Pyrrolidone Derivatives in Water Solution: An Experimental and

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Ind. Eng. Chem. Res. 2009, 48, 1036–1050

Pyrrolidone Derivatives in Water Solution: An Experimental and Theoretical Perspective Marıá J Da´vila, Rafael Alcalde, and Santiago Aparicio* Department of Chemistry, UniVersity of Burgos, 09001 Burgos, Spain

In this paper thermophysical properties of 2-pyrrolidone, N-methyl-2-pyrrolidone, and N-cyclohexyl-2pyrrolidone, in binary and ternary liquid mixtures with water are reported at 298.15 K and 0.1 MPa over the whole composition range. Derived excess and mixing properties were used to analyze mixtures’ structure. To infer a detailed picture of the intermolecular interactions in the studied fluids, attenuated total reflection infrared absorption spectroscopy was used. To study fluid properties from a molecular viewpoint, density functional theory computations were carried out, leading to structural and energetic information. Formation of H-bonding and dipolar interactions between cyclic amides and with water were observed, resulting in complex fluid structures that were stronger in binary mixtures containing water than in ternary ones. Intermolecular forces are weakened upon mixing by the presence of a second cyclic amide in the mixture. 1. Introduction Throughout twentieth century, solvents’ importance was obvious in biomacromolecules because of the structural effects of the solvent in proteins that affects their thermodynamic properties. Currently, computational and experimental chemistry permit the identification and analysis of the influence of solvents in complex processes, developing new models useful to interpret the behavior of complex fluids.1 Moreover, industry is searching for new solvents for production optimization purposes. Reducing the use of solvents is difficult; for this reason the search for nonharmful solvents, both from an environmental and toxicological viewpoint, and the reduction of emissions from volatile organic solvents are new purposes. Thus, changes in the composition of solvents, cost, and social benefit evaluations are being made to optimize products and processes. Therefore, determination of thermophysical properties of fluid mixtures is necessary in the industry because of the innumerable number of processes in which they take part. A better knowledge of the physical and chemical properties of fluid mixtures is required to allow a greater development of physical models, and consequently, design improvements for industrial processes.2 In this work, solvents within the 2-pyrrolidone family were selected because of their importance for biological and industrial purposes and because of the presence in their molecular structures of the very important amide group together with different additional groups allowing the formation of homo and heteorassociations through H-bonding. N-Methyl-2-pyrrolidone (NMP) (Figure S1, Supporting Information), is a powerful selective solvent, with very good thermal and chemical stability, low volatility, and a high flash point, completely miscible with water at all temperatures.3 NMP is recyclable by distillation, biodegradable, and nontoxic to aquatic life. NMP is used in many industrial processes: pure hydrocarbons recovery in the petrochemical process; extractive distillation of aromatic compounds due to the high selectivity of this solvent;4 liquid-liquid extraction, as for n-paraffins, isoparaffins and cycloparaffins separation because of the presence of π electron-systems;3 liquid chromatography; desulfuration of gases; and recrystallization for purification of synthesis products.5 2-Pyrrolidone (PYR) (Figure S1, Supporting Information) is used in many industrial * To whom correspondence should be addressed. E-mail: sapar@ ubu.es.

applications3 and as a model for complex molecules with biological interest because of the CO-NH peptide bond. PYR is able to self-associate in the pure state through H-bonding.6-10 PYR in solution retains this ability even at low amide concentrations.11 N-Cyclohexyl-2-pyrrolidone (CHP) (Figure S1, Supporting Information) is used in a large number of chemical processes. CHP structure shows a nonpolar region and a donor-acceptor CO-NH peptide bond, providing as useful a model for proteins as the other lactams. CHP has a low vapor pressure12 and it is miscible with water below its lower consolute temperature (LCT). In this way, CHP is frequently used in solvent extractions because of its great selective precipitation ability, due to its hydrophobicity (cyclohexyl ring), and high donicity (carbonyl oxygen).13,14 Thus, CHP is considered for processes where partial solubility between solvents is necessary,5 such as happens with water.15,16 Considering the importance of cyclic amides aqueous solutions, theoretical and experimentally studies have been carried out to determinate thermophysical properties and to clarify their structure.11,15,17-31 It is well-known that solvent mixtures are important to the improvement of solubility of low soluble substances in pure solvents. Properties of solvent blends change with the composition, and in this way, selectivity and efficiency can be controlled in numerous processes.32 The NMP/W solvent mixture is also useful to study the critical effect of hydration of the amide group.33 Aqueous PYR solvent blends show a strongly organization with formation of a H-bonding network, with a cyclic dimer maintained even at low lactam concentration.11 Moreover, this solvent mixture is highly useful in industry as kinetic inhibitors of crystallization of gas hydrates. On the other hand, lactams in solvent blends are useful in interpreting the properties of peptide compounds in aqueous solutions. CHP is a nonmiscible solvent with water under certain conditions of temperature, so this two-phase system allows a characterization of the role of hydrophobic interactions in proteins.34 PYR, NMP, and CHP are strongly polar molecules, with carbonyl oxygen acting as H-bonding donor with water, the polar character being greater in aqueous solutions than in the pure state.31 These molecular interactions formed between cyclic amides and water lead to a complex behavior in thermophysical properties. Therefore, in this paper we report properties for PYR + W, CHP + W, NMP + CHP, and CHP + PYR binary and PYR +

10.1021/ie800911n CCC: $40.75  2009 American Chemical Society Published on Web 12/08/2008

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 1037

CHP + W and CHP + NMP + W ternary mixtures, in the full composition range at 298.15 K and 0.1 MPa. Attenuated total reflection infrared (ATR-IR) spectroscopy was used to study the PYR + W, NMP + W, and CHP + W binary mixtures in the full composition range at 298.15 K and 0.1 MPa. Experimental spectra were deconvoluted allowing us to propose structural organizations at the molecular level and their evolution with composition. Moreover, full geometry optimizations were carried out for these three lactams through density functional theory (DFT) in gas phase and water solution. The properties, structural and energetic, of self-association complexes in PYR, and lactams/ water complexes were analyzed through DFT calculations in gas phase and water solution. These results confirm the main trends of the mixtures’ behavior inferred from thermophysical and ATR-IR data. 2. Materials and Methods 2.1. Compounds. Pure solvents PYR (Fluka), NMP (Fluka), and CHP (Sigma-Aldrich) were degassed with ultrasounds and kept out of the light (NMP is photosensitive) over Fluka 0.3 nm molecular sieves and thus used without further purification. Degassed ultrapure water (Millipore Milli-Q, resistivity 18.2 mΩ · cm) was used in all experiments. The purity of cyclic amides was checked by GC using a Perkin-Elmer 990 gas chromatograph (PYR, 99.95%; NMP, 99.80%; and CHP, 99.70%) and by comparison with the literature thermophysical properties (Supporting Information, Table S1, refs 35-42 contained therein). Mixture samples were prepared by weighing the sample with a Mettler AT261 microbalance ((1 × 10-5 g) and syringing it into vials with the same mixture volume to avoid preferential evaporation; thus, a precision of (1×10-4 in the mole fraction is obtained. Ternary systems were studied in the whole composition range by a procedure described previously.11 2.2. Density Functional Theory (DFT) Calculations. Molecular modeling was carried out for NMP, PYR, and CHP molecules, and for complexes formed by these cyclic amides and water, in gas phase and water solution, to clarify the molecular interactions existing in the studied systems. DFT computations, using the Becke gradient corrected exchange functional43 and Lee-Yang-Parr correlation functional44 with three parameters (B3LYP),45 together with 6-311++G** basis sets, were carried out with the Gaussian 03 series of programs.46 Atomic charges were calculated to fit the electrostatic potential47 according to the Merz-Singh-Kollman (MK)48 scheme. Aqueous-phase calculations were carried out using the self-consistent reaction field approach (SCRF), in which the solvent is treated as a continuum, using the integral formalism of the polarization continuum model (IEF-PCM),49 thus, providing a quantitative estimation of contributions to the total free energy of solvation. IEF-PCM calculations were done with the solute located in a cavity built using the united atom model; a value of 1.2 was used to scale the radii and there were 70 tesserae per sphere. Energy of complexes was calculated as the difference among the complex and monomers energies with the basis set superposition error (BSSE) corrected according to the counterpoise procedure.50,51 Full structure optimizations were carried out for monomers and complexes, both in gas phase and water solution. Optimization results were checked by the absence of imaginary frequencies in the calculated vibrational spectra. 2.3. Instrumentation. Density (F) and speed of sound (u) were measured simultaneously with an Anton Paar DSA 5000 instrument, where density was measured by an oscillating U-tube

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((5 × 10 g cm-3) and speed of sound by measuring the traveling time of a sound pulse from the piezoelectric transducer to the detector ((0.5 m s-1). The cell temperature was controlled by a built-in solid state thermostat and measured by internal platinum resistance thermometers ((1 × 10-2 K). Calibration was carried out using two reference standards, n-nonane (Fluka, purity >99.5%) and toluene (Sigma-Aldrich, purity >99.5%). Density values for these standards were obtained from the literature.39 Dynamic viscosity (η) was measured using an automated AMV200 Anton Paar rolling ball microviscometer. The cell temperature was controlled through a Julabo F25 external thermostat and measured with a platinum resistance thermometer ((1 × 10-2). The rolling time was measured to (1 × 10-2 s; precision for dynamic viscosity was (5 × 10-3 mPa s. Calibration was carried out using n-dodecane (Aldrich, >99.5%), hexan-1-ol (Fluka, >99.5%), octan-1-ol (Fluka, >99.5%), and decan-1-ol (Fluka, >99.5%) as standards.39 Refractive indices (nD) were measured in relation to the sodium D line by an automated Leica AR600 refractometer to (5 × 10-6. Temperature was controlled by a Julabo F32 external circulator and measured by a platinum resistance thermometer ((1 × 10-2 K). Calibration was performed using double degassed water (Millipore Milli-Q) and a standard oil (nD ) 1.51416) supplied by the manufacturer. Isobaric molar heat capacities (cp) were measured to (1 × 10-1 J mol-1 K-1 using a Setaram micro DSC III calorimeter. It consists of two vessels (reference and measuring) surrounded by an array of highly sensitivity Peltier elements ((1 × 10-2) and lodged in a calorimetric block surrounded by a thermostatic liquid (nundecane) that ensures a temperature homogeneity. The calorimeter works under the Calvet principle, determining the variation of the heat flow to/from liquid, with both cells maintained to the same temperature. Measurements were performed according to the isothermal step method described in the literature,52 using n-hexane (Fluka, >99.5%) as reference material and butan-1-ol (Aldrich, >99.5%) as calibration liquid, whose cp values were obtained from Zabransky et al.42 ATR-IR spectra for liquid phases were obtained with a Nicolet Nexus spectrometer together with a Smart Thermal ARK device. The ATR accessory contains a zinc selenide crystal and its temperature is controlled through a built-in temperature controller, with the temperature measured through a RTD temperature sensor to (1 °C. All ATR-IR experiments were carried out at 298.15 K. 2.4. Excess and Mixing Properties. Excess molar volume (VEm), mixing viscosity (∆mixη), mixing refractive index (∆mixnD), *E ), were and excess molar free energy of activation (∆Gm evaluated from the experimental data according to well-known thermodynamic expressions.11,53 Isentropic compressibilities (kS), were calculated from Laplace equation. Excess isentropic compressibilities (kSE) and excess molar isobaric heat capacities (CPE) were determined according to the ideal behavior criterion defined by Benson et al.54 Redlich-Kister equation55 was used to correlate excess and mixing properties for binary systems (XE): k

XE ) x(1 - x)

∑ A (2x - 1)

j

j

(1)

j)0

where x stands for the mole fraction and Aj are the fitting coefficients obtained by a least-squares procedure, with the proper number of coefficients, k, determined by a F-test.56 E Ternary excess and mixing properties (XTER ) were fitted to 57 Cibulka equation:

1038 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 E XETER ) XBIN + x1x2x3(B0 + B1x1 + B2x2)

(2)

E represents the sum of properties for the three where XBIN corresponding binary constituents obtained from eq 1, and the last term in eq 2 is the ternary contribution to the corresponding property. Partial molar properties for binary and ternary systems were calculated from fitting equations, eqs 1 and 2, and parameters reported in Tables S6 and S8, Supporting Information, according to the intercept method using well-known thermodynamic relationships.53 The limiting values at infinite dilution for these properties were evaluated as the values at mole fraction approaching to zero. For binary systems, a large number of experimental points were measured in the vicinity of both pure solvents (Table S5, Supporting Information) to define more accurately the values of the fitting coeficients for eq 1 for these concentration ranges and thus to improve the accuracy of the calculated properties at infinite dilution. Thus, the uncertainties in the calculated partial molar volume and partial excess isobaric molar heat capacity are (1% for both properties in the whole composition range. This is calculated considering the error rising from fitting eqs 1 and 2, as well as systematic errors from experimental measurements of density and isobaric molar heat capacity.

3. Results and Discussion 3.1. Molecular Modeling. Results for optimized structures of studied monomers, in gas phase and water solution, at B3LYP/6-311++g** theoretical level are reported in Figure S1 (Supporting Information), and Tables S2 and S3 (Supporting Information, results for NMP were reported in a previous paper31). The five-membered rings containing the amide group are not perfectly planar for the three studied molecules, an envelope configuration is obtained both in gas phase and water solution, Tables S2 and S3 (Supporting Information). Another remarkable structural feature rises from the orientation of the group attached to the amide nitrogen. For NMP, the preferred configuration of the methyl group involves an hydrogen atom almost eclipsing the carbonyl oxygen,31 in contrast with the preferred staggered configuration in the solid phase.58 This eclipsed configuration is stabilized by the interaction between hydrogen and the strongly negative carbonyl oxygen. Results reported in Figure S1 (Supporting Information) for CHP show the cyclohexyl group in a perpendicular arrangement with the amide group ring, both in gas phase and water solution; the calculated torsional barrier, Figure 1, shows that this configuration is clearly preferred to the in-plane one (in which both rings are coplanar). In this perpendicular arrangement, an efficient interaction among the hydrogen of the cyclohexyl group and the strongly negative carbonyl oxygen is obtained, thus resulting in a lower energy, as for NMP. The relaxed potential energy scans reported in Figure 1 show a large torsional barrier, more remarkable in water solution; thus, free rotation of the cyclohexyl group is energetically hindered. The interaction among the cyclohexyl hydrogen and the carbonyl oxygen in the eclipsed configuration provides an additional stabilization of 1.32 (gas phase) and 1.98 (IEF-PCM water) kcal mol-1 compared with the staggered one; these values are slightly larger than the ones previously reported for the configuration of methyl hydrogens in NMP.31 The cyclohexyl ring in CHP shows a chairlike structure with hydrogen atoms alternating axialequatorial directions. In Table 1, we report the main molecular parameters for the three studied monomers. The bond length among carbon and oxygen in the carbonyl group is almost the

Figure 1. Relaxed potential energy scannings computed at B3LYP/6311++g** theoretical level for CHP in gas phase, and water solution (IEFPCM). ∆E is the energy relative to the conformer with lower energy. Scanned dihedral angle D(1-10-13-14); atom numbering as in Figure S1 (Supporting Information). We indicate in the Figure the position of the oxygen atom (O) for guiding purposes. Table 1. Main Parameters of the Monomers Calculated at B3LYP/ 6-311++g** Theoretical Level in Gas Phase and Water Solution (IEF-PCM)a

PYR NMPb CHP

q (MK)

CdO (Å)

C(O)-N (Å)

N-C(O)-O (deg)

-0.605 -0.759 -0.592 -0.731 -0.623 -0.763

1.2152 1.2346 1.2181 1.2344 1.2181 1.2361

1.3700 1.3491 1.3712 1.3542 1.3712 1.3555

125.99 125.88 125.77 125.70 125.77 125.94

a q (MK) ) Merz-Singh-Kollman charges for oxygen. Bold numbers: water solution. b Data from ref 31.

same for the studied lactams, this bond is slightly elongated on going to water solutions. Thus, the interaction with the selfconsistent reaction field, which simulates the surrounding water solvent, slightly weakens the carbonyl oxygen bond. The charge in the carbonyl oxygen is also larger in absolute value in water solution, Table 1; this larger charge separation justifies the increase in the dipole moment on going to solution, Figure S1 (Supporting Information). The bond among C(O) and amide nitrogen is larger than the one with oxygen, as we may expect from its lower bond order, but on going to water solution the nitrogen bond length decreases, and thus a reinforcement is inferred for the three amides. Thus, in solution oxygen bonds are weakened, whereas nitrogen ones are reinforced. The structures obtained for the three monomers show that the carbonyl oxygen has a clear ability to develop hydrogen bonds with other molecules acting as an acceptor position. The hydrogen attached to nitrogen in PYR is a donor position able to form hydrogen bonds. Nitrogen is in theory another position able to form hydrogen bonds, for PYR the preference of acting as an hydrogen-bond donor through the hydrogen attached to nitrogen is clearly preferred over the acceptor one through the nitrogen atom, whereas the nitrogen position is highly sterically hindered both for NMP and CHP by the presence of the hydrophobic methyl and cyclohexyl groups. We have to remark on the large dipolar moments of the three lactams, which are reinforced in water solution (increasing more than 40%); thus, remarkable dipolar interactions will also be present in the studied fluid mixtures. Electrostatic potential surfaces (EPS) (Figure S1, Supporting Information) show a strongly negative region close to the carbonyl oxygen corresponding to the negative calculated charges for the three lactams, Table 1. For PYR, we have to

Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 1039 Table 2. Changes of Energy (∆E ) Esol - Egas), Free Energies of Solvation (∆Gsol), and Electrostatic (∆Gelec) and Nonelectrostatic (∆Gnon - elec) Contributions to the ∆Gsol Calculated at B3LYP/ 6-311++g** Theoretical Level for PYR and CHP in Water Solution (IEF-PCM). All Energies in kcal mol-1

PYR NMPa CHP a

∆E

∆Gelec

∆Gnon-elec

∆Gsol

-11.38 -7.82 -8.52

-11.99 -8.18 -8.81

4.60 7.07 8.50

-7.39 -1.11 -0.31

Data from ref 31.

remark the positive region on the hydrogen attached to the amide nitrogen, which corresponds to the large positive charge of this atom (+0.338 in gas phase increasing to +0.386 in water solution); this points again to this position as an adequate hydrogen bond donor position. For NMP a positive region extends from the ring to the hydrophobic methyl group, the nitrogen atom is clearly immersed in the hydrophobic moiety, thus hindering the possible development of H-bonding with water through this position. For CHP, a large positive region extends across the molecule, leading to a remarkable large hydrophobic region. The three studied molecules are stabilized on going from gas phase to water solution, Table 2, pointing to a very favorable interaction with the aqueous environment, although theoretical results in water solution are obtained considering the solvent as a continuum, and thus effects like H-bonding are not treated specifically. The stabilization is remarkably larger for PYR than for the N-substituted lactams, because of the larger polarization of PYR molecules, Figure S1 (Supporting Information). A correspondence among ∆E () Esol - Egas) and free energies of solvation (∆Gsol) is observed in Table 1. Results show a great water affinity by PYR as the large negative ∆Gsol show, thus PYR is more efficiently solvated by the water environment. On the contrary, we have to remark on the close to zero ∆Gsol values for NMP and CHP monomers despite the remarkably negative ∆E values for both molecules. An analysis of the electrostatic and nonelectrostatic contributions to ∆Gsol, Table 2, shows that NMP and CHP have large electrostatic contributions (lower than PYR because of their lower polarization) but also large positive nonelectrostatic contributions (almost double for CHP than for PYR). These large nonelectrostatic contributions rise from the remarkable work required to build the PCM cavity owing to its molecular size and shape; the hydrophobic moieties on the methyl group for NMP and the very large one across the molecule for CHP make solvation difficult in water solution. The analysis of lactam/lactam and lactam/water complexes from a structural and energetic viewpoint allows studying shortrange interactions in the fluid mixtures. Only PYR is able to form lactam/lactam complexes through H-bonding. Two possibilities may be inferred for these complexes: (i) open linear dimer and (ii) hexagonal cyclic dimer, Figure 2. The cyclic dimer is clearly preferred over the linear one, both in gas phase and IEF-PCM water solution, which is in agreement with literature results.10 This preference for the six-membered ring complex may be justified by the cis configuration of amide group in PYR, which is maintained for lactams with up to 10membered rings.59 The prevailing presence of cyclic dimer in PYR discards the presence of higher association complexes for this molecule both in pure fluid or in water solutions, hence, an equilibrium monomer/cyclic dimmer may be expected. On going from gas phase to IEF-PCM solution, binding energies decrease remarkably, although hydrogen bonding distances remain almost unchanged, thus dimerization process is affected by the surrounding media.

Figure 2. Highest binding energy PYR dimers calculated at B3LYP/6311++g** theoretical level in gas phase and IEF-PCM water solution. Distances are in Å, counterpoise corrected binding energies (∆E) are in kcal mol-1: (black) gas phase, (bold) IEF-PCM water solution. We report only structures for IEF-PCM water solution, gas phase ones are indiscernible. We indicate in the figure the position of the oxygen atom (O) for guiding purposes.

Complexes PYR + 3W, NMP + 2W,31 and CHP + 2W have been optimized in gas phase and in aqueous solution to analyze short-range lactam/water interactions, Figure 3. Large binding energies are obtained for water complexes for the three studied lactams in gas phase but they decrease remarkably on going to water solution. Oxygen-hydrogen distances reported in Figure 3 are short enough (∼1.90 Å) to ensure the formation of hydrogen bonds among studied lactams and W molecules. Carbonyl oxygen is able to hold two hydrogen bonds with W molecules but the H-C(O)-H angle decreases remarkably for the three studied lactams on going to water solution thus hindering this interaction. PYR is able to develop interactions with three molecules, giving rise to a very large binding energy in gas phase but a very important weakening of the complex is inferred on going to water solution. Hence lactam/W complexes are energetically favorable, although this effect decreases upon going to water solution. We should consider that IEF-PCM calculations are obtained considering that complexes are totally surrounded by W medium, thus they would be describing highly diluted lactam solutions. As the lactam concentration increases, and the polarity of the surrounding media decreases,60,61 the binding energy of the complexes should be larger than the values calculated for IEF-PCM water solutions because of the weaker interaction of the surrounding polar medium with the studied complexes. 3.2. ATR-IR. ATR-IR vibrational spectroscopy provides valuable information on the structural changes that arise upon mixing. The combined analysis of experimental and DFT calculated IR spectra allow assigning different bands to molecular level features, thus leading to a more reliable, and physically meaningful, experimental spectral deconvolution. This fact is important considering that in the deconvolution process of experimental spectra different sets of fitting parameters could fit the same spectra equally well.62 Hence, the deconvolution of the obtained spectra was carried out avoiding the ambiguity of the fitting considering fits that lead to a reliable physical picture of the fluids’ structure. The experimental ATR-IR and calculated vibrational spectra for PYR in gas phase and water solution, for the N-H stretching and amide spectral regions, are reported in Figure 4. Spectral data are in agreement with literature data,63-65 a wide peak with a maximum at 3327.6 cm-1 for the N-H stretching and a narrower peak at 1673.1 cm-1 corresponding to the amide region. We focused in the analysis of the band in the N-H stretching region, which was deconvoluted to study the fluid structure, Figure 5. A very intense peak at 3221.9 cm1 and two weaker ones at lower and higher wavelengths describe accurately

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Figure 3. Highest binding energy (a) PYR + 3W, (b) NMP + 2W and (c) CHP + 2W complexes calculated at B3LYP/6-311++g** theoretical level in gas phase and IEF-PCM water solution. Distances are in Å, counterpoise corrected binding energies (∆E) are in kcal mol-1: (black) gas phase, (bold) IEF-PCM water solution. We report only structures for IEF-PCM water solution, gas phase ones are indiscernible. We indicate in the figure the position of the oxygen atom (O) in the lactam molecule for guiding purposes.

Figure 5. Least squares fit to Gaussian-Lorentzian functions for the IR spectrum of PYR in the N-H stretching region. Gaussian-Lorentzian functions in the 2800-3000 cm-1 are not shown to improve visibility: continuous line, experimental ATR-IR spectrum; dashed lines, fitted peaks; the fitted spectrum is over the experimental one. Deconvolution data are given in Table S4 (Supporting Information).

Figure 4. IR spectrum of PYR in the N-H stretching and amide I regions. Experimental: pure liquid PYR spectrum at 298.15 K. DFT calculated at B3LYP/6-311++g** theoretical level in gas phase for PYR monomer and cyclic dimer. DFT harmonic wavenumbers scaled with a factor 0.96. Numerical values show NH stretching and amide I maxima.

the experimental spectrum. The features for 2800-3000 cm-1 correspond to contributions from the stretching of C-H groups in the cycle, and thus they have scarce importance for the analysis of the intermolecular forces. The two small deconvolution peaks reported in Figure 5 correspond to the overtones of the CdO stretch and to the hydrogen-bonded CdO.66 The comparison among experimental and DFT calculated spectra allows us to assign the bands; the most intense band in the N-H stretching region at 3221.9 cm-1 is very close to the theoretical value of the cyclic dimer, and the absence of a band around 3482.8 cm-1 shows that there is no presence of monomeric, nonassociated, PYR molecules. The presence of a linear dimer or higher association complexes should be discounted because the predicted B3LYP/6-311++G** wavenumber for the N-H stretching of the linear dimer is 3200 cm-1. These results are confirmed by the behavior of the amide I region band that also discounts the presence of a monomeric species. Thus, pure PYR fluid structure is characterized by the prevailing presence of a cyclic dimer, which is in agreement with literature results.59

Before we analyze the behavior of the studied binary mixtures through ATR-IR spectroscopy we should clarify some aspects of their phase behavior. The system PYR + W forms solid hydrates with melting points higher than 298.15 K (the selected temperature of this study) for water mole fractions in the 0.35-0.65 range.67 Thus, some of the studied mixtures are solid at 298.15 K. We report in Figure S2 (Supporting Information) the properties of the monohydrate formed for xPYR ) 0.50, for this composition the higher melting point is obtained (30.4 °C).67 The solid hydrate appears a little after the sample is prepared at ambient temperature (∼22 °C). We have measured the melting point of this mixture using a simple visual method (placing the sample in an equilibrium cell where temperature is controlled through a circulating bath and measured with a Pt-100 thermometer), and the temperature agrees with the literature value, Figure S2 (Supporting Information). Once the hydrate formed, we melted it at around 50 °C and then we cooled it again to 25 °C. At this final temperature the mixture remains liquid (subcooled melt) for a certain time, thus allowing the measurement of its liquid phase properties (Figure S2, Supporting Information). It then freezes again to the solid monohydrate. This freezing-melting behavior is obtained for all the mixtures in the 0.35-0.65 range, and thus although they finally form solid phases it is also possible to measure the liquid phase properties to analyze their behavior in the whole composition range. Previous works reported by several authors18,68 have also reported studies for PYR + W at 298.15 K in the whole composition range. Moreover, although solid dihydrates are also


Figure 6. Effect of mixture composition on the main spectral features obtained from the deconvolution of experimental ATR-IR spectra for xPYR + (1 x)W at 298.15 K. Numeric data are given in Table S4 (Supporting Information).

formed for the NMP/water system, their melting points are wellbelow 25 °C.69 The behavior of PYR in W solution is very complex, as the experimental spectra for the whole composition range reported in Figure S3 (Supporting Information) show. The DFT predicted wavenumbers also reported in Figure S3 (Supporting Information) may help to analyze the mixed fluid structure although they should be considered with caution because they were calculated considering a surrounding IEF-PCM media, and thus predicted values would be valid for water-rich regions. For W-rich regions a wide and flat peak is obtained, very similar in shape to that for pure water, whereas for PYR-rich regions the peak is sharper. A sudden change is observed in the spectra for 0.4-0.5 mol fractions pointing to remarkable structural changes. Peak deconvolution was carried out focusing in the N-H stretching region because the wider peaks obtained in this region allow a less ambiguous deconvolution, Table S4 (Supporting Information). Water peaks superimpose with those obtained from N-H stretching and amide I bands of PYR molecules. The pure water spectrum was fitted using two main components, following the generally accepted assignments, a peak positioned at 3210.9 cm-1 (the icelike peak) and a second one at 3394.4 cm-1 (the liquidlike peak); these assignments are in agreement with literature values.70,71 The icelike peak corresponds to a well-ordered H-bonding network, whereas a more disordered structure leads to the liquidlike one. The bending peak at 1644 cm-1 superimposes with the peak rising from the CdO stretching. As PYR is added to W-rich solutions new spectral features arise. At low PYR concentrations the lactam molecules are not associated with other PYR molecules, these monomers interact with W molecules forming complexes. Hence, for W-rich solutions we have four main spectral features in the N-H stretching regions: (i) W icelike peak, (ii) W liquidlike peak, (iii) a peak rising from the stretching vibration of W OH groups hydrogen-bonded to PYR monomers, and (iv) a peak rising from N-H stretching in PYR molecules. DFT calculations for PYR molecules in IEF-PCM W solutions, Figure S3 (Supporting Information), show that PYR monomers, free or associated to W molecules, should lead to a peak at ∼3200 cm-1. As the PYR concentration increases a peak rising from the PYR dimer should appear, this peak according to DFT calculations should appear around 3250 cm-1, Figure S3 (Supporting Information). In Figure 6, we report the evolution of the main spectral features with mixture composition. Water structure is strongly affected by the presence of PYR molecules.

As PYR molecules are added to pure W, the icelike peak quickly decreases and blueshifts. On the contrary, although the liquidlike peak also decreases quickly, it appears to redshift. Hence, a weakening of the W H-bonding network is experienced by PYR addition. The frequencies of the water peaks change in a steeped way up to xPYR ≈ 0.2 and then in a more subtle way (Figure 6 panels a and c) thus showing that remarkable structural changes happen around this composition. W peak areas (Figure 6 panels b and d) decrease remarkably with increasing composition, but three changing zones with limits at xPYR ≈ 0.2 and ≈ 0.5 are obtained. The third peak caused by water molecules is produced by the stretching vibration of molecules bonded to PYR monomer sites (both CO and NH sites). This peak shows a sudden blueshift as PYR concentration rises, but from xPYR ≈ 0.2 it appears to redshift. The stretching vibrations of water molecules bonded to PYR ones appear at higher frequencies than the usual OH molecules in water, thus these hydrogenbonds in W/PYR should be weaker than the ones in pure water. From these results, we propose a molecular picture of the effect of PYR on water structure. (i) The addition of PYR clearly weakens the icelike structure of water but the redshift of the liquidlike peak shows that a more disordered H-bonding is formed, hence PYR acts as an structure breaker leading to more dynamic water H-bonding networks able to solvate the PYR molecules. (ii) Icelike structure seems to dominate up to xPYR ≈ 0.2, whereas from xPYR ≈ 0.2- 0.5 liquidlike is dominating. This show that PYR molecules are in monomeric form, and thus their solvation is improved through a dominating liquidlike water structure. (iii) At xPYR ≈ 0.5 another structural feature rises. From this concentration PYR is dominating and PYR dimer prevails over monomers, hence water molecules tend to cluster, and thus the relationship of area icelike/area liquidlike is larger than 1. (iv) The peak of water molecules bonded to PYR points again to interaction with monomers up to xPYR ≈ 0.2 and then to water clusters interacting with surrounding PYR molecules, the redshift of this peak shows stronger W/PYR hydrogen bonds for PYR-rich fluids, hence surrounding water molecules hinder the interaction with PYR monomers. The NH stretching peak suffers a sudden blueshift and then it remains almost constant, whereas its intensity first increases and then decreases with xPYR ≈ 0.2-0.5 as limiting composition. This behavior is in agreement with the monomer/dimer relationships as PYR concentration increases. Finally, the NH stretching vibration in the PYR dimer leads to a peak from xPYR ≈ 0.5


Figure 7. (a) Least squares fit to Gaussian-Lorentzian functions for the IR spectrum of NMP in the CO stretching region. Continuous line, experimental ATR-IR spectrum at 298.15 K; dashed lines:, fitted peaks. The fitted spectrum is over the experimental one. (b) Ratios of the peak areas of NMP1 (associated) and NMP2 (monomer) peaks obtained from experimental peak deconvolution for xNMP + (1 - x)W at 298.15 K.

Figure 8. Effect of mixture composition on the spectral features in the water stretching region obtained from the deconvolution of experimental ATR-IR spectra for xNMP + (1 - x)W at 298.15 K.

showing a slight blueshift (14 cm-1) as the PYR mole fraction increases; hence, water seems to slightly reinforce this bonding. ATR-IR results for NMP and NMP/W mixtures are reported in Figures 7 and 8. The stretching CO peak in NMP appears in the 1550-1800 cm-1 region, whereas no remarkable features are obtained in the 3000-3500 cm-1 range. A narrow and intense peak is obtained for CO stretching, for which the deconvolution is reported in Figure 7a. Although NMP is not able to self-associate through H-bonding, its large dipolar moment, Figure S1(Supporting Information), leads to a remarkable ordering and to the formation of associates competing with free monomers.72 This is in agreement with the spectrum reported in Figure 7a, the analysis of the second derivative of the spectrum revealed the existence of two main peaks that may be attributed to NMP dimer (NMP1, the most intense one at 1672.3 cm-1) and to NMP monomers (NMP2, less intense at 1689.2 cm-1). Although Dyrkacz72 proposed the existence of higher oligommers in pure NMP, the narrow character of the peak hinders the extraction of more information, and thus we propose a simple equilibrium among monomer/dimer structures to explain the NMP spectrum. The peak reported in Figure 7a could be properly fitted using a larger number of GaussianLorentzian functions, but in our opinion this would be a merely mathematical fitting exercise, and thus physical information could not be extracted. Results reported for pure NMP show that pure liquid NMP is strongly associated as the ratio among the area of NMP1 and NMP2 peaks reported in Figure 7b shows. The addition of W has a strong effect on NMP monomer/dimer equilibria, Figure 7b; the population of monomers increases remarkably upon water addition because in this way the formation of complexes NMP/W through H-bonding is clearly

favored. The effect of NMP molecules on water structure is reported in Figure 8, this analysis is carried out through the OH stretching region, where wider peaks allow a better deconvolution. The icelike and liquidlike water peaks show a blueshift as NMP concentration increases (Figure 8a), thus a clear weakening of water H-bonding is produced by NMP molecules. A third remarkable peak appears even for low NMP mole fractions (Figure 8a) that is assigned to stretching of water OH groups involved in H-bonding with NMP carbonyl oxygens. This peak appears at larger wavenumbers, thus NMP/W heteroassociations are weaker than W/W homoassociations as we may expect. NMP molecules disrupt the water H-bonding network as seen in Figure 8b. The addition of NMP to water decreases the ratio among the icelike/liquidlike peaks, thus NMP favors the liquidlike H-bonding structure because with this arrangement the development of NMP/W heterocomplexes is more favorable. Results reported in Figure 8b for the area of the OH stretching in the NMP/W complexes peak show that it leads to a maximum at x ≈ 0.25. This value is in agreement with the value obtained from the thermophysical properties of NMP/W mixture (Figure 11). Hence, heteroassociations in NMP/W mixtures are clearly favored with the formation of heterocomplexes being more remarkable for water-rich mixtures. Results for CHP containing mixtures are reported in Figures 9 and 10. The spectra of pure CHP for the CO stretching region reported in Figure 9a shows two main peaks corresponding to associated molecules through dipolar interaction (CHP1) and to CHP-free monomers (CHP2). Although CHP1 is dominating, and thus CHP is preferentially associated in the pure state, the ratio among the CHP1/CHP2 peaks’ areas is lower than for pure NMP (see Figures 9b and 11b). Hence, although a


Figure 9. (a) Least squares fit to Gaussian-Lorentzian functions for the IR spectrum of CHP in the CO stretching region. Continuous line, experimental ATR-IR spectrum at 298.15 K; dashed lines, fitted peaks. The fitted spectrum is over the experimental one. (b) Ratios of the peak areas of CHP1 (associated) and CHP2 (monomer) peaks obtained from experimental peak deconvolution for xCHP + (1 - x)W at 298.15 K.

Figure 10. Effect of mixture composition on the spectral features in the water stretching region obtained from the deconvolution of experimental ATR-IR spectra for xCHP + (1 - x)W at 298.15 K.

E Figure 11. Excess molar volume, Vm , mixing viscosity, ∆mixη, excess isentropic compressibility, kSE, mixing refractive index, ∆mixnD, excess isobaric molar *E heat capacity, CPE, and excess molar free energy of activation, ∆Gm , at 298.15 K and 0.1 MPa: (b) xPYR + (1 - x)W, (O) NMP + (1 - x)W, (2) xCHP + (1 - x)W, (∆) xPYR + (1 - x)NMP, (9) xPYR + (1 - x)CHP, and (() xNMP + (1 - x)CHP. Experimental points are the symbols; fitting equations are the lines (Table S6, Supporting Information, and ref 11).

remarkable dipolar ordering is present in pure CHP, the presence of the bulky cyclohexyl groups leads to a certain degree of steric

hinderance that makes dipolar arrangement difficult in comparison with NMP. The addition of water increases the monomer


E Figure 12. Partial molar excess volume, V m , and partial molar excess E isobaric heat capacity, CP,m , of the i-compound in the binary mixtures reported in Figure 13 at 298.15 K and 0.1 MPa: (s) xPYR + (1 - x)W; (---) xCHP + (1 - x)W; (-•-) xNMP + (1 - x)W; (-••-) xNMP + (1 - x)CHP; (-•••-) PYR + (1 - x)CHP; (- -•-) xPYR + (1 - x)NMP.

population (Figure 9b) although as water concentration increases the ratio of areas evolves through a minimum, which is in contrast with the behavior of NMP/W mixtures. This can be justified considering the highly hydrophobic character of the cyclohexyl groups that for water rich mixtures tend to selfaggregate leading to the formation of micelles in these solutions.69 The effect of CHP on water H-bonding is reported in Figure 10. The peak in the water OH-stretching region is deconvoluted using three peaks rising from icelike, liquidlike, and OH associated to CO features. The wavelengths of these three peaks show blueshift, thus a weakening of the H-bonding is produced by CHP molecules. The icelike/liquidlike ratio is also strongly affected by CHP molecules. As the CHP concentration increases the liquidlike structure prevails thus allowing a better interaction among water and CHP molecules. 3.3. Thermophysical Properties of Lactam/W and Lactam/Lactam Binary Systems. Experimental properties for the binary systems and fitting coefficients for the excess and mixing properties are reported in Tables S5 and S6 (Supporting Information) and in Figures 11 and 12. Aqueous systems containing PYR, NMP, or CHP show strong deviations from ideality, being less noticeable for PYR + W, although heteroassociations are very remarkable for all the studied mixtures. The presence of water molecules lead to larger effects in NMP and CHP containing mixtures than for PYR ones because of the self-association through H-bonding in pure PYR in contrast with the dipolar interaction in NMP and CHP. Excess volumes for the studied lactam/water mixtures are negative in the whole composition range, with the largest values for NMP blends. Strong heteroassociations are present in the mixtures, therefore a contraction is made in the mixing process. For NMP systems a remarkable structural difference rises upon water mixing, evolving from a highly structured dipolar fluid to the developing of H-bonding, thus turning the structure to a compact one.31 CHP shows dipolar interaction and a certain degree of structuring in the pure state, as the ATR-IR results

have showed. Upon mixing, CHP aqueous solvation is not so effective as for NMP, due to hydrophobic character of the cyclohexyl group that leads to a less compact structure and a greater geometry impediment for H-bonding formation with water molecules, so excess volumes are less negative. PYR aqueous blends show the same packing effect but weaker than in the other cases because of the self-association of PYR fluid that is perturbed on going upon water addition, therefore excess volume values are less negative. Mixing viscosities for the studied aqueous systems are positive in the whole composition range. The smaller values are obtained for PYR blends, and so, the effect caused by the presence of water molecules by H-bonding is weaker in this case. The larger values obtained for CHP mixtures are justified considering the presence of the bulky cyclohexyl group that hinders the fluid’s flow. Isentropic compressibility shows strong minimums for all the aqueous systems, showing H-bonding formation, but the lower minimum in the CHP + W system is due to the absence of H-bonding and to the weaker dipolar ordering, compared with NMP, in pure CHP. Low concentrations of amide cause a weakening of water structuring, increasing the liquidlike Hbonding, and thus reducing the compressibility. The partial molar properties reported in Figure 12 and the infinite values of these properties shown in Table 3 support the efficient solvation of the three studied lactams in water mixtures and the development of remarkable lactam/water heteroassociations in mainly water-rich mixtures. Thus, to recap, excess properties for lactam/water systems show the formation of heteroassociation by H-bonding, PYR aqueous blends values being the lowest due to remaining PYR-PYR interaction in the mixture. NMP and CHP + W mixtures show strong heteroassociations compared with the H-bonding absence in the pure fluids. Position of the properties maxima or minima, around xPYR ) 0.25 and xNCP ) 0.33, show the formation of NMP or CHP + 2W and PYR + 3W heteroassociations, which are energetically and sterically favored as previously shown by DFT computations. Thermophysical properties for lactam/lactam systems show low deviations from ideality. Excess volumes for PYR + CHP and PYR + NMP are negative and small in the whole composition range,11 therefore a relatively efficient packing is inferred upon mixing. On the other hand, an expansive effect is shown for NMP + CHP systems, due to the packing effectivity lower than in the two pure components. Mixing viscosities are positive in the whole composition range for PYR/ CHP because of steady dipolar interaction and more efficient packing, while this property is negative for NMP/CHP and PYR/ NMP in the whole range, showing that the mixing process weakens the dipolar NMP and CHP structure. These behaviours are confirmed by the other excess and mixing determined properties. Therefore, three factors determine the mixture behavior in cyclic lactams blends: molecule geometry, steric factors, and an effective packing with weak interactions. 3.4. Thermophysical Properties of PYR/CHP/W Ternary Mixture. Excess and mixing properties for this ternary system are reported in Figures 13-15 and Tables S7-S9 (Supporting Information). Excess molar volume is negative in the whole composition range (Figure 13a) with minimum values in the water-rich zone. The ternary contribution to this property (Figure 13b) and Table S9 (Supporting Information) is positive in the whole composition range, thus upon the formation of the ternary mixture an expansive effect arises. The simultaneous presence of PYR, CHP, and W molecules seems to hinder the packing,


E E E Figure 13. (a) Excess molar volume, Vm,TER , (b) ternary contribution of excess molar volume, Vm,TER - Vm,BIN , (c) mixing viscosity, ∆mixηTER, (d) ternary contribution of mixing viscosity, ∆mixηTER - ∆mixηTER, (e) mixing refractive index, ∆mixnD,TER, and (f) ternary contribution of mixing refractive index, ∆mixnD,TER - ∆mixnD,BIN, for the x1PYR + x2CHP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

E E E Figure 14. (a) Excess isentropic compressibility, kS,TER , (b) ternary contribution of excess isentropic compressibility, kS,TER - kS,BIN , (c) excess isobaric molar E E E heat capacity, CP,TER , and (d) ternary contribution of excess isentropic compressibility, CP,TER - CP,BIN , for the x1PYR + x2CHP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

Table 3. Infinite Dilution Propertiesa E,∞ Vm (cm3 mol-1)

(1) (1) (1) (1)

PYR + (2) W NCP + (2) W PYR + (2) CHP NMP + (2) CHP

CPE,∞ (J mol-1 K-1)

(1)

(2)

(1)

(2)

-0.8499 -4.8515 -0.5944 0.4733

-1.3774 -1.1074 0.1905 0.5351

78.37 195.12 35.10 -2.05

27.41 36.72 -4.86 -5.57

a The numbers in brackets show the compound for which the property is reported for each mixture.

and thus the development of heteroassociations due to the bulky and hydrophobic character of the cyclohexyl group in CHP. The maximum of the ternary contribution, Table S9 (Supporting Information), appears when almost equal quantities of the three molecules are present, thus confirming the disruptive effect of the simultaneous presence of PYR and CHP molecules.

Mixing viscosity (Figure 13c) is positive in the whole composition range. Ternary contribution to this property (Figure 13d) is positive for water-rich mixtures and negative for waterpoor blends. The maximum of the ternary contribution (Table S9 Supporting Information) appears for 1PYR/1CHP/2W mixtures; hence, for high W concentrations the disruptive effect of CHP is less remarkable and the development of ternary heteroassociations may be possible. This fact is in agreement with the largest negative excess molar volume for this composition range. Moreover, ternary contribution to mixing viscosity is negative for very rich PYR mixtures (Table S9 Supporting Information) hence the addition of small quantities to PYRdominated mixtures produced a very important disruptive effect because of the efficient PYR/PYR homoassociations reinforced by the presence of small quantities of water.


E E Figure 15. (top) Partial excess molar volume of i-compound, Vm,i , and (bottom) partial excess isobaric molar heat capacity, CP,i , in the x1PYR + x2CHP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

E E E Figure 16. (a) Excess molar volume, Vm,TER , (b) ternary contribution of excess molar volume, Vm,TER - Vm,BIN , (c) mixing viscosity, ∆mixηTER, (d) ternary contribution of mixing viscosity, ∆mixηTER - ∆mixηTER, (e) mixing refractive index, ∆mixnD,TER, and (f) ternary contribution of mixing refractive index, ∆mixnD,TER - ∆mixnD,BIN, for the x1CHP + x2NMP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

Mixing refractive index is positive with a positive ternary contribution, which is in agreement with the behavior of excess molar volume: more compact mixtures and thus less compressible mixtures (Figure 14a) lead to larger refractive indices. Excess isobaric heat capacity shows a complex behavior, although it is predominantly positive, thus evidencing H-bonding (Figure 14c) The ternary contribution to the property (Figure 14d) is positive for PYR-rich mixtures and negative for NCPrich ones. Hence, intermolecular forces in the ternary system are favored by the presence of PYR molecules in excess, whereas for CHP dominated fluids, the development of heteroassociations is clearly hindered by the bulky and hydrophobic character of the cyclohexyl group. Partial molar properties are reported in Figure 15. Partial excess molar volumes are negative for the three mixture compounds in the full composition range, except for CHP in rich zones of PYR, thus pointing to remarkable intermolecular interactions and steric effects upon mixing. At high W concen-

tration, interactions are more intense and water H-bonding network seems to allow an efficient packing of studied lactams despite their sizes and shapes. In turn, partial excess molar volume of W shows very negative values in the full composition range due to the formation of H-bonding and to the efficient packing of W molecules into fluids rich in lactam. Thus, water and lactams lead to efficient packing and interaction in fluids dominated by the other molecules thus allowing the development of remarkable heteroassociations. This is in agreement with the positive partial excess heat capacity reported in Figure 12, with the only exception of CHP at high PYR concentration. The highest values of the property are obtained for CHP in PYR/W mixtures and increase as the W mole fraction. PYR molecules show almost equal affinity for CHP and W molecules, showing a minimum for the partial property close to 0.5 mol fraction. Values for W show affinity both for CHP and PYR with a maximum for the property around 0.33 mol fraction showing that heteroassociations are developed with CHP and PYR.


E E E Figure 17. (a) Excess isentropic compressibility, kS,TER , (b) ternary contribution of excess isentropic compressibility, kS,TER - kS,BIN , (c) excess isobaric molar E E E heat capacity, CP,TER , and (d) ternary contribution of excess isentropic compressibility, CP,TER - CP,BIN , for the x1CHP + x2NMP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

E E Figure 18. (Top) Partial excess molar volume of i-compound, Vm,i , and (bottom) partial excess isobaric molar heat capacity, CP,i , in the x1CHP + x2NMP + (1 - x1 - x2)W ternary system at 298.15 K and 0.1 MPa.

3.5. Thermophysical Properties of CHP/NMP/W Ternary Mixture. Properties for this ternary system are reported in Figures 16-18 and Tables S10-S12 (Supporting Information). Excess molar volume is negative in the full composition range, except for poor water mixtures (Figure 16a) of the ternary contribution (Figure 16b) being positive values in the whole composition range. Thus, the presence of both amides hinders the H-bonding formation, producing system expansion in relation to the binary systems, because of the different size and molecular orientation. Mixing viscosity, Figure 16c, is positive in rich W zones and negative in rich lactams zones, showing that low W concentration is not able to counteract the dispersive forces in the amides. The large negative ternary contribution, Figure 16d, discards the development of ternary interactions, this property contrast with the positive values reported in Figure 13d for the PYR/ CHP/W mixtures and shows how the simultaneous presence of

NMP and CHP molecules with the methyl and cyclohexyl hydrophobic moieties hinders the development of heteroassociations. The minimum of the ternary contributions appears for a 1NMP/1CHP/2W composition ratio showing the large disruptive effect of both lactams on water H-bonding network. Mixing refractive index (Figure 16e) shows positive values, although the ternary contribution (Figure 16f) is mainly negative in the whole composition range, with a minimum value at the same composition ratio of the remaining properties. Excess isentropic compressibility and its ternary contribution (Figure 17) are in agreement with the contractive mixture process and with the expansive ternary effects in comparison with the binary mixtures. Excess isobaric heat capacity, Figure 17c, is always positive in the full composition range except water poor mixtures, being the ternary contribution to this property, Figure 17d, negative,


thus pointing to a weakening of the intermolecular forces upon the formation of the ternary system. Finally, partial molar properties are shown in Figure 18. Excess partial molar volumes are negative except for the lactams in water-poor blends, therefore, although the development of heteroassociations among the three involved molecules is inferred in the mixtures, as the water content decreases these interactions are weakened by steric effects. Excess partial heat capacity is positive for the three molecules, showing the highest value for low CHP concentrations. Infinite dilution values for NMP in CHP/W mixtures increase with W concentration owing to higher CHP concentrations hindering interaction among NMP and W molecules. Values for W show that it is able to develop heteroassociation both with NMP and CHP, with a maximum value of the property around 0.33 in both cases. 4. Conclusions The studied binary and ternary systems show strongly nonideal behavior. From the combined experimental-theoretical approach used to characterize the studied fluids we may infer the next remarkable conclusions: (i) DFT results shows that PYR molecules develop cyclic dimmer complexes through H-bonding, whereas linear cyclic ones should be discarded. This is confirmed by the ATR-IR results. (ii) PYR, NMP, and CHP may develop energetically favorable heterocomplexes with water molecules although the binding energy of these complexes decreases for water-rich mixtures. (iii) ATR-IR results shows a strong effect of amides on water H-bonding network, that is disrupted evolving from the prevailing icelike structure to a more flexible liquidlike one upon lactam addition. This structuring allows a more efficient packing of lactams in water-dominated fluids and thus a more feasible development of H-bonding among lactams and water molecules. (iv) NMP and CHP are not able to self-associate through H-bonding but their large dipolar moments lead to dipolar complexes. Thus, their fluid structure may be explained through a simplified dimer/monomer equilibrium. Dimer dominate over monomers, this effect being more remarkable for NMP. (v) The addition of water to lactams has also a strong effect on the structure of fluids, decreasing the populations of associated dimer, both for H-bonded ones in PYR and dipolar ones in NMP and CHP. In this way, interaction with water through H-bonding may be developed more efficiently. (vi) In binary mixtures, interaction of water with lactams is clearly favored, thus giving rise to an efficient packing. This effect is also remarkable for CHP despite the presence of the bulky and hydrophobic cyclohexyl group. (vii) For the studied ternary systems, the simultaneous presence of two lactams and water molecules seems to hinder the development of intermolecular forces and thus ternary interaction should be discarded. This effect is more remarkable when NMP and CHP are simultaneously present in the mixture. Thus solvation and H-bonding is more effective for binary lactam/water systems than for ternary ones. Acknowledgment The financial support by Junta de Castilla y Leoń, Project BU020A07, and Ministerio de Educacioń y Ciencia, Project CTQ2005-06611/PPQ, (Spain), is gratefully acknowledged. Supporting Information Available: Table S1 (thermophysical properties of pure solvents), Figure S1 (gas phase optimized

structures for PYR and CHP with atom numbering and ESP surfaces for PYR, NMP and CHP), Table S2 (calculated molecular parameters of PYR), Table S3 (calculated molecular parameters of CHP), Figure S2 (solid hydrates for PYR + W binary system), Figure S3 (IR spectrum of x PYR + (1-x) W in the N-H stretching and amide I regions), Table S4 (leastsquares fit to Gaussian-Lorentzian functions for the ATR-IR spectrum of x PYR + (1-x) W), Table S5 (binary mixtures properties), Table S6 (fitting coefficients for binary mixtures excess and mixing properties), Table S7 (x1PYR + x2CHP+ (1 - x1 - x2)W ternary mixture properties), Table S8 (x1 CHP + x2 NMP+ (1 - x1 - x2) W ternary mixture properties), Table S9 (fitting coefficients for x1 PYR + x2 CHP+ (1 - x1 - x2) W ternary mixture excess and mixing properties), Table S10 (fitting coefficients for x1 CHP + x2 NMP+ (1 - x1 - x2) W ternary mixture excess and mixing properties), Table S11 (maxima and minima for x1 PYR + x2 CHP+ (1 - x1 - x2) W for the ternary contributions to excess and mixing properties) and Table S12 (maxima and minima for x1 CHP + x2 NMP + (1 - x1 - x2) W for the ternary contributions to excess and mixing properties). This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Silla, E.; Arnau, A.; Tunõń, I. Fundamental Principles Governing Solvents Use. Handbook of SolVents; ChemTec: Toronto, Canada, 2001. (2) Wypych, G. New Trends Based on Patent Literature. Handbook of SolVents; Chemtec: Toronto, Canada, 2001. (3) Hradetzky, G.; Hammerl, I.; Bittrich, H. J.; Wehner, K.; Kisan, W. Selective Solvents. Physical Sciences Data; Elsevier: Amsterdam, The Netherlands, 1989; Vol. 31. (4) Mueller, E.; Hoehfeld, G. Aromatic Recovery by Extractive Distillation with N-Methylpyrrolidone. In Proceedings of the 8th World Petroleum Congress; Applied Science: London, 1971; pp 211-219. (5) Marcus, Y. Solvent Mixtures. Properties and SelectiVe SolVation; Marcel Dekker: New York, 2002. (6) Walmsey, J. A. Self-association of 2-Pyrrolidinone. 2. Spectral and Dielectric Polarization Studies of Benzene Solutions. J. Phys. Chem. 1978, 82, 2031–2035. (7) Dachwitz, E.; Stockhausen, M. Dielectric-Relaxation of 2-Pyrrolidinone and Some n-Substituted, Related-Compounds in the Liquid State. Ber. Bunsen. Phys. Chem. Phys. 1985, 89, 959–961. (8) Jadzyn, J.; Malecki, J.; Jadzyn, C. Dielectric Polarization of 2-Pyrrolidinone-Benzene Solutions. J. Phys. Chem. 1978, 82, 2128–2130. (9) Beine, A. H.; Dachwitz, E.; Wodniok, L.; Stochhausen, M. On the Assessment of Dielectric-Relaxation Parameters of Liquids. Z. Naturforsch. 1986, 41, 1060–1070. (10) Yekeler, H. Solvent Effects on Dimeric Self-Association of 2-Pyrrolidinone: An Ab Initio Study. J. Comput.-Aided Mol. Des. 2001, 15, 287–295. (11) Alcalde, R.; Aparicio, S.; Garcia, B.; Davila, M.; Leal, J. M. SoluteSolvent Interactions in Lactams-Water Ternary Solvents. New. J. Chem. 2005, 29, 817–825. (12) Specialty Pyrrolidones. BASF Technical Brochure; BASF: Shakopee, MN, 1995. (13) Morita, Y.; Kawata, Y.; Mineo, H.; Koshino, N.; Asanuma, N.; Ikeda, N.; Yamasaki, K.; Chikazawa, T.; Tamaki, Y.; Kikuchi, T. A Study on Precipitation Behavior of Plutonium and other Transuranium Elements with N-Cyclohexyl-2-pyrrolidone for Development of a Simple Reprocessing Process. J. Nucl. Sci. Technol. 2007, 44, 354–360. (14) Ikeda, Y.; Wada, E.; Harada, M.; Chikazawa, T.; Kikuchi, T.; Mineo, H.; Morita, Y.; Nogami, M.; Suzuki, K. A Study on Pyrrolidone Derivatives as Selective Precipitant for Uranyl Ion in HNO3. J. Alloys Comp. 2004, 374, 420–425. (15) Lou, A.; Pethica, B. A.; Somasundaran, P.; Yu, X. Phase Behavior of N-Alkyl-2-Pyrrolidones in Aqueous and Nonaqueous Systems and the Effect of Additives. J. Colloid Interfaces Sci. 2002, 265, 190–193. (16) Aparicio, S.; Alcalde, R.; Da´vila, M. J.; Garcia, B.; Leal, J. M. Liquid-liquid Equilibria of Lactam Containing Binary Systems. Fluid Phase Equilib. 2008, 266, 90–100. (17) Pirilla-Honkanen, P. V.; Ruostesuo, P. A. Thermodynamic and Spectroscopic Properties of 2-Pyrrolidinones. 1. Excess Molar Volumes of

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ReceiVed for reView June 10, 2008 ReVised manuscript receiVed October 10, 2008 Accepted October 22, 2008 IE800911N

Pyrrolidone Derivatives in Water Solution: An Experimental and

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