Density and Molar Volume Predictions Using ... - ACS Publications

Ind. Eng. Chem. Res. 2007, 46, 6041-6048

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GENERAL RESEARCH Density and Molar Volume Predictions Using COSMO-RS for Ionic Liquids. An Approach to Solvent Design Jose´ Palomar,*,† Vıćtor R. Ferro,† Jose´ S. Torrecilla,‡ and Francisco Rodrı´guez‡ Seccio´ n de Ingenierıá Quı´mica (Departamento Quı´mica Fı´sica Aplicada), UniVersidad Auto´ noma de Madrid, Cantoblanco, 28049 Madrid, Spain, and Departamento de Ingenierıá Quı´mica, UniVersidad Complutense de Madrid, 28040 Madrid, Spain

The specific density and molar liquid volume of 40 imidazolium-based ionic liquids were predicted using the COSMO-RS method, a thermodynamic model based on quantum chemistry calculations. A molecular model of ion pairs was proposed to simulate the pure ionic liquid compounds. These ion-paired structures were generated at the B3LYP/6-31++G** computational level by combining the cations 1-methyl- (Mmim+), 1-ethyl- (Emim+), 1-butyl- (Bmim+), 1-hexyl- (Hxmim+), and 1-octyl-3-methylimidazolium (Omim+) with the anions chloride (Cl-), tetrafluoroborate (BF4-), tetrachloroferrate (FeCl4-), hexafluorophosphate (PF6-), bis(trifluoromethanesulfonyl)imide (Tf2N-), methylsulfate (MeSO4-), ethylsulfate (EtSO4-), and trifluoromethanesulfonate (CF3SO3-). Satisfactory agreement with the available experimental measurements was obtained, showing the capability of the current computational approach to describe the effect of the anion nature and cation substituent on the volumetric properties of this family of ionic liquids. Thus, calculated and experimental density values of ionic liquids (and also other common solvents) were fitted by linear regressions with correlation coefficients R > 0.99 and standard deviations SD < 20 kg/m3. Consequently, molar liquid volumes were also predicted very accurately by COSMO-RS, indicating the suitability of the ion-pair model to describe intermolecular interactions of pure ionic liquids. In this sense, the σ-profiles of the ion-paired molecules were used to qualitatively analyze the influence of cation and anion natures of ionic liquids on their volumetric properties. As a result of the analysis, we propose the charge distribution area below the σ-profile (Sσ-profile) as a simple a priori parameter to characterize the contributions of cation and anion to the ionic liquid behavior as tool to design solvents. 1. Introduction Room-temperature ionic liquids (ILs) have gained popularity in recent years as suitable green solvents due to their unique properties: negligible vapor pressure, high thermal and chemical stabilities, wide liquid-state range, and high solvent capacity.1,2 Thus, the application of these novel solvents to organic synthesis, catalysis, electrochemistry, biocatalysis, materials science, and separation processes is being extensively investigated.3-5 A main advantage of these new solvents is that the cation and anion can be selected from among a huge diversity to obtain an appropriate ionic liquid for a specific purpose. Therefore, ILs are often referred to as designer solVents. However, the practical application to industrial processes of the large number of possible ionic liquids is still limited by the scarce available experimental data. A tremendous amount of work is being carried out in the experimental determination of thermophysical properties of ionic liquids and their mixtures. In this sense, the recently presented IUPAC Ionic Liquid database (ILThermo) provides up-to-date information on IL publications with more than 29 000 total data points at the moment.6 At this stage of development of ionic liquids, the application of predictive theoretical models to estimate their thermophysical properties * To whom correspondence should be addressed. Tel.: 34 914 972 859. Fax: 34 914 973 516. E-mail: [email protected]. † Universidad Auto ´ noma de Madrid. ‡ Universidad Complutense de Madrid.

is of great interest. First, the design of efficient ILs for a chosen chemical reaction or a separation process by appropriate computational simulations would be less time-consuming and less cost-intensive. Second, these models should provide better understanding of the behavior of ILs as solvents. At the moment, the application of classical thermodynamic models, such as NRTL, UNIQUAC, and UNIFAC, to IL systems has been limited in most cases to correlating the experimental equilibrium data, even when the increasing amount of experimental data has recently allowed the determination of their interaction parameters.7,8 To our knowledge, no applications to predict volumetric properties of ILs have been reported. In contrast, quantum chemical methods have already been applied by Banerjee et al.9 to derive volume and surface parameters of the isolated molecules of imidazolium-based ILs, using a polarizable continuum model (PCM). Thereby, these structural parameters were used to estimate the UNIQUAC interaction parameters for a number of ILs.9 On the other hand, densities of ILs have been simulated using molecular dynamics to validate new force fields.10,11 Klamt and co-workers12-16 proposed a completely new perspective in fluid-phase thermodynamics. These researchers have developed a quantum chemical approach (COSMO-RS) for the prediction of the thermodynamic properties of pure and mixed fluids using only structural information of the molecules. In contrast to group contribution methods, which depend on an

10.1021/ie070445x CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007

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extremely large number of experimental data, COSMO-RS calculates the thermodynamic data from molecular surface polarity distributions, which result from quantum chemical calculations of the individual compounds of the mixture. The different interactions of molecules in a liquid, that is, electrostatic interactions, hydrogen bonding, and dispersion, are represented as functions of surface polarities of the partners. Using an efficient thermodynamic solution for such pairwise surface interactions, COSMO-RS finally converts the molecular polarity information into standard thermodynamic data. Concerning the application of COSMO-RS to the study of ionic liquids, several publications demonstrate the general suitability of this a priori method to calculate activity coefficients and phase equilibrium data (vapor-liquid equilibria (VLE), liquid-liquid equilibria (LLE)) of these systems.13,17-24 Tendencies due to temperature changes or the modification of IL concentrations are described in accordance with experimental results. Equally, the modification of phase equilibria, caused by the variation of the anion or the cation of an IL, is qualitatively simulated by COSMO-RS. In addition, this computational procedure was used to perform a thermodynaminc optimization of ILs as entrainers in distillative separations.19 In cited works, a molecular model of independent counterions was applied, in which the screening charge densities of the cation and anion are calculated separately and then combined to a single sigma profile to define the ionic liquid compound.13,17-22 A different computational approach consists of considering the ion-paired structure to simulate the ionic liquid compound. In this case, the sigma profile of the ionic liquid compound is obtained for the molecule as a whole. This alternative molecular model has successfully been applied to predict the VLE of ionic liquid systems using COSMORS.23,24 The current version of COSMOtherm software25 allows for computing the pure compound liquid density of a given substance at 298 K. The liquid density Fi of a pure compound i is computed from the corrected molar liquid volume V ˜ i of the compound (MWi is the molecular weight of the compound and NA is Avogadro’s constant):

Fi )

MWi V ˜ iNA

(1)

The corrected molar liquid volume V ˜ i is computed from a quantum structure-property relationship (QSPR): COSMO V ˜ i ) cHMFHMF + cHHBHHB + cM2Mi2 + i i + cVCOSMOVi

+ c0 + cNringNring i

∑A cAk

element

Aki (2)

The descriptors for the corrected molar liquid volume are the pure compound misfit interaction enthalpy HMF i , the pure compound hydrogen-bonding enthalpy HHB i , the COSMOVolume of the compound as given in the compound COSMOfile VCOSMO , the second σ-moment of the compound M2i , the i number of ring atoms in the compound Nring i , and the areas of surface in a given compound that belong to atoms of the same element type Aki , where k is the element number.26 In this work, we perform for the first time COSMO-RS calculations to predict volumetric properties, as density and molar liquid volume, of pure ionic liquid solvents. For this purpose, the ion-pair geometries of 40 imidazolium-based ILs have been optimized at a suitably high quantum-chemical level.27 In the current molecular model, the surface polarity information of both counterions interacting as a whole is used

in COSMO-RS calculations. Then, COSMO-RS parameters related to charge distribution of these molecular structures are analyzed with the aim of (i) obtaining some light on the solvent behavior of IL compounds and (ii) establishing a simple quantitative parameter to characterize the counterion natures as tool in designing these solvents. 2. Computational Details The molecular geometry of all compounds (common solvents and ionic liquids) were optimized at the B3LYP/6-31++G** computational level in the ideal gas phase using the quantum chemical Gaussian 03 package.28 As a molecular model to simulate the pure ionic liquid, ion-paired structures including both counterions were optimized as a whole. Vibrational frequency calculations were performed for each case to confirm the presence of an energy minimum. Then, the standard procedure was applied for COSMO-RS calculations, which consists of two steps: First, Gaussian 03 was used to compute the COSMO files. The ideal screening charges on the molecular surface for each species were calculated by the continuum solvation COSMO model using the BVP86/TZVP/DGA1 level of theory.29-31 Subsequently, COSMO files were used as input in COSMOtherm9 statistical thermodynamic code to calculate the specific density and molar liquid volume of the chemical species studied at 298 K. According to our chosen quantum method, the functional and the basis set, we used the corresponding parametrization (BP_TZVP_ C21_0105) that is required for the calculation of physicochemical data and contains intrinsic parameters of COSMOtherm as well as element-specific parameters. The experimental data of pure ionic liquids used in this work to validate the computational predictions are available, in most cases, in the recently presented IUPAC Ionic Liquid database (ILThermo)6 and were chosen on the basis of the lowest assigned uncertainty among the reported data. In this work, molar liquid volume (V ˜ ) is expressed in this work as the volume of one molecule in Å3, being related to molar volume (Vm) (cm3/mol) by the expression

Vm [cm3/mol] ) (V ˜ [Å3/molecule])(10-24 [cm3/Å3]) × (6.02 × 1023 [molecules/mol]) (3) 3. Results First, in order to validate the capability of the COSMO-RS computational approach to predict volumetric properties of common organic solvents, the specific density is calculated at 298 K for a series of alcohols and water, which allows covering a wide range of values: from ethanol (F ) 789 kg/m3) to trichloroethanol (F ) 1560 kg/m3). The results of these density calculations are collected in Table 1. As can be seen, the correspondence between experimental32 and COSMO-RS predictions is very good, for which the root-mean-square deviation (rmsd) is less than 1.7%. Accordingly, as density is directly related to molar volume, the computed molar liquid volumes of these compounds are in satisfactory agreement with the experimental one, as is clearly evident from Table 1 (rmsd < 1.8%). In view of these promising preliminary results, the next step was to evaluate the suitability of simulating the pure ionic liquids by a simple molecular model as the ion-paired structure. For this purpose, the molecular geometries of ion pairs for two very common types of imidazolium-based ionic liquids (those including BF4- or PF6- anion) were optimized at the quantum chemical level described above. The structures obtained are

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Figure 1. Molecular structure of ion-paired species calculated at B3LYP/6-31++G** computational level in gas-phase environment for BmimBF4 (A), BmimPF6 (B), BmimCl (C), and BmimCF3SO3 (D). Table 1. Density (G) and Molar Liquid Volume (V ˜ ) Predictions Using COSMO-RS for Alcohols and Water at 298 K F (kg/m3) ethanol butanol octanol 1,2-hexanediol water 1,3-propanediol ethylene glycol chloroethanol glycerol dichloroethanol trichloroethanol

V˜ (Å3)

exptl32

COSMO-RS

% deviation

exptl32

COSMO-RS

% deviation

789 810 827 951 997 1053 1113 1201 1281 1404 1560

769 795 821 935 995 1051 1121 1161 1261 1385 1544

2.5 1.8 0.7 2.1 -0.2 0.2 -0.7 3.3 1.6 1.3 1.0

97 152 261 206 30 120 93 111 119 136 159

100 155 263 210 30 120 92 116 119 138 161

-3.1 -2.0 -0.8 -1.9 0.0 0.0 1.1 -4.5 0 -1.5 -1.3

Table 2. Density (G (kg/m3)) Predictions Using COSMO-RS for 1-Alkyl-3-methylimidazolium ILs at 298 K, Employing an Ion-Pair Model To Simulate the Pure Compounda Mmim COSMO ClBF4EtSO4MeSO4CF3SO3PF6FeCl4Tf2Na

1290 1306 1332 1377 1466 1478 1572 1578

exptl

132741 (-3.6%)

157047 (-0.5%)

Emim COSMO 1235 1260 1284 1325 1399 1413 1506 1531

Bmim exptl

125236 (-0.6%) 123940 (-3.5%) 138543 (-1.0%) 152137 (-0.6%)

Hxmim

Omim

COSMO

exptl

COSMO

exptl

COSMO

exptl

1169 1194 1222 1244 1319 1321 1423 1453

108033 (-7.6%) 120537 (0.9%)

1110 1147 1174 1193 1242 1266 1352 1395

104034 (-6.3%) 114838 (0.1%)

1073 1107 1140 1162 1199 1216 1300 1345

100735 (-5.9%) 110339 (-0.4%)

121242 (-2.6%) 130137 (-1.4%) 136744 (3.3%) 138046 (-3.0%) 143648 (-1.1%)

129445 (2.2%) 133346 (-1.4%) 137037 (-1.8%)

123739 (1.7%) 128046 (-1.5%) 132537 (-1.5%)

Percent experimental - predicted deviations in parentheses.

reported in parts A and B of Figure 1 for BmimBF4 and BmimPF6, respectively. Therein, the optimized ion pairs were used in COSMO-RS calculations to estimate the specific density of 10 1-alkyl-3-methylimidazolium ionic liquids (MmimBF4, EmimBF4, BmimBF4, HxmimBF4, OmimBF4, MmimPF6, EmimPF6, BmimPF6, HxmimPF6, and OmimPF6; where Mmim+ ) 1-methyl-, Emim+ ) 1-ethyl-, Bmim+ ) 1-butyl-, Hxmim+ ) 1-hexyl-, and Omim+ ) 1-octyl-3-methylimidazolium; BF4) tetrafluoroborate; PF6- ) hexafluorophosphate). All the predicted density values in this work are collected in Table 2, together with the available experimental data of ionic liquid

densities.6,33-48 Results of density prediction for these ionic liquids are compared to those of common organic solvents in Figure 2. As can be seen, the density of these ionic liquids is predicted with similar high accuracy as of alcohols and water. In fact, an excellent linear relationship is found between experimental and calculated data for these compounds, including 11 common organic solvents and seven imidazolium-based ionic liquids. The linear regression fit presents an excellent correlation coefficient, R ) 0.999, and a slope of 1.01. The root-meansquare deviation of the 18 calculated densities is again less than 1.7%. It is important to note that predictions for ionic liquids

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Figure 2. Comparison of experimental and COSMO-RS calculated values of specific density for some common solvents (alcohols and water) and XmimBF4 and XmimPF6 ionic liquids at 298 K.

follow the experimental tendencies related to cation and anion effects on densities. Thus, the increase of the alkyl chain length of the cation rises to significantly lower densities (calculations predict density decreasing about 15-20% on going from 1-methyl to 1-octyl substituent in BF4- and PF6- cases). In addition, the choice of the anion has also a noteworthy effect on ionic liquid densities, since predictions for compounds with the same cation but different anion (BF4- or PF6-) describe density differences around 10-15%. Based on the above satisfactory results, the application of the current computational approach has been generalized to estimate the volumetric properties of a wide collection of representative 1-alkyl-3-methylimidazolium ionic liquids (including five cations: Mmim+, Emim+, Bmim+, Hxmim+, and Omim+; and eight anions: tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), chloride (Cl-), tetrachloroferrate (FeCl4-), bis(trifluoromethanesulfonyl)imide(Tf2N-),methylsulfate(MeSO4-), ethylsulfate (EtSO4-), and trifluoromethanesulfonate (CF3SO3-)). The analysis of Table 2 indicates a very good prediction of the density of ionic liquids by COSMO-RS, with a value of rmsd ) 3.0% for a data sheet with 23 ionic liquids whose experimental density is available in the bibliography. Figure 3 can be used to see the good linear relationship between experimentally measured and COSMO-RS calculated densities of imidazoliumbased ionic liquids (compounds with BF4- and PF6- are not included here). Again, the linear fit presents a high correlation coefficient (R ) 0.995) with a slope slightly higher (1.14) and an intercept value higher (and negative) than those shown in Figure 2. In sum, for the wide range of imidazolium compounds with Cl-, FeCl4-, Tf2N-, MeSO4-, EtSO4-, and CF3SO3anions, we find a satisfactory prediction of specific density with accuracies of 0.1-8% (see Table 2), but with a linear relationship between experimental and calculated values somewhat different with respect to that of alcoholic solvents and BF4and PF6- imidazolium based ionic liquids. The reason for this deviation is not evident at the moment and implies absolute density differences lower than 9%. Further calculations are now being performed in our laboratory extending the type of cation and anion, but also including more than one conformation, in order to obtain a more general density prediction using COSMORS. In any case, current predictions of ionic liquid densities are very reliable (rmsd < 3%) and they are able to reproduce the tendencies related to anion and cation nature effects. In

Figure 3. Comparison of experimental and COSMO-RS calculated values of specific density (kg/m3) for 23 ionic liquids based on 1-alkyl-3methylimidazolium cation at 298 K.

Figure 4. Comparison of experimental and COSMO-RS calculated values of molar liquid volume for common and IL solvents at 298 K.

addition, the molar liquid volumes computed by COSMO-RS at current calculation levels present an excellent correspondence with the experimental available data for common solvents32 and ILs49,50 (see Figure 4 and Table 3). The counterion effects on IL volumetric properties are analyzed on the basis of data in Tables 2 and 3: (i) It seems clear that the choice of the anion has a primary effect on the ionic liquid density, increasing for a fixed cation in the sequence Cl- < BF4- < EtSO4- < MeSO4- < CF3SO3- < PF6- < FeCl4- < Tf2N- for each 1-alkyl-3-methylimidazolium series studied. This computational evidence reproduces the tendency of the most complete experimental available data corresponding to 1-butyl-3-methylimidazolium series (see Table 2). As a result, we infer that the densities of these ionic liquids increase with the molecular weight of the anion, but this is not a general rule. For example, imidazolium ionic liquids with EtSO4- anion are less dense than those with MeSO4- anion. (ii) There is also a significant density dependence on the alkyl substituent size in the imidazolium cation ring. Thus, the increase of the number of carbon atoms in the chain gives a minor calculated density for each series of imidazolium-based ionic liquids studied. These


Figure 5. Comparison of computed molar liquid volumes and molecular weights for some characteristic imidazolium (Xmim) based ILs at 298 K. Table 3. Predicted Molar Liquid Volume (V ˜ (Å3)) by COSMO-RS for 1-Alkyl-3-methylimidazolium ILs at 298 K, Employing an Ion-Pair Model To Simulate the Pure Compounda Mmim

Emim

Cl-

BF4EtSO4MeSO4CF3SO3PF6-

171 234 277 251 279 272

197 261 306 279 309 301

FeCl4Tf2N-

311 397

340 424 (429)16a

a

Bmim 248 314 (312)16a 359 334 363 (367)16b 357 (340)16a 393 479 (485)16a

Hxmim 303 368 414 387 423 410

Omim 357 423 467 438 477 465

448 502 533 (543)16a 587 (601)16a

Experimental values in parentheses.

results agree with the experimental measurements collected in Table 2, as is evident from the complete series of Tf2N- anion. Therefore, it is clear that the increase of the density of these ionic liquids is not proportional to the molecular weight of the imidazolium cation. This evidence indicates that the physicochemical intermolecular interactions must be determinant for the volumetric properties of pure ILs. Since density values result from the ratio of molecular weight and molar liquid volume (eq 1), the comparison of both parameters should easily manifest how the molecular weight of the anion influences differently the IL density from that of the cation. Such a comparison is performed in Figure 5, where it can be observed that the molar liquid volume of all ionic liquids increases with the molecular weight of both anion and cation, but in different proportions. Thus, if one considers an IL series with a fixed anion, the effect of the cation is described by tendency lines with a slope greater than 1. Taking into account that both axes present the same numerical scale, it implies a MWi/V ˜ i ratio less than 1. As a consequence, the densities of these ionic liquids must decrease with the cation size. On the contrary, the anion effect is related to tendencies with MWi/V ˜ i ratios greater than 1. Then, an anion with higher molecular weight generally implies higher density of the ionic liquid. However, as we noted above, there are exceptions to this rule: for example, those anions with unsubstituted alkyl groups (MeSO4- and EtSO4-) are slightly out of the general anion effect shown in Figure 5. In sum, current COSMO-RS results reveal that the chemical nature of the counterions has a relevant role in the volumetric

properties of ILs, which should be ascribed to different contributions to the intermolecular interactions in the fluid. An additional advantage of COSMO-RS methodology is that it also provides the charge distribution (σ, sigma) on the molecular surface, easily visualized in the histogram function σ-profile. The sigma profile shows the amount of polar surface charge on the molecular surface and can be used to anticipate the possible interactions of the compound in a fluid.16 Then we may analyze the σ-profile of the ion pairs for the purpose of understanding cation effects on IL densities. Figure 6a compares the σ-profile for 1-octyl-, 1-butyl-, and 1-methyl-3-methylmidazolium tetrafluoroborate compounds. The σ-profile of these ILs can be qualitatively divided in three main regions, which are separated in Figure 6a by two vertical lines located at the cutoff values for the hydrogen bond donor (σHB < -0.0082 e/Å2) and acceptor (σHB > 0.0082 e/Å2).16 The peak at ∼0.01 corresponds to the negatively charged BF4- anion and, consequently, is almost not modified by cation. Since this peak is at the high-polarity region σHB > 0.0082 e/Å2, BF4- anion can be considered a hydrogenbond segment.16 In the negative side, we observe some unresolved and low peaks at lower values than the cutoff -0.0082 e/Å2, which are not affected by the alkyl chain. These peaks are related to the hydrogen atoms of imidazolium ring; thereby they may contribute to hydrogen bonds as acceptors. Finally, the distribution of the charge densities around zero (-0.0082 e/Å2 < σ < 0.0082 e/Å2) corresponds to the nonpolar alkyl groups of the cation, being those for positive and negative signals assigned to carbon and hydrogen atoms, respectively. As Figure 6a shows, the higher number of carbon atoms in alkyl chain implies the increasing of the distribution of the charge densities around the nonpolar area. As a consequence, the σ-profile becomes less symmetric when the length of the imidazolium substituent increases, indicating more repulsive interactions between polar and nonpolar groups, i.e., lower capacity of the ionic liquid to interact with itself.16 Interestingly, we observe a quantitative relation between the predicted molar volume and the values of the charge distribution area below the σ-profile (Sσ-profile) in the nonpolar region (-0.0082 e/Å2 < σ < 0.0082 e/Å2) for all the 1-alkyl-3-methylmidazolium tetrafluoroborate compounds, and also for the compounds with other anions such as PF6- and Tf2N-. Thus, the linear fit among all data of Figure 6b provides the next linear relationship V ˜i ) 67 + 1476Sσ-profile with an excellent correlation coefficient, R ) 0.998. This suggests that Sσ-profile is an adequate a priori parameter to quantitatively characterize the cation nature. In addition, we can analyze the σ-profile of 1-butyl-3methylimidazolium (Bmim) ILs with the aim of interpreting the anion effects. Thus, Figure 7a presents the σ-profiles of BmimCl, BmimBF4, BmimPF6, and BmimFeCl4. As can be observed, the distribution of the charge densities in the negative sides assignable to the cationsis very similar for all compounds. In contrast, the peaks corresponding to anionic fragments present very different intensities and locations at the σ-profile, i.e., very different polarities and electronic charges. For example, chloride is described with the highest hydrogen bond donor capacity among all the anions studied; in contrast, it presents the lowest value of Sσ-profile. As a consequence, in this case, we should consider at least two different regions in the σ-profile, in order to characterize the anion nature on the basis of the proposed charge distribution area (Sσ-profile) parameters. Thus, as shown in Figure 7b, we find that COSMO-RS molar volumes are reasonably well correlated by multilinear regression to two parameters based on areas under the σ-profile curve estimated in the nonpolar (-0.0082 T +0.0082 e/Å2) and the hydrogen

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Figure 6. (a) Sigma profiles employed for benchmarking 1-alkyl-3-methylimidazolium tetrafluoroborate compounds, using the probabilistic surface charge distribution px(σ). (b) Comparison of predicted molar volumes with the values of charge distribution area (Sσ-profile) below σ-profile in the range (-0.0082 T +0.0082 e/Å2) for 1-alkyl-3-methylimidazolium with BF4-, PF6-, and Tf2N- anions.

Figure 7. (a) Sigma profiles employed for benchmarking 1-butyl-3-methylimidazolium compounds with different anions, using the probabilistic surface charge distribution px(σ). (b) Comparison of molar volumes predicted by COSMO-RS to those estimated by a multilinear regression using the values of charge distribution area (Sσ-profile) below the σ-profile curve in the nonpolar region (-0.0082 T +0.0082 e/Å2) and the hydrogen bond donor region (+0.0082 T +0.02 e/Å2).

bond donor (+0.0082 T +0.02 e/Å2) regions, respectively. The multilinear relationship is performed using the eight anions studied, and the equation obtained is V ˜ i ) -21 + 1008Sσ-profile (nonpolar) + 1613Sσ-profile (H-bond donor), with a correlation coefficient of R ) 0.94. We note that if the points corresponding to EtSO4- and MeSO4- anions are not included in the fit, then the correlation coefficient improves to 0.99. Clearly, a more exhaustive partition of the σ-profile is needed for adequately differentiating alkyl fragments from weak polar fragments. In summary, we obtain that the charge distribution area (Sσ-profile) below σ-profiles of ionic liquid structures could be used to characterize the chemical nature of both cation and anion. These results suggest new possible applications of COSMO-RS methodology, since Sσ-profile is a simple a priori parameter to be implemented in IL design tools for obtaining desired cation or anion effects.

4. Conclusions Accurate specific densities and molar liquid volumes of 40 ionic liquids have been calculated using the COSMO-RS method with a molecular model that simulates the pure compounds in terms of ion-paired structures. The capability of the volumetric property prediction for ILs by the proposed computational approach is reported to be as high as that for common organic solvents. COSMO-RS is also revealed as a valuable computational tool to describe the intermolecular interaction of ionic liquids containing a variety of functional groups. In addition, the charge distribution area value below the σ-profile is suggested as a simple molecular parameter to characterize the contribution of both cation and anion to IL volumetric properties, which could be applied in the near future toward designing IL solvents with appropriate characteristics for a specific use.


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ReceiVed for reView March 27, 2007 ReVised manuscript receiVed June 14, 2007 Accepted June 19, 2007 IE070445X

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