Interactions of Ionic Liquids and Acetone: Thermodynamic Properties

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Interactions of Ionic Liquids and Acetone: Thermodynamic Properties, Quantum Chemical Calculations and NMR Analysis Elia Ruiz, Victor R Ferro, José Palomar, Juan Ortega, and Juan J. Rodríguez J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp402331y • Publication Date (Web): 20 May 2013 Downloaded from http://pubs.acs.org on May 22, 2013

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The Journal of Physical Chemistry

Interactions of Ionic Liquids and Acetone: Thermodynamic Properties, Quantum Chemical Calculations and NMR Analysis. Elia Ruiz,a Victor R. Ferro,a Jose Palomar,*a Juan Ortegab and Juan Jose Rodriguezb a

Departamento de Química Física Aplicada (Sección de Ingeniería Química),

Universidad Autónoma de Madrid, Madrid, Spain. b

Laboratorio de Termodinámica y Fisicoquímica de Fluidos, Parque Científico-

Tecnológico, Universidad de Las Palmas de Gran Canaria, Canary Islands, Spain.

Abstract The interactions between ionic liquids (ILs) and acetone have been studied to obtain further understanding on the behavior of their mixtures, which generally give place to exothermic process, mutual miscibility and negative deviation of Raoult’s law. COSMO-RS was used as suitable computational method to systematically analyze the excess enthalpy of IL-acetone systems (>300), in terms of the intermolecular interactions contributing to the mixture behavior. Spectroscopic and COSMO-RS results indicated that acetone, as polar compound with strong hydrogen-bond acceptor character, in most cases establishes favorable hydrogen-bonding with ILs. This interaction is strengthened by the presence of an acidic cation and an anion with dispersed charge and non-HB acceptor character in the IL. COSMO-RS predictions indicated that gas-liquid and vapor-liquid equilibrium data for IL-acetone systems can be finely tuned by the IL selection, i.e., acting on the intermolecular interactions between the molecular and ionic species in the liquid phase. NMR measurements for 1

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IL-acetone mixtures at different concentrations were also carried out. Quantum chemical calculations by using molecular clusters of acetone and IL species were finally performed. These results provided additional evidences on the main role played by hydrogen-bonding in the behavior of systems containing ILs and HB acceptor compounds, as acetone. Keywords: Ionic Liquids, acetone, COSMO-RS, NMR, excess enthalpy, molecular interactions

2


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1. Introduction Ionic liquids (ILs) are being investigated by academia and industry in a variety of fields involving chemistry, biochemistry, engineering, pharmacology and many other areas, due to their exceptional properties as almost negligible vapor pressure and tunable solvent capacity. The optimal ion selection among a huge number of cations and anions is a determinant step in the development of IL-based applications. The efficiency of several potential IL applications depends on the intermolecular interactions between the compounds in the liquid mixture, such as absorption, distillation, liquid-liquid extraction, and others. 1 , 2 , 3 An outstanding variety of organic compounds (aliphatic, aromatic, alkanolic, ketonic, etc.) has been studied in separation process involving ILs.4 Depending on the chemical nature of both IL and organic solvent, different intermolecular interactions can be established in the mixture fluid. Hydrogen bonding has been demonstrated playing an important role in pure ILs and in mixtures of ILs with polar solvents presenting hydrogen bond donor (HBD) and acceptor (HBA) character. 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 It has been observed that relevant properties of mixtures including HBD or HBA compounds and ILs, such as phase equilibrium: gas-liquid (GLE), vapor-liquid (VLE) or liquid-líquido (LLE), can be drastically affected by the cation-anion selection.14,15,16,17 Among the growing variety of mixtures studied in the last years formed by ILs and polar organic compounds with HBA character, those including acetone present attractive applications in new processes. Thus, ILs are being investigated for removing acetone from water, 18 for improving the kinetic and selectivity effects in heterogeneous hydrogenation of acetone19 and for analyzing the presence of acetone by microextraction-gas chromatography.20 In addition, ILs can act as entrainer in ternary systems with acetone,21,22,23,24 can be also used to reduce the IL viscosity and the IL consumption in solvent extraction25 and to enhance the kinetic and 3



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solvation effects in the synthesis of imidazolium-based ILs.26 Moreover, acetone has been successfully used as regenerating agent of exhausted adsorbents saturated by ILs. 27 , 28 As consequence, several properties for the acetone-IL system have been measured, including volumetric and excess properties, LLE and VLE data, etc.18,29,30,31As remarkably property, acetone behaves as an almost universal solvent for ILs, dissolving as hydrophilic as hydrophobic ILs, only being reported a few cases of immiscibility with acetone (ILs based on sulfates or phosphate anions).32,33 An important property in the studies on thermodynamic of solutions is the excess enthalpy hE, which is of great utility when analyzing the nature of the molecular interactions in fluid solutions; therefore its knowledge is also interesting for ILcontaining systems. The hE obtained by experimentation are a consequence of the disruption of interactions in the pure compounds and the establishment of new interactions in the mixture. Several research groups have reported experimental excess enthalpies for systems containing ILs and organic solvents such as water, alcohols, ketones, etc.17,34,35,36,37,38,39,40,41,42,43,Error! Bookmark not defined.,44,45 Brennecke et al.41 found both exothermic and endothermic behaviors for water-IL systems depending on the IL counterions presented in the aqueous mixture, which were attributed to the capability of the IL capability to form hydrogen bonds with water molecules. In this sense, Piras et al

44

found

that

IL-water

interactions

became

less

important

as

the

hydrophobic/hydrophilic ratio increased, along with the length of the alkyl chain in the ILs. Navas et al.38 obtained that the hydrophobic [bmim][PF6] (HBD solvent) mixed endothermically with water and alcohols (polar HBD solvents), whereas a highly exothermicity behavior is observed when ILs are mixed with acetone (HBA solvent), due to favorable hydrogen-bonding interactions. These approaches suggest that hE is affected by the nature of both the ionic liquid and the solvent, being the mixture 4


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behavior determined by the different interactions occurring in solution.41,42,

45

Nevertheless, considering the huge amount of possible organic solvent–IL combinations, the available experimental hE data are still limited for a systematic analysis. In this context, a priori computational methods, such as the quantum-chemical approach COSMO-RS, 46 capable of predicting thermodynamic data without needing previous experimental data, are of great utility. 47 COSMO-RS has been shown as a valuable method for predicting a variety of thermodynamic properties of ILs mixtures, 48 , 49 , 50 , 51 , 52 providing a useful computational tool for designing ILs with specific properties.53,54 It has also been reported the suitability of COSMO-RS to predict the hE of the binary mixtures formed by the ionic liquids 1-butyl-X-methylpyridinium tetrafluoroborate (X=2, 3 and 4) with alkanols or water.38,39 A important feature of COSMO-RS is that provides a description of the different intermolecular interactions (electrostatic forces, hydrogen bonding and van der Waals forces) contributing to the effects on the hE.36 Furthermore, COSMO-RS has been successfully applied to analyze the GLE,55,56,57,58,59 VLE,60 LLE36,39 and solid-liquid equilibria (SLE)27 in terms of the COSMO-RS description of the intermolecular interaction contributions to the excess enthalpies of corresponding IL-based mixtures. This work studies the interactions between ionic liquids (ILs) and acetone by means of computational and spectroscopic analysis, with the aim of achieving further understanding on the particular behavior of mixtures including IL and HBA polar compounds. For this purpose, COSMO-RS method was applied for the systematic evaluation of the excess enthalpy behavior of IL-acetone mixtures. Firstly, COSMO-RS capability in predicting hE is evaluated for 49 organic compound-IL systems, through comparison with available experimental data. Then, a computational screening of 312 IL-acetone mixtures is performed, considering different IL structures by varying the 5



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type of anions (12) and cations (26, including imidazolium, pyridinium, pyrrolidinium, phosphonium and ammonium). In addition, the contributions to hE from the intermolecular interactions (Polar-Misfit, hydrogen-bond and van der Waals) between the components of the mixtures are also estimated, in order to evaluate the type of interactions determining the behavior of IL-acetone mixtures. Then, the obtained hE values were related to other significant thermodynamic properties of the mixture, such as GLE, LVE and LLE data, obtaining insights regarding the selection of IL to design new technological processes involving acetone in the mixture. Finally, in order to obtain the aim further insights on the local interactions between IL and acetone components in the mixture, more detailed molecular simulations and

1

H-NMR spectroscopic

measurements were performed. Thus, quantum chemical calculations with molecular models including clusters of acetone with the IL ions were conducted, in order to establish interaction energies and bond distances between mixed species. In addition, 1

H-NMR spectroscopic measurements were carried out for selected IL-acetone mixtures

at different concentrations, providing experimental evidences on the role of hydrogen bonding on the properties of IL-acetone systems.

2. Computational and experimental details In this work, COSMO-RS calculations were carried out following a multistep procedure. First, the software Gaussian03 61 was used for the quantum-chemical calculation to generate the COSMO files for each compound studied. For this purpose, the molecular geometry for every compound (acetone and ILs counterions, see IL abbreviation and IL name listed in Table S1) was optimized at B3LYP/6-31++G** computational level in the ideal gas phase. A molecular model of independent ions was 6


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applied in COSMO-RS calculations, where ILs were treated as equimolar mixture of cation and anion. Vibrational frequency calculations were performed in each case to confirm a minimum energy state. After that, the ideal screening charges on the molecular surface for each species were calculated by the continuum solvation COSMO model using BVP86/TZVP/DGA1 level of theory. Subsequently, COSMO files were used as input in COSMOtherm statistical thermodynamic calculations. In COSMO-RS, the excess enthalpy of a binary mixture is calculated by the algebraic sum of several contributions associated with different types of interactions, including electrostaticmisfit (MFt), Van der Waals (vdW) and hydrogen bonds (HB), so:

h E = h E (MF) + h E (HB) + h E (vdW)

(1)

The summands of Equation 1 result from the contribution of each component of the mixture to the interaction, such that:

h E (MF) = x1[ h1E (MF)] + x2[ h2E (MF)]

(2)

h E (vdW) = x1[ h1E (vdW)] + x2[ h2E (vdW)]

(3)

h E (HB) = x1[ h1E (HB)] + x2[ H 2E (HB)]

(4)

The contributions of each summand in Equation 2-4 to the final values of H mE are obtained from the expression:

hiE (Interaction) = hi,Emixture (Interaction) - hi,Epure (Interaction)

(5)

where the enthalpy of the compound i at a given mixture composition, hiE , is computed as the expectation value of the partition sum of the microscopic segment-segment interaction enthalpies.47 The keyword HE_SPLIT toggles the printing of the three contributions to the total hE (hEHB, hEMisfit and hEvdW) to the COSMOtherm table and output files. 7



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The version C21_0111 of the COSMOthermX program package 62 and its implicit (BP_TZVP_C21_0111) parametrization was used to accomplish the calculations of all the thermodynamic properties (excess enthalpy of the IL-acetone mixtures, Henry’s Law constant for acetone in ILs and total pressure of binary IL-acetone solution) of this work. 1

H-NMR spectra were obtained at room temperature with a Bruker DRX-500

spectrometer (500 MHz), using TMS as calibration standard. Acetone-D6 was used as deuterated solvent for all the measurements. The operating conditions varied were acetone concentration in the ionic liquid (from neat IL to 0.95 % mole of acetone in the mixture. Molecular simulations consist of gas phase geometry optimizations at DFT computational level of different clusters composed by the IL ions and the acetone molecule.) The M05-2X density functional and the 6-31++G(d,p) basis sets have been selected to perform the simulations. M05-2X functional is one of the newest DFT methods that is assumed to correctly reproduce weak interactions 63 . Harmonic vibrational frequency calculations are always carried out on the optimized structures to establish the nature of the stationary points. 3. Results and Discussion COSMO-RS methodology was applied to predict the hEs of the IL-organic compound mixtures with experimental data reported in the literature. A sample of systems chosen contains 49 mixtures and includes 7 different polar solvents (water, methanol, ethanol, 1-propanol, 1-butanol, acetone and 3-pentanone) and 20 different ILs with a variety of cations and anions. Experimental and COSMO-RS values for the maximum (or minimum) of hE curves are compared in Figure 1. As can be seen on the 8


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graph, an appropriate goodness-of-fit is achieved for the wide range of hE values for ILorganic solvent mixture at different temperatures presented in bibliography. The linear fit between experimental and calculated data presents square correlation coefficient R2=0.97 and standard deviation SD=2%. It should be remarked that the reported ILmolecular solvent mixtures present as endothermic as exothermic behaviors depending on the nature of the solvent and the IL forming the mixture, which would determine the change of intermolecular interactions of the mixture components respect to the pure compounds. Some authors have explained the significance of the hE in IL-organic systems in terms of preferential solvation effects between the molecules of the mixtures, for the case of polar compounds as water, alcohols or ketones.13 In order to anticipate the different solvation effects in the mixtures collected in Figure 1, we can consider the empirical solvent scales of dipolarity (SdP), polarizability (SP), basicity (SB) and acidity (SA) developed by Catalan et al.64 whose normalized values for water, acetone and alcohols were collected in Table 1. Analyzing the water-IL mixtures in Figure 1, it was clear that that the hEs of the mixture strongly depends on both the cation and the anion selected for the IL, i.e. the strength of the interactions between water and IL species and the rupture of like forces in pure compounds. Water is an amphoteric compound, which presents high polarity (SdP=0.681 and SP=0.997), high acidity/HBD character (SA=1.062) and weak basicity/HBA character (SB=0.025) as solvent. As a result, the hEs for the binaries containing water and ILs with common cation (1-ethyl-3methyl-imidazolium, [emim]+) goes from endothermic for slightly hydrophobic anion, as [BF4] -, 43 to strongly exothermic for hydrophilic anion, as [CF3CO2]- or [CH3SO3 ]-.41 Similar endothermic/exothermic behaviors are observed for mixtures of fixed alcohol (also amphoteric polar compounds, see Table 1) with different ILs. On the other hand, when the analysis is focused on one IL mixed with alcohols of increasing alkyl chain, it 9



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is observed that the decrease of solvent polarizability of alcohol (see SP values in Table 1) implies lower exothermicity (or higher endothermicity) of its mixture with IL, i.e., less effective interactions between alcohol and IL species. Therefore, the mixing enthalpies of the pairs IL-organic solvent also depends on the molecular structure of organic compound. Remarkably, all the available hE data for IL-ketone systems evidenced exothermic mixture behavior, indicating favorable intermolecular interactions between IL and HBA organic compounds as acetone. Solvent scale parameter of acetone indicates its high polarity (SdP and SP in Table 1) and basicity (SB) associated to the HBA carbonyl group of this compound, but negligible acidic character as solvent (SA in Table 1). These results suggest a different behavior of mixtures involving IL and organic solvent presenting remarkably HBA character, as acetone. An additional advantage of COSMO-RS methodology is that provides the 3D polarized charge distribution (σ, sigma) on the molecular surface, easily visualized on the histogram function σ-profile, which can be used to anticipate the possible interactions of a compound in a fluid. Figure 2 shows the σ-profile of examples for the three kind of molecular solvents collected in Figure 1: alcohol, water and ketones. Thus, the σ-profile of water is dominated by two peaks presented in strongly negative polar regions of the electron lone-pairs of the oxygen atom (peak located at +0.018 e Å-2) and in strongly positive polar hydrogen atoms (peak located at -0.016 e Å-2), in agreement with the polar amphoteric character of this solvent (see SdP, SP, SB and SA parameters in Table 1). The σ-profile of methanol also corresponds to an amphoteric compound, but in this case, containing higher amount of electronic density located in the non-polar regions. It accords empirical solvent scale parameters where polarity for methanol -or other alcohol- is lower than water. A different COSMO-RS description is shown for acetone. Acetone presents an oxygen fragment in polar positive region (peak at +0.013 e 10


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Å-2), some charge concentration in the non-polar region and the absence of atomic fragments with acidic character, anticipating its HBA behavior and relative polarity, in good agreement with the solvent effects measured for this compound (see SdP, SP, SB and SA parameters in Table 1). Further analysis in this work will be centered on analyzing the interactions between IL and acetone, in order to understand the particular behavior of those solutions. Figure 3 compares experimental hE points and those estimated by COSMORS at 353.15 K for the two cases of IL-acetone mixtures reported in bibliography.17 As can be observed, COSMO-RS also provides a reasonable estimation of hE over the whole range of acetone compositions in the mixture, comparable with the predictability observed for other mixtures, as those of IL and alcohols.36,42 It was noted above that the σ-profiles of molecular solvents provided by COSMO-RS seem to anticipate the solvent effects described by the empirical scales. For this reason, we have analyzed the σprofiles of the cation and anion of ILs provided by COSMO-RS in order to evaluate their possible behaviors in the mixture with acetone. In Figure 4, σ-profile of cations are dominated by a main peak with charge distribution in the non-polar region, corresponding to the aliphatic and aromatic groups of the cationic alkyl chain and head group, and some other peaks at lower values than the cutoff -0.082 e Å-2 related to the hydrogen atoms of the aromatic ring. These fragments are responsible for HBD capability of cations, which present an acidity trend of [eim]+ > [bpyr]+ > [emim]+. Hence, the σ-profiles provided by COSMO-RS reveal that hydrogen-bond interactions are possible between the acetone and the cations, involving the –O- atom of the acetone and the acidic hydrogen atoms of the cations, whose HB strength would increase in the order H2 > H3 > H4 ≈ H5 (Figure 1S of Supplementary Material). On the other hand, 11



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regarding COSMO-RS description of anions in Figure 5, the σ-profile of [BF4]corresponds to a peak located at the negative polar region (0.011 e Å-2), which indicates a weak HBA group character. The [C2H5SO4]- anion shows a stronger HBA fragment (SO4 group, at 0.014 e⋅Å-2) and electronic charge located in non-polar regions (-C2H5 chain). Similar description can be done for [CH3SO3]- anion, but in this case the anion presents a still stronger HBA group. Hence, COSMO-RS description suggests different anion capacity to establish hydrogen bond interaction with the cation, which would compete with cation-acetone interactions in the mixture. Figure 6 presents the hE values of 312 acetone-IL equimolar mixtures predicted by COSMO-RS at 298 K (collected in Table S3 of Supplementary Information). Qualitatively the mixing process for the acetone-IL mixtures goes from slightly endothermic to highly exothermic depending on the selection of both cation and anion constituting the IL. Thus, the exothermicity of those binaries clearly increases by the presence of HBD groups linked to cation head group, such as is shown for the imidazolium-based series [eim]+ > [OHemim]+ > [emim]+ > [emmim]+. In this sense, for a common anion, it is generally observed more exothermic mixtures in the order [pyrrolidinium]+ > [pyridinium]+ > [imidazolium]+ > [phosphonium]+ > [ammonium]+, according to the acidic character of the cation family. In addition, the selection of the anion also determines the excess enthalpy behavior of IL-acetone mixtures. It is observed that big anions with disperse charge, lacking the HBA ability, such as [C6F18P]-, [FeCl4]- or [NTf2]-, are those giving the most negative values for excess enthalpies of IL-acetone mixtures; whereas anions with HBA groups, as [CH3SO3]- and [C2H5SO4]-, are the ones providing endothermic systems, suggesting rupture of favorable cation-anion HB interactions with the mixture. As a consequence, the most

12


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negative hE values for IL-acetone mixtures (the white and light grey areas on Figure 6), i.e., the most favorable interactions between IL and acetone, are found for ILs

presenting a cation with strong HBD character, which favors cation-acetone interactions, in combination with non-HBA anions, which are not able to promote strong cation-anion hydrogen bonding. In order to study in detail the intermolecular interactions established in these systems, contributions to hE of IL-acetone mixtures, due to electrostatic (MisFit), van der Walls and Hydrogen Bond interactions, were calculated by using COSMO-RS. Figure 7 presents the particular energetic contributions (MF), (HB) and (vdW) to the excess enthalpies for equimolar mixtures of acetone with imidazolium-based ILs presenting the common anion [NTf2]-, as reference of weakly basic specie. As can be seen, hydrogen bond interactions dominate the behavior of the mixtures, increasing the exothermicity with the acidity of the cation. Polar misfit contributions are also relevant, nearly following the order of H-bond interactions, whereas the corresponding of van der Waals interactions are negligible for these mixtures. Figure 8 depicts the contributions of the different intermolecular interactions to the excess enthalpy of mixtures with acetone and ILs including a common cation [eim]+, which presents a strong HBD character, as we noted above (see discussion on Figure 4). In this case the ILs formed by anions without HBA groups, as [C6F18P]-, [FeCl4]- or [NTf2]-, give place to highly exothermic mixtures with acetone, determined by attractive cation-acetone hydrogen bond interactions. On the contrary, as increases the anion basicity the hE(HB) contribution becomes less relevant or even slightly endothermic, which means that cation-anion interactions compete with acetone-cation interactions, decreasing the affinity of IL for the solvent.

13



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Once hE values for the generally favorable IL-acetone mixtures have been analyzed in terms of the intermolecular interactions between acetone molecules and counterionic species of IL, their possible relationships to macroscopic properties of interest in these mixtures -such as GLE and VLE data- have been evaluated. Previous works showed that hE analysis by COSMO-RS can be successfully used to design ILs with improved thermodynamic properties, for example in certain process such as absorption,55,56,57,58,59 adsorption27 and liquid extraction36,39 with the participation of ILs. As can be seen on Figure 9A, as more exothermic acetone-IL mixture as higher gas solubility of acetone in IL, in concordance agreement with the trends obtained for other solutes as CO2, NH3 and toluene.55,56,57,58,59 The VLE data are also relevant information for practical application as distillation involved IL mixtures. Figure 9B related the HE of mixing to the pressure ratio coefficient (Pmixture/Ppure acetone), as parameter of reference for the volatility change of acetone because of IL mixing, considering the negligible partial pressure of IL at these conditions. The curve shows that while increasing in the exothermicity of the system, the pressure ratio tends to zero. This means that the favorable interactions with IL significantly decrease the partial pressure of acetone in the mixture. In contrast, when acetone-IL interactions are weaker (hE values close to zero), the pressure ratio increases toward Pmixture/Ppure

acetone

values > 1, indicating

systems favorable to IL regeneration by distillation. In order to obtain further insights on the acetone-IL interactions, quantum chemical molecular simulations including clusters of acetone and IL ionic species as well as spectroscopic analysis based on 1H-NMR measurements were performed. Two models of cationic structures were considered in cluster optimizations: the single 1- and the di 1,3-substituted imidazolium cations, [eim]+ and [mmim]+, respectively, along

14


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with [PF6]- anion. As we noted in above, the σ-profiles provided by COSMO-RS for the considered species ([eim]+ and [mmim]+ cations and acetone) reveal that HB donor/acceptor interactions are possible between the acetone and the cations involving the =O atom of the acetone and the –H atoms of the cations. This conclusion can also be drawn from their σ-surfaces shown in Figure 1S of Supplementary Material. Charge distribution in the sigma surfaces suggests that where some potential HBD hydrogen are available, the acidic character decreases in the sequence H3 > H2 > H4. This result is also supported by the atomic Mülliken charges calculated for the gas phase optimized structures of the cations (also shown in Figure 1S). It is interesting to note that atomic charges at the H2 atom is the same for both the [mmim]+ and the [eim]+ cations, advising that their HB donating properties are very similar. The quantum-chemical optimization of clusters including acetone and IL species shows that the total electronic energy of the overall system decreases respect to that of isolated molecules as a result of the [cation-acetone]+ complex formation (Figure 10), which are coherent with the computed negative excess enthalpies for the mixtures of acetone with ILs. Changes in the total electronic energy, ∆E, for the [cation-acetone]+ complex formation are very similar, as expected, for [mmim]+ and [eim]+ cations when the observed HB interaction takes place with H2 proton involvement. Furthermore, the optimized -H…O= nonbonding distances are close in both cases. ∆E is less negative for [eim-acetone]+ complexes in the sequence H3>H2>H4, in good agreement with the HBD character of these groups, previously discussed. Moreover, the -H…O= distance is noticeable shorter in H3-complex (1.735 Å) than in H2-complex (2.041 Å). It is important to underline that [eim-acetone]+ complexes through H4 proton have not been obtained in molecular optimizations. The corresponding final optimized structure in this case is the same to that obtained when acetone directly interacts with H3 proton. Optimized structures of 15



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the of the [eim][PF6] ion pairs (Figure 11) show that [PF6]- anion is roughly placed at the same cation positions that acetone molecule in the corresponding complexes (Figure 10). This means that [PF6]- anion and acetone molecule compete by the same proton donor center at the [eim]+ in mixtures of the [eimPF6] ionic liquid and acetone. Moreover, optimized structures of the [eim-acetone]+ complexes and the [eimPF6] ion pair show that cation-acetone interactions have a specific character, as correspond to hydrogen bonding interactions. Competence of the acetone molecules and [PF6]- anion by the acid centers at the [eim]+ cation is likewise observed in the optimized structures of the [eimPF6-acetone] clusters (Figure 12), where it is found that acetone is able to displace the anion from its interactions with the most acid H3 center (Figure 12). Nuclear magnetic resonance (NMR) is one of the most widely used spectroscopic technique to investigate 1-alkyl-3-methylimidazolium ionic liquids,11,65,66 since it allows obtaining a better understanding about the nature of cation-anion or solvent-IL interactions appearing in solution. Previous 1H-NMR study by Zhai et al.67 provided experimental evidences on the weakness of the cation-anion interaction as a result of the acetone addition in some 1-alkyl-3-methylimidazolium PF6 and BF4 ILs. In this study, 1

H-NMR spectra were obtained at room temperature for IL-acetone binary mixtures

with different composition. Figure 13A shows 1H-NMR spectra for [bmim][NTf2] where most relevant peaks are located between 6 and 10 ppm belonging to aromatic hydrogen atoms (see H2, H4, H5 in Table 2). As it can be observed, the value of 1H chemical shift of the most acidic ring proton (H2) progressively increases with acetone concentration (∆δ≈1.5 ppm from neat IL to 5% w/w of IL), assignable to the formation of local interactions (hydrogen bonding) between the acetone’s carbonyl group and this cation-acidic hydrogen atom.15 Figure 13B and Table 2 shows similar displacement of

16


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the [H2] signal in [eim][NTf2] mixtures with acetone. These 1H-NMR spectra are consistent with the results obtained by both COSMO-RS and cluster simulations regarding the presence of hydrogen-bonding between acetone and IL compounds in the mixture. Figure 13B also presents a significant larger displacement at higher 1H chemical shifts for the most acidic proton of cation [H3] of [eim]+ when acetone concentration increases (∆δ≈2.8 ppm from neat IL to 5% w/w of IL), revealing stronger HB interactions between acetone and [eim]+ cation than with [bmim]+ specie, also in agreement with the conclusions achieved by COSMO-RS and cluster quantum-chemical simulations. To complete the analysis, Table 3 collects the difference between the computed 1H- and

16

O- NMR spectra of the complexes [cation-acetone]+ and the

individual (cations and acetone) species at GIAO/M052X/6-311++G(d,p) quantumchemical level. The predicted displacements of the chemical shifts (Table 3) due to cation-acetone interactions reproduce reasonably well the experimental trends, obtaining additional evidences on the presence of hydrogen-bonding between acetone and IL, which were strengthened by the presence of more acidic centers in the cation.

Conclusions The interactions between ionic liquids (ILs) and acetone have been studied to obtain further understanding on the behavior of their mixtures, which generally give place to exothermic process, mutual miscibility and negative deviation of Raoult’s law. COSMO-RS was used as suitable computational method to systematically analyze the excess enthalpy of IL-acetone systems (>300), in terms of the intermolecular interactions contributing to the mixture behavior. Spectroscopic and COSMO-RS results indicated that acetone, as polar compound with strong hydrogen-bond acceptor 17



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Page 18 of 38

character, in most cases establishes favorable hydrogen-bonding with ILs. This interaction is strengthened by the presence of an acidic cation and an anion with dispersed charge and non-HB acceptor character in the IL. COSMO-RS predictions indicated that gas-liquid and vapor-liquid equilibrium data for IL-acetone systems can be finely tuned by the IL selection, i.e., acting on the intermolecular interactions between the molecular and ionic species in the liquid phase. NMR measurements for IL-acetone mixtures at different concentrations were also carried out. Quantum chemical calculations by using molecular clusters of acetone and IL species were finally performed. These results provided additional evidences on the main role played by hydrogen-bonding in the behavior of systems containing ILs and HB acceptor compounds, as acetone. The interactions between ionic liquids and acetone were studied with the aim of achieving further understanding on the particular behavior of these mixtures including IL and hydrogen-bond donor polar compounds. The suitability of COSMO-RS method to reasonably predict the excess enthalpy (hE) of IL-organic solvent mixtures was first validated by comparison to experimental available data for 49 systems. A computational COSMO-RS screening of 312 IL-acetone mixtures, including 12 anions and 26 cations of different families, was performed to evaluate the hE behavior, observing higher exothermic mixing when increasing the acidic character of the cation in combination with great and charge disperse anion, whereas the presence of hydrogen-bond acceptor anion promotes endothermic mixing phenomena. The COSMO-RS analysis of the intermolecular interactions contributing to hE values indicated that hydrogen bonding plays a main role in the behavior of IL-acetone mixtures, which determine their thermodynamic properties such as GLE, LVE and LLE data. Spectroscopic measurements by 1H-NMR technique at different concentrations demonstrated the 18


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presence of local hydrogen-bond interactions between acetone molecule and the hydrogen-bond donor groups of the IL cation, increasing the chemical shifts of NMR signals with acidic character of such cationic groups. Further molecular simulations were carried out on clusters of acetone and IL ions at high quantum-chemical computational level, obtaining interaction energies and bond distanced as additional evidences on the main role of hydrogen bonding on the properties of IL-acetone systems. The results of this work provide useful insights regarding the selection of IL to design new technological processes involving acetone in the mixture. Acknowledgements The authors are grateful to the ‘‘Ministerio de Economía y Competitividad’’ and ‘‘Comunidad de Madrid’’ for financial support (projects CTQ2011-26758 and S2009/PPQ-1545, respectively). Authors are also very grateful to “Centro de Computación Científica de la Universidad Autónoma de Madrid” for computational facilities. Supporting Information Available: Figure 1S: Sigma surfaces of the [eim]+ and [mmim]+ cations and the acetone calculated by COSMO-RS method. Mulliken atomic charges (calculated at M052X/6-311**G(d,p) computational level) of some selected atoms are also shown. Table S1: Names and abbreviations of the studied IL counterions. Table S2: Comparison between hE experimental data collected from literature and those predicted by COSMO-RS. Table S3: Predicted hE values of 312 acetone-IL equimolar mixtures computed by COSMO-RS at 298 K.

19



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water

alcohol

ketone

5 R² = 0.9753

hE EXP. (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

-5 -5

0

5

hE COSMO-RS (kJ/mol)

Figure 1. Comparison of experimental and COSMO-RS calculated values for the maximum (or minimum) of hE curves of IL-organic compound mixtures at different temperatures (ranging between 298.15 and 353.15 K). Water (white), alcohols (grey) and ketones (black).

1 ACS Paragon Plus Environment


12

acetone

9

p(σ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6

water

methanol

3

0 -0.03

-0.02

-0.01

0

0.01

0.02

0.03

σ [e/A2] Figure 2. σ-Profiles of pure compounds: methanol, acetone and water obtained by COSMO-RS.


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Experimental

COSMO-RS

0

hE (kJ/mol)

-0.5

-1

-1.5

-2 0

0.25

0.5

0.75

1

xacetone

Experimental

COSMO-RS

0

-0.5

hE (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

-1.5

-2 0

0.25

0.5

0.75

1

xacetone Figure 3. Experimental () and COSMO-RS () curves for excess molar enthalpy of 3 ACS Paragon Plus Environment


(A) [bmim][NTf 2 ] + acetone and (B) [hmim][NTf 2 ] + acetone mixtures at T = 353.15 K.

40

p(σ)

30

[emim]+

20

10

[bpyr]+ [eim]+

0 -0.03 -0.02 -0.01

σ

0.0

[e/A2]

0.01

0.02

0.03

Figure 4. σ-Profiles of different cations obtained by COSMO-RS: [eim]+ (light solid line), [bpyr]+ (dashed line) and [emim]+ (dark solid line).

[PF6]-

40

[NTF2]30

p(σ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 38

[BF4]-

20

[C2H5SO4][CH3SO3]-

10

0 -0.03 -0.02 -0.01

0.0

0.01

0.02

0.03

σ [e/A2] Figure 5. σ-Profiles of different anions obtained by COSMO-RS. 4 ACS Paragon Plus Environment

0

0.5

-2.5

-5.0

C2H6PO4

C8H14SO3N

Anions

CH3SO3

tol-SO3

C2H5SO4

DCN

BF4

BC4N4

PF6

NTf2

-10 FeCl4

eeee-P eeee-N prmpyr emmim bbbb-N bbbb-P bmmim ommim meO-emim omim hxmim mmim epy emim OH-prmim opy OH-omim bmim OH-prpy OH-emim mim eim bpyr opyr b-N m-N

C6F18P

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cations

Page 31 of 38

Figure 6. Screening of COSMO-RS predicted excess molar enthalpies hE (kJ/mol) for 312 equimolar acetone-IL mixtures at T= 298 K.



hE (MF) HE' (MF)

hE (HB) HE' (HB)

hE (vdW) HE' (VDW)

2.5

eim

emmim

meO-emim

emim

bmim

hxmim

-5

OH-emim

-2.5

omim

0

hE (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 38

-7.5 -10

Cation acidity cation + NTf2

Figure 7. Description of the cation effect on the excess molar enthalpy of acetone-IL equimolar mixture in terms of the intermolecular interaction contributions [hE(MF), hE (HB) and hE (vdW)] computed by COSMO-RS at T=298 K.


-10

C8H14SO3N

C4H10SO4

C2H6PO4

CF3CO2

BC4N4

HE' (VDW) hE (vdW)

FeCl4

NTf2

-5

PF6

0

C6F18P

hE (kJ/mol)

HE' (HB) hE (HB)

CH3SO3

HE' (MF) hE (MF)

5

Anion basicity eim + anion

-15

Figure 8. Description of the anion effect on the excess molar enthalpy of acetone-IL equimolar mixtures in terms of the intermolecular interaction contributions [hE(MF), hE (HB) and hE (vdW)] computed by COSMO-RS at T=298 K.

5

5

0

0

hE (kJ/mol)

hE (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DCN

Page 33 of 38

-5 -10

-5 -10 -15

-15

(A) -20 -0.02

(B) -20

0.00

0.02

0.04

0.06

0.08

0.10

-0.5

0

0.5

KH acetone (MPa)

1

1.5

2

2.5

3

Pmixture/Ppure (kPa)

Figure 9. Relationship between the hE of mixing and A) the acetone Henry’s constant in IL; B) the vapor pressure ratio coefficient for all the studied acetone-IL systems, computed by COSMO-RS at 298 K.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[mmim-acetone]+

[eim-acetone]+ (-H 2 …O=)

0.53

Page 34 of 38

[eim-acetone]+ (-H 3 …O=)

0.51

-0.47

-0.47

0.20

0.23

0.48 -0.11

-0.36

-0.18

0.42 -0.11 -0.19 0.40

-0.08

ΔE = -15.6 kcal/mol



Figure 10. Optimized structures of the [cation-acetone]+ complexes for both [mmim]+ and [eim]+ cations at M052X/6-311++G(d,p) computational level. Computed nonbonding interatomic distances (Å) and Mülliken atomic charges for some selected atoms are also included. Energy differences correspond to the Total Electronic Energy (E) being

defined

as:

∆E = Ecation − acetone − (Ecation + Eacetone ) for

the

model

process:

[cation]+ + [ acetone ] 0 → [cation − acetone]+ .


Page 35 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[eimPF 6 ] (-H 3 …F- interaction)

(-H 2 …F- interaction)

(-H 4 …F- interaction)

-0.75

-0.79

0.23

-0.75

-0.06

0.29

-0.18

0.03 -0.21

-0.75

0.28

0.17

-0.23

-0.70 -0.17

0.49

0.39




Figure 11. Optimized structures of the [eimPF 6 ] ion pairs at M052X/6-311++G(d,p) computational level. Computed non-bonding interatomic distances (Å) and Mülliken atomic charges for some selected atoms are also included. Energy differences correspond

to

(

the

Total

Electronic

Energy

(E)

being

defined

as:

)

∆E = EeimPF6 − Eeim − EPF6 for the model process: [eim]+ + [ PF6 ] − → [eimPF6 ] .



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 38

[eim-PF 6 -acetone]

A 0.40 0.35 -0.48 0.28 -0.13 -0.17 -0.15

-0.18

0.52

0.22

-0.72

2.229

-0.73

ΔE = -98.2 kcal/mol B -0.77 -0.71 0.24

-0.78 0.39

0.25

-0.44

-0.27 -0.14

-0.19

0.53


Figure 12. Optimized structures of the [eim-PF 6 -acetone] aggregates at M052X/6311++G(d,p) computational level. Computed non-bonding interatomic distances (Å) and Mülliken atomic charges for some selected atoms are also included. Energy differences correspond to the Total Electronic Energy (E) being defined as:

ΔE = E aggregate − (E cation + E anion + E acetone ) for

the

model

process:

[cation]+ + [ anion ] − + [ acetone ] → [cation − anion − acetone].


Page 37 of 38

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


A) [bmim][NTf2]

B) [eim][NTf2]

[bmim+] [H2] [H4] [H5]

[H2]

Neat IL

[eim+] [H2] [H4] [H2] [H5]

[H3]

[H5] [H5]

[H3]

[H4]

[H4]

5% acetone

50% acetone

95% acetone

12

10

8

ppm

6

12

10

8

6

ppm

Figure 13. 1H-NMR spectra of (A) [bmim][NTf 2 ] and (B) [eim][NTf 2 ]: pure IL and IL-acetone mixtures with increasing mass concentration of acetone.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

Table 1. Empirical scales of dipolarity (SdP), polarizability (SP), basicity (SB) and acidity (SA) for organic solvents, developed by Catalan et al.64 Solvent water methanol ethanol 1-propanol 1-butanol acetone

SdP

SP

0.681 0.608 0.633 0.658 0.674 0.651

0.997 0.904 0.783 0.748 0.655 0.907

SB 0.025 0.545 0.658 0.782 0.809 0.475

SA 1.062 0.605 0.400 0.367 0.341 0.000

Table 2. Experimental 1H-NMR chemical shifts (δ, ppm) of neat [bmim][NTf 2 ] and [eim][NTf 2 ] and mixtures with acetone at increasing mass concentration of acetone. 1

H-NMR δ (ppm)

Neat IL 5% Acetone 50% Acetone 95% Acetone

[eim][NTf 2 ]

[bmim][NTf 2 ]

H2 7.5 7.6 8.4 9.0

H3 -

H4 6.4 6.5 7.2 7.7

H5 6.2 6.4 7.2 7.6

H2 7.5 7.6 8.5 9.0

H3 10.5 10.9 12.7 / 12.5 / 12.3 13.3 / 13.1 / 12.9

H4 6.4 6.5 7.3 7.7

H5 6.3 6.4 7.2 7.7

Table 3. Difference between 1H- and 17O-NMR chemical shifts (Δδ) of [cationacetone]+ complexes and individual species calculated at GIAO/M052X/6-311++G(d,p) level. TMS and H 2 O are used as reference compound to obtain 1H- and 17O-NMR Δδ values, respectively. Δδ, ppm +

[mmim-acetone] [eim-acetone]+ (-H 2 … O=) [eim-acetone]+ (-H 3 … O=)

H2 2.8 2.9

H3 --0.2

H4 -0.1 -0.1

O -96.9 -98.2

0.3

6.0

-0.01

-131.1


Interactions of Ionic Liquids and Acetone: Thermodynamic Properties

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