to light, as well as some weaknesses where more work is needed. Nevertheless, at this point enough information appears to be available to speculate on the nature of the CS2/NMP solvent system and its relationship to coal extraction. First, let us summarize the results of the various experiments to put them into perspective. The negative excess volume data for CSNP could be interpreted in terms of either complex formation between CS2 and NMP, the ability of CS2 to fit into the interstitial regions between the NMP molecular associates, or a tighter structure for some intermediate NMP oligomers. This last possibility seems to be the least likely of the three based on other binary systems with NMP. The viscosity data were consistent with weak complex formation between CS2 and NMP. The data were also consistent with a mechanism where the associated NMP oligomers undergo a major reorganization into a new structure. Especially the large change near a 1:1 volume ratio of solvents suggests a transition from one type of oligomer to another. This could be either cyclic or chain oligomers becoming larger or interconversion of forms, such as cyclic dimer to chain dimer. The solvatochromic measurements on CSNP suggest that the dipolar state of the CS2/NMP solutions is higher at roughly a 1:1 volume ratio then for either solvent separately. Again, this is consistent with a CS2-NMP donor-acceptor complex with a moderate separation of charge. But it is also consistent with a change to a system with higher dipolar character for the oligomers. The FTIR data helped to clear up some aspects of the problem (and also created more questions). No concrete evidence of a long-lived CS2-NMP complex was found. For all practical purposes, CS2 can be considered as an inert solvent with only non specific van der Waals interactions toward NMP. NMP association was directly observed. The complexity of the carbonyl band changes clearly show that several types of oligomers exist in solution. There are indications that the dipolar nature of the solutions may be changing, with one or more oligomers having an increased dipole compared to NMP. At a 1:1 ratio, there are quite a few different types of associates in the solution as well as monomer. The picture that emerges from all these observations: A major component of the synergetic extraction

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of CS2 and NMP is the ability of CS2 to alter the association behavior of NMP. In this respect, the suggestions of Iino1 and Painter18 concerning the role of CS2 to alter the association of NMP, and the current work are all in concert. However, the current information suggests some additional speculation on the mechanism of CSNP extraction. The solvatochromic data and the possibility of dipole-dipole transition effects being observed in the FTIR data, suggest that the reason for extraction enhancement may be related to major changes in the dipolar character of the solution. Thus, it is not just the presence of association and the balance between monomer and oligomers that is important as suggested by Painter and co-workers. The very structure of the oligomers themselves may be critical to fully understanding the extraction process. Oligomers that have larger dipoles relative to the NMP appear to be a necessary component in the synergetic binary solvent extraction. This is not to say that solution electrostatics are the only factor involved in extraction enhancement, but the current information suggests they must play an important role. In addition, the critical role of CS2 may be in stabilizing these more polar NMP oligomers. The higher polarizability of CS2 could promote favorable interactions through van der Waals forces. The same should be true of more polar solvents, but it is possible that these may interact too strongly with the enhanced oligomer dipoles and defeat the necessary solvating interactions with coal macromolecules. The exact identity of these higher dipole oligomers cannot be determined solely on the basis of data here. However, the source of the increased dipole can be inferred by considering the possible structures that might exist for the oligomers. Examining the dimer structures, the dipole of the cyclic structure where the dipoles will be antiparallel will be less than that for the chain dimer where at least a portion of the dipoles at any instant will be parallel. Thus, it would appear that chain species are the most likely candidate in assisting extraction. Why should the increased dipolar species lead to enhanced extraction? One possible answer to this is that the larger dipole will interact more strongly with the π cloud of the aromatics than for the NMP monomer. There is one nagging problem with the hypothesis that the dipolar state of some NMP oligomers may be important to understanding the coal extraction mechanism. The nearly identical behavior of the FTIR data for CS2 and for cyclohexane is hard to reconcile with the solvatochromic results for the two solvents. The finding that only CS2 solutions showed a higher dipolar/ polarizability state for intermediate solvent ratios in the solvatochromic data does not fit with the nearly identical spectral changes observed for the two solvents in the FTIR NMP carbonyl data. The latter suggests nothing special about the CS2 system over the cyclohexane system. One possible explanation is that the higher dipolar oligomers exist in both solvents with similar equilibrium constants. However, for some reason, in cyclohexane the higher dipolar NMP oligomers cannot approach as close to the dye molecule as in the CS2 case. Certainly, the rather large size of cyclohexane compared to CS2 must influence the solvent properties. It may be that there is some low level of order or disorder between the NMP oligomers and cyclohexane


compared to CS2, that prevents the NMP high dipolar oligomers from moving into the reaction field of the dye. Despite this problem, the high dipole/polarizability found in the solvatochromic data at a 1:1 volume ratio for CSNP still stands, and still seems to offer the best explanation for the behavior of CSNP on coal extraction. The proposed explanation for synergetic extraction of coal in CSNP solutions here also may provide an (even more highly speculative) answer to another little understood phenomena in CSNP extraction of coal. Additives are know to enhance the coal extraction. Particularly, it has recently become clear that the addition of substances such as di-N-methylpyrrolidinonium pentacyanopropenide can significantly enhance extractions.3-5,73 Usually only very small amounts are necessary to cause rather large changes in extract yield. Chen et al. suggested that the reason pentacyanopropenide salts, and several other salts, can enhance extraction (73) Chen, C.; Kurose, H.; Iino, M. Energy Fuels 1999, 13, 11801183.


is that they may induce a polarization in the aromatic systems in coal, leading to disruption of low energy interactions, such as π-π and van der Waals forces.73 However, it would also be expected that these charged salts would also have an impact on charged oligomers. Could these additives alter or promote the formation of chain over cyclic species, thereby enhancing extraction? This also might explain why only relatively small amounts of additive are needed to produce markedly high extract yields. Small amounts of ionic additives could act as catalysts to produce large change in oligomer equilibria. Acknowledgment. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Sciences, U.S. Department of Energy, under contract number W-31-109-ENG-38. EF010003C

Carbon Disulfide

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