2678
Ind. Eng. Chem. Res. 2009, 48, 2678–2690
Experimental Thermodynamic Properties of 1-Butyl-2-methylpyridinium Tetrafluoroborate [b2mpy][BF4] with Water and with Alkan-1-ol and Their Interpretation with the COSMO-RS Methodology Ana Navas, Juan Ortega,* Remko Vreekamp, and Elena Marrero Laboratorio de Termodina´mica y Fisicoquı´mica, Parque Cientı´fico-Tecnologico, UniVersidad de Las Palmas de Gran Canaria, Canary Islands, Spain
Jose´ Palomar Seccio´n de Ingenierı´a Quı´mica (Dpto. de Quı´mica-Fı´sica Aplicada), UniVersidad Auto´noma de Madrid, Cantoblanco, 28049-Madrid, Spain
This work provides experimental data on liquid-liquid equilibria in the range 288-328 K and data of HEm and VEm at temperatures of 298.15 and 318.15 K, for binary mixtures corresponding to 1-butyl-2methylpyridinium tetrafluoroborate, [b2mpy][BF4], with alkanols (methanol to pentan-1-ol) and water. The [b2mpy][BF4] is completely miscible in water, and all the mixtures give positive values of HEm and VEm, giving increasing values of excess quantities with temperature. For mixtures with alkanols, values of HEm > 0 are obtained with VEm < 0, endothermic effects accompanied by contraction, with temperature changes represented by (dHEm/dT)p > 0 and (dVEm/dT)p > 0. Experimental data are correlated with a suitable equation and are presented together with those obtained in previous works for another two isomers, [b3mpy][BF4] and [b4mpy][BF4], and the properties of all of them are compared and discussed along with their behavior. The quantum-chemical method COSMO-RS is applied for the first time to predict excess properties of these mixtures containing ionic liquids, and the results show a significant contribution of hydrogen bond interactions in the behavior of the mixtures studied here, generically referred to as [bXmpy][BF4] (X ) 2, 3, 4) + H2O or + CVH2V+1(OH) (V ) 1-5). 1. Introduction This work forms part of a broader study that aims to analyze the behavior of some ionic liquids (IL) derived from pyridine and alkanols, which have been found to be interesting solvents. Hence, the behavior in solution of two of the isomers has been studied previously in a systematic manner, such as 1-butyl-3methylpyridinium tetrafluoroborate1 ([b3mpy][BF4]) and 1-butyl-4-methylpyridinium tetrafluoroborate2 ([b4mpy][BF4]) with several alkanols and water, to establish the effect of the hydrogen bond of these compounds in IL solutions. The effect of the alkanol chain is studied and also the position of the methyl group in the cation [bXmpy]; see Figure 1. In previous studies,1,2 this latter aspect was clearly demonstrated in alkanol mixtures, which were interpreted to be more energetically favorable in systems with [b4mpy][BF4] than with [b3mpy][BF4], undoubtedly because of the more linear molecular structure of the former, with a larger surface. Structurally, the mixtures with alkanols present less contractions in the case of [b4mpy][BF4], and these increase with temperature, probably for the same reason as that given before. Most of the information about these systems is provided by systematic research into isomeric ILs, so in this work an experimental study was carried out with 1-butyl-2-methylpyridinium tetrafluoroborate ([b2mpy][BF4]), carrying out a preliminary miscibility analysis in several alkanols, from methanol to pentan-1-ol, hence establishing a range of compositions and studying the possibility of obtaining measurements of solutions at the temperatures selected here, of 298.15 K and of 318.15 K. We are also interested in determining the interactional effects * To whom correspondence should be addressed. E-mail:jortega@ dip.ulpgc.es.
between the IL and water, owing to the hydrophilic nature of the IL. The properties of the pure substances involved, such as densities, volumes, and enthalpies, and those of the corresponding binary mixtures, will be determined experimentally. The results obtained will provide more knowledge about the effect of the alkanol chain and the position of the -CH3 group in the pyridine ring, and the influence of temperature on the properties of solutions. For [b2mpy][BF4] only one article has been found in the literature3 relating to a cytotoxicity study. Another important aspect for these solutions is the theoretical modelization. Although the value of its application has not yet been verified, it can provide more knowledge about the structural interpretation of these mixtures. More specifically, it was not possible to apply some classical group contribution models, which are recognized in the field of chemical engineering, because some of the interaction parameters between the corresponding functional groups of ILs are unknown. We are planning to study this in future works. However, at present, models based on quantum-chemical approaches are available, such as the ab initio model of the COSMO-RS methodology4 to estimate the properties of pure fluids and mixtures from the structural information of individual molecules. More precisely, we use this model because its application is indicated when the experimental data is incomplete5 or, as also occurs here, when additional information is required about the behavior of the systems studied;6 it could also help to improve the development of other models. The COSMO-RS model has been used to estimate thermodynamic properties of ILs, including liquidliquid equilibria data (LLE) and activity coefficients of its mixtures with water or alkanols.7-15 However, to our knowledge, there are no published studies about application of the COSMO-RS method to estimate properties of mixtures contain-
10.1021/ie8009878 CCC: $40.75 2009 American Chemical Society Published on Web 01/27/2009
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2679
ing ILs. For this reason, in this work we will study the value of E of binary mixtures of the method to estimate the Hm [b2mpy][BF4] with water and alkanols. After evaluating the E values, it will be capacity of the method to estimate the Hm used to analyze molecular interactions that affect the mixing properties of ILs, providing a theoretical model to apply when a specific IL is required for a specific purpose, which behaves in a specific way in solution.16,17 2. Experimental Section 2.1. Apparatus and Procedures. Miscibility regions in the binary mixtures [b2mpy][BF4] + alkanol were determined with a LLE cell, placed on a magnetic stirrer, using a method of continuous dilution with visual detection of turbidity changes. The measurements were taken at regular temperature intervals between 275 and 355 K and at atmospheric pressure, and the cell was thermostated with an external circulation bath (Haake, model Phoenix II) that provided a close control of the water temperature used to keep the liquids in the cell at quasiisothermal conditions. The temperature that indicates the merging of the two phases was measured with a precision thermometer, model ASL-F25, with a reading error of (1 mK, using a PT100 probe immersed in liquid, and the inaccuracy of the measurement was estimated to be (0.02 K. Initially, a known quantity of [b2mpy][BF4] was introduced into the cell, and accurately known quantities of alkanol were added and determined by weight differences with a Hamilton TLL syringe of 100 µL. The two-phase liquid mixture was stirred and slowly heated/cooled until it became homogeneous, at which moment the temperature change was recorded. Another known quantity of alkanol was added, and the operation was repeated. The molar fraction in [b2mpy][BF4], xIL, calculated for each point presented an uncertainty of (0.001. In this way, pairs of values were obtained (xIL, T) over a wide interval of compositions, 0.5 < xIL < 1. The process was repeated in the opposite direction, starting with a known amount of alkanol in the cell and adding known quantities of [b2mpy][BF4], obtaining paired values in the interval 0.5 > xIL > 0 which must be matched with those found in the first stage. Excess volumes VEm were calculated from density values, both of pure products and of mixtures, determined with an AntonPaar DMA58 vibrating-tube digital densimeter, with a reading error of (0.02 kg · m- 3. In spite of the fact that the densimeter has its own thermostatic system with a Peltier effect, the apparatus was connected to the above-mentioned water bath kept at a temperature of (T - 0.5) K, where T was the working temperature, 298.15 or 318.15 K. In this case, the different binary mixtures were prepared synthetically by weighing them into airtight closed vials and placing them in an ultrasound bath for 30 min to homogenize the sample. The imprecision in the molar fractions in [b2mpy][BF4] was estimated to be (0.0002. The refractive indices of the pure compounds were measured with a Zuzi Abbe type refractometer, model 320, with an inaccuracy of (0.0002 units in nD, maintained at the temperature specified above. A ST-1000 Selecta rotational viscometer was used to measure the viscosity of the pure compounds, with a precision of (1% at the bottom of the scale and a reproducibility of 0.2%. Thermostatization of the special viscosimeter cell, with an incorporated heat exchanger, was carried out with the water bath described and resulted in an imprecision in the temperature control in the viscosimeter cell of (0.05 K. The apparatus was calibrated previously at each temperature using a standard of reference provided by Brookfield Engineering Laboratories.
Figure 1. Structure of 1-butyl-2-methylpyridinium tetrafluoroborate ([b2mpy][BF4]). E For direct measurement of the enthalpies of the mixture Hm , an MS80D Calvet conduction calorimeter was used from Setaram, Lyon (France), calibrated electrically by a Joule effect with an EJ3 power source, simulating analogous thermograms to the mixing process when different powers are applied to a calibration cell. Electrical calibration of the apparatus was checked at the two working temperatures with the mixture propan-1-ol + benzene,18 with an inaccuracy lower than 1% E . For the mixtures of this work with [b2mpy][BF4], a for Hm special cell was used equipped with a light stirring system, described in a previous article,1 keeping the stirring constant during the mixing time. The net energy of the operation was calculated from the difference between the thermogram of the process and that corresponding to the stirring, estimated previously in a preliminary experiment on the IL. 2.2. Materials. The [b2mpy][BF4] used in this work was provided by IOLITEC (IOnic LIquid TEChnologies), GmbH & Co. KG with a commercial purity > 99% and a water content < 580 ppm, and checked with a Karl Fischer titrator, Mettler DL18, and had a halide content, according to the manufacturer, of 93 ppm. The alkanols (methanol to pentanol) of maximum purity, supplied by Fluka and Aldrich, were checked using a GC fitted with a FD, model HP6890, to verify manufacturers’ values. The [b2mpy][BF4] was stored and handled in a dry chamber with a constant relative humidity lower than 5%. Before preparing the samples, all the products used here were degassed with ultrasound for several hours, and the alkanols were kept over a molecular sieve of 0.3 nm, from Fluka. The water used, both in the experiments and to calibrate the equipment, was bidistilled and degassed in our laboratory and had an electrical conductance lower than 1.5 µS. To obtain the maximum information about the products used in order to subsequently apply the results in the data treatment, the densities F and the refractive indices nD were measured for all the substances over a wide range of temperatures. Also, the dynamic viscosity η of [b2mpy][BF4] was measured using the rotational viscosimeter, for purposes of characterization. In this way, the coefficients that determine the gradients of each property M could be calculated as a function of temperature, (∂M/∂T)p, the values of which are presented in Table 1 in the interval 288-328 K. However, this table only gives the values found for [b2mpy][BF4] at the temperatures selected in this work for measuring the mixing properties, 298.15 and 318.15 K, since the values found for water and alkanols were almost identical to those recorded in previous works.1,2 More specifically, it was of interest to us to record experimental values for densities at different temperatures, in order to evaluate the parameter kv(T) and its influence on the treatment of excess quantities, which we will study next. Hence, Figure 2a shows the variation in molar volumes of the pure compounds, IL, water, and alkanols, with temperature, recording in Table 1 the coefficients of the function F ) F(T). Figure 2b shows the variation with temperature of the parameters kv(T) ) V2°(T)/V1°(T) for the different mixtures studied in this work. In all cases, the gradients are almost zero, although a slightly negative gradient is observed in the mixtures of [b2mpy][BF4] with water and a slightly
2680 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 Table 1. Physical Properties of Pure Compounds, Density, G, Refractive Index, nD,Viscosity, η, and Coefficients of Correlations compd
T/K
F/kg · m- 3
nD
η/mPa · s
[b2mpy][BF4]
298.15 318.15
1202.17 1188.71
1.4545 1.4500
403.3 99.9
YEm ) zIL(1 - zIL)
F ) Ae- RT
i i IL
(1)
i)0
where zIL )
Thermal Coefficients in the Range 288-328 K nD ) a + bT
∑yz
ln(η) ) A + B/T
compd
a
104(-b)
A
103(R j)
A
B
[b2mpy][BF4] methanol ethanol propan-1-ol butan-1-ol pentan-1-ol water
1.5272 1.4465 1.4824 1.5015 1.5025 1.5258 1.3648
2 4 4 4 4 4 1
1422.82 1146.68 1105.90 1095.42 1085.72 1075.73 1102.77
0.57 1.26 1.15 1.05 1.00 0.95 0.34
-16.5
6737.3
positive gradient for mixtures with alkanols that increases slightly with the number of -CH2 groups of the alkanol. In any case, it appears clear that in this work we can consider the parameter kv to be independent of temperature, and also the same consideration for kh, according to expression 6 of the following section. 3. Results 3.1. LLE Data. The diagram of partial miscibility (see Figure 3) of the binary systems [b2mpy][BF4] + alkanol shows the different regions where the measurements of the corresponding solutions, both of HEm and of VEm, can be obtained at temperatures of 298.15 and 318.15 K. Table 2 shows the pairs of values (xIL,T) obtained by applying the previously described procedure to mixtures [b2mpy][BF4] + alkanol, from ethanol to pentan-1ol. The diagram in Figure 3 shows a clear asymmetry toward regions poorer in [b2mpy][BF4], and the upper critical solution temperature (UCST) of each system varies with the alkanol chain, as shown in the inset. Also, this inset shows how the miscibility and the UCST decrease for the same alkanol with the nature of the IL isomer in the order [b2mpy][BF4] > [b3mpy][BF4] > [b4mpy][BF4]. The position of the -CH3 group on the pyridine ring (see Figure 1) produces a strong CH3-N attraction, hindering the interstitial accommodation of the alkanol in its midst, with this effect decreasing in the order indicated. On the other hand, the increase in alkanol chain length with the corresponding reduction in its polar nature increases the immiscibility in the IL, as shown in the inset of Figure 3. For the work proposed, after studying the miscibility curve in Figure 3, it seems that the [b2mpy][BF4] + pentan-1-ol system should be totally rejected, since it is almost completely immiscible at a temperature of 318.15 K and is only miscible (see Table 3) where xIL < 0.008. Both methanol and ethanol are completely miscible at temperatures of 298.15 and 318.15 K while propan-1-ol and butan-1-ol present only partial miscibility. Table 3 shows the numerical values of the immiscibility regions for each system together with the pairs of values (xIL, UCST) for the systems of [b2mpy][BF4] + alkanol. 3.2. Excess Properties. The paired values (xIL, HEm) and (xIL, E ) for the mixtures [b2mpy][BF4] + alkanol or + water at Vm 298.15 and 318.15 K are presented in Tables 4and 5, respectively. The experimental data sets were correlated with a simple polynomial equation, a function of the so-called actiVe fraction of [b2mpy][BF4], zIL(xIL). If we represent the excess properties E E , that could correspond either to Hm (in by a generic form Ym -1 3 -1 E J · mol ) or Vm (in m · mol ), the equation used is
xIL xIL + k(T)(1 - xIL)
with the coefficients dependent on T in the form 2
yi )
∑Y T
j-1
(2)
ij
j)0
substituting in each case the coefficients Yij by those corresponding to correlations of enthalpies hij or excess volumes Vij. This is done to try and obtain a single correlation of the properties as a function of the temperature, which later permits others to be estimated by derivation. Clearly, the data correlation implies correlation of the excess property with two variables, molar fraction and temperature, YEm(xIL,T), for which a powerful algorithm for nonlinear functions must be applied. Calculation of the parameter k(T) depends on whether the E E E or Vm . Hence, in correlations of Vm , this correlation is of Hm parameter is the quotient of molar volumes of pure compounds of the mixture under the same conditions of p and T, which we denote kv and which, at a reference temperature, that could be T0, takes the form kV(T0) )
Mi FIL(T0) (Ri-RIL)T0 Vi°(T0) (e ) ) VIL°(T0) MIL Fi(T0)
(3)
where Ri and RIL are, respectively, the coefficients of thermal expansion of compound i and of the IL. Now, for any other temperature T, eq 3 takes the form kV(T) ) kV(T0)e(Ri-RIL)(T-T0)
(4)
(xIL, HEm),
the parameter However, to correlate the paired values k can be indicated by kh, as a function of the temperature would have a form analogous to eq 4, and can be written as kh(T) ) kh(T0)e(2⁄3)(Ri-RIL)(T-T0) where: kh(T0) )
( )( ) (
S i° qi rIL ) SIL° qIL ri
2⁄3
Vi°(T0) VIL°(T0)
(5)
) ( ) 2⁄3
) kq
kv(T0) kr
2⁄3
(6)
To be able to use these expressions, the preliminary study of section 2.2 was first carried out, and the values of Vi°(T) were represented in Figure 2a and the parameter kv(T) in Figure 2b for the binary systems of this work, and in the interval 288-328 K. The coefficients of thermal expansion in the same temperature interval are presented in Table 1 for pure compounds. However, as mentioned in section 2.2, in this work the kv parameter is considered not to be dependent on temperature, because of the almost insignificant thermal gradient value of kv. To estimate the kh value, values are needed for the area parameters, qk, and volume, rk, for each compound, and for each of these a group contribution method can be used with the parameters given by Bondi.19 For the alkanols, the calculation is simple, but this is not the case for the ILs, although for [b2mpy][BF4] we could adopt the same values that are calculated for the other isomers, [b3mpy][BF4] and [b4mpy][BF4], of rIL ) 7.29 and qIL ) 6.69. Nonetheless, the calculation of these parameters was repeated, since the quotient of volumes in eq 3 is similar to the quotient of ri/rIL, from which the value for rIL indicated previously is obtained. To calculate qIL, eq 6 was used,
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2681
Figure 2. (a) Variation of Vi° with T for [b2mpy][BF4] (4) and for alkanols and water (b). (b) Values obtained for kv ) V2°/V1° versus T for binary mixtures formed by [b2mpy][BF4] (1) + alkanols (2) or + water (2). Labels indicate number of carbon atoms in the alkanols. Table 2. Experimental xIL-T Data for xIL[B2mpy][BF4] + (1 xIL)Alkan-1-ol xIL 0.009 0.014 0.020 0.026 0.031 0.039 0.048 0.058
T/K
xIL
T/K
xIL[b2mpy][BF4] + (1 - xIL)Ethanol 281.49 0.069 293.94 0.196 283.82 0.083 294.28 0.220 285.95 0.096 294.50 0.248 288.43 0.112 294.56 0.276 290.27 0.129 294.62 0.313 291.58 0.148 294.48 0.350 292.82 0.167 294.17 0.396 293.49 0.176 293.82
xIL[b2mpy][BF4] + (1 - xIL)Propan-1-ol 0.006 293.76 0.072 319.81 0.009 301.67 0.094 320.03 0.013 306.49 0.118 320.24 0.017 309.13 0.144 320.26 0.021 311.22 0.172 320.19 0.027 313.84 0.202 320.04 0.033 315.68 0.235 318.89 0.041 317.10 0.263 317.68 0.056 318.82 0.292 316.36
Figure 3. Plots of T Vs xIL for the binaries xIL[b2mpy][BF4] + (1 xIL)alkanols: 9, ethanol; 2, propan-1-ol; b, butan-1-ol; ×, pentan-1-ol. The inset represents the upper critical solution temperature (UCST) Vs the number of alkanol carbon atoms, u, and for different IL isomers. Labels indicate the X-values in [bXmpy][BF4].
applying Connolly’s empirical method20 as part of the commercial software PCMODEL, calculating, on the one hand, the sum of the areas of the anion and the cation, SIL°, and, on the other hand, that of the second component, Si°, the alkanol. Introducing the quotient in eq 6, the mean value indicated previously for qIL can be obtained. With the area parameters, eqs 5 and 6 can now be used to determine the coefficient kh(T) from kh(T0), considering the coefficients of thermal expansion of Table 1 for the pure compounds. Table 6 shows the values obtained for kv and kh, which, according to the method specified, are not very different at temperatures of 298.15 or 318.15 K. E To obtain coefficients of eq 1 applied to the values of Hm and 21 E Vm, Marquardt’s algorithm was used, as part of the MATLAB commercial software for nonlinear functions. The values
xIL
T/K 293.19 292.36 290.78 288.94 286.91 283.42 279.40
0.323 0.357 0.392 0.426 0.461 0.500 0.540 0.581
314.04 311.55 309.17 307.04 304.42 299.79 296.91 294.01
xIL[b2mpy][BF4] + (1 - xIL)Butan-1-ol 0.007 315.38 0.088 336.51 0.010 319.93 0.111 336.86 0.013 323.18 0.138 336.80 0.017 325.16 0.174 336.83 0.021 327.24 0.202 336.57 0.028 330.14 0.222 335.94 0.039 332.82 0.250 335.27 0.052 334.58 0.284 334.47 0.070 335.96 0.323 332.96
0.365 0.411 0.452 0.491 0.532 0.574 0.616 0.660 0.704
330.89 328.30 325.31 322.19 318.08 315.12 312.89 310.62 309.95
xIL[b2mpy][BF4] + (1 - xIL)Pentan-1-ol 0.005 316.68 0.090 351.12 0.009 324.85 0.122 351.39 0.012 330.97 0.156 352.26 0.015 335.34 0.194 351.90 0.019 338.50 0.232 351.61 0.024 341.38 0.280 350.69 0.029 343.75 0.314 350.06 0.038 346.38 0.337 349.44 0.050 348.68 0.352 349.09 0.057 350.17 0.373 347.97
0.394 0.406 0.432 0.452 0.472 0.487 0.512 0.578 0.623 0.394
347.94 346.46 344.76 343.35 342.44 341.39 340.25 337.56 337.26 347.94
obtained in each case are compiled in Table 6. The specific coefficients for each property Yij are high-powered, although the coefficients yi resulting from applying eq 2 are of the same
2682 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 Table 3. Inmiscibility Intervals and USCT for the Binaries xIL[b2mpy][BF4] + (1 - xIL)Alkan-1-ol inmiscibility intervals mixture [b2mpy][BF4] [b2mpy][BF4] [b2mpy][BF4] [b2mpy][BF4] [b2mpy][BF4]
+ + + + +
298.15 K
318.15 K
UCST T/K
methanol ethanol 294.6 propan-1-ol [0.009; 0.540] [0.056; 0.263] 320.3 butan-1-ol [0; 1] [0.009; 0.532] 336.9 pentan-1-ol [0; 1] [0.008; 1] 352.3
xIL
E Table 4. Excess Molar Enthalpies, Hm , for the Binaries xIL[b2mpy][BF4] + (1 - xIL)Alkan-1-ol or + (1 - xIL)Water at T ) 298.15 K and T ) 318.15 K
xIL
E Hm /J · mol- 1
xIL
E Hm /J · mol- 1
xIL
E Hm /J · mol- 1
xIL[b2mpy][BF4] + (1 - xIL)Water 0.129 0.144 0.110 0.156
order of the correlated magnitude. The correlations of mixtures with water will be mentioned in the following section. 3.2.1. Binary Mixtures IL + Water. The binary system [b2mpy][BF4] + water is completely soluble over the entire range of compositions at temperatures of 298.15 and 318.15 E E ) and (xIL,Vm ), respectively, K. Data for the properties (xIL,Hm are shown in Tables 4 and 5, and the values obtained for the coefficients of eq 1 are used to correlate these properties at two temperatures in Table 6. However, for the system [b2mpy][BF4] + water, the data fit was carried out differently from the abovedescribed procedure, since the value of kv, as shown in eq 3, is strongly influenced by the quotient of the molecular weights (Mi/MIL), with the following distinction: if Mi , MIL, then (Mi/ MIL) , 1, and in some cases kv < 0.1, giving rise to nonreal E in regions of extreme mole inflections of the function Ym fraction. For this reason, in the correlation of all the mixtures [b2mpy][BF4] + water (as was also done with the other ILs, see refs 1 and 2), the parameter k, both kv and kh, was submitted to the correlation process as an additional parameter to the coefficients of eq 1, applying a simultaneous fit of the property E E (xIL) and on the other hand Vm (xIL), data on the one hand Hm with temperature. E (xIL) at 298.15 Figure 4a shows experimental values for Hm and 318.15 K in this work for the mixture of [b2mpy][BF4] + water and the curves obtained using the coefficients obtained by the procedure described; the quality of the fits is acceptable. E , a similar energetic In the inset, with equimolar values of Hm behavior is observed for the mixtures [b2mpy][BF4] + water E for the system and [b3mpy][BF4] + water. The values of Hm [b4mpy][BF4] + water are slightly higher and show the same order of variation as the surfaces exposed for the ILs in the final mixture, according to the values obtained for the corre0 , as obtained by Connolly’s empirical procedure.20 sponding SIL E The slope (dHm /dT)p > 0 in all cases. E Figure 4b shows an equivalent representation for Vm (xIL), at the two working temperatures. As can be observed in the inset of Figure 4b, the values of systems [b2mpy][BF4] + water and [b4mpy][BF4] + water are much higher than those of [b3mpy][BF4] + water, such that the position of the -CH3 group in this latter system favors contraction effects more than in the other two cases, due to a better interstitial accommodation of the water molecule. This can be seen even more clearly in the inset. The behavior is similar for both working temperatures, E E with temperature, and hence, (dVm / with a clear rise in the Vm dT)p > 0 in all cases. 3.2.2. Binary Mixtures IL + Alkanol. Analyzing Figure 3, we can observe that the systems [b2mpy][BF4] + methanol or + ethanol are completely miscible at the temperatures selected for this work. The mixtures with the other alkanols present zones of immiscibility in which the excess properties cannot be evaluated, and the intervals are shown in Table 3. E E and Vm , Figures 5 and 6 show the experimental data of Hm respectively, with the corresponding correlation curves obtained for each system, identifying the interval of compositions with immiscibility in systems in which this occurs. These representations show some aspects of the behavior of these systems which
0.0764 0.0871 0.0988 0.1138 0.1315 0.1526
768 845 921 1010 1101 1201
0.0801 0.0923 0.1072 0.1255 0.1482
1064 1185 1323 1472 1624
T ) 298.15 K 0.1780 1273 0.2071 1367 0.2392 1416 0.2742 1464 0.3137 1494 0.3686 1494 T ) 318.15 K 0.1755 1795 0.2078 1942 0.2558 2096 0.3296 2133 0.4389 2037
0.4300 0.5129 0.6130 0.7260 0.8524
1457 1369 1190 933 558
0.4925 0.5883 0.6399 0.7806 0.8577
1955 1699 1542 1069 746
xIL[b2mpy][BF4] + (1 - xIL)Methanol 0.0655 0.1301 0.1715 0.2223 0.2512 0.2832
751 1195 1379 1559 1637 1696
0.0789 0.1257 0.1708 0.1927 0.2208 0.2576
1124 1516 1868 1989 2094 2189
T ) 298.15 K 0.3212 1735 0.3652 1766 0.4145 1752 0.4658 1670 0.5239 1582 0.5900 1481 T ) 318.15 K 0.2878 2241 0.3279 2246 0.3737 2244 0.4258 2162 0.4784 2074 0.5373 1973
0.6623 0.7423 0.8190 0.8928 0.9592
1275 1047 789 473 173
0.5984 0.6701 0.7477 0.8334 0.9112 0.9778
1842 1648 1423 1096 689 199
xIL[b2mpy][BF4] + (1 - xIL)Ethanol 0.0880 0.1095 0.1357 0.1477 0.1640 0.1832
1100 1260 1466 1556 1652 1758
0.0891 0.1114 0.1546 0.1741 0.1954 0.2207
1330 1560 1917 2048 2178 2314
T ) 298.15 K 0.2046 1871 0.2309 1984 0.2625 2058 0.3000 2095 0.3459 2090 0.4016 2063 T ) 318.15 K 0.2819 2530 0.3220 2601 0.3691 2600 0.4221 2568 0.4828 2454 0.5534 2304
0.4649 0.5385 0.6273 0.7223 0.8190 0.9158
1976 1850 1622 1330 974 499
0.6095 0.6744 0.7470 0.7923 0.8588 0.9333
2161 1888 1617 1415 1067 561
xIL[b2mpy][BF4] + (1 - xIL)Propan-1-ol 0.5405 0.5779
2065 1969
0.0350 0.2685 0.2961 0.3263 0.3621 0.4028
606 2585 2640 2682 2707 2690
T ) 298.15 K 0.6444 1779 0.7131 1558 T ) 318.15 K 0.4475 2623 0.4956 2502 0.5464 2369 0.6020 2161 0.6599 1991 0.7138 1770
0.8128 0.8980
1098 686
0.7660 0.8214 0.8691 0.9183 0.9598
1564 1235 941 652 378
xIL[b2mpy][BF4] + (1 - xIL)Butan-1-ol 0.5094 0.5661 0.6400
2599 2476 2249
T ) 318.15 K 0.7175 1946 0.7995 1555 0.8821 1122
0.9536
616
will be discussed. Hence, parts a and b, respectively, of Figure E 5 show the values of Hm at 298.15 and 318.15 K, with endothermic effects in each case, which increase with increasing alkanol chain length, as can also be observed in the inset, in E (x)0.5) are compared, for a same alkanol, which values of Hm taking the following order depending on the IL: [b4mpy][BF4] > [b2mpy][BF4] > [b3mpy][BF4] at 298.15 K. More specifi-
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2683 Table 5. Excess Molar for Binaries xIL[b2mpy][BF4] + (1 - xIL) Alkan-1-ol or + (1 - xIL) Water at T) 298.15 K and T) 318.15 K E Volumes,Vm ,
xIL
E F/kg · m- 3 109Vm /m3 · mol- 1
xIL
E F/kg · m- 3 109Vm /m3 · mol- 1
xIL[b2mpy][BF4] + (1 - xIL)Water 0.0502 0.1010 0.1517 0.1998 0.2833 0.3386 0.4100 0.5112
1066.29 1102.15 1124.07 1138.48 1155.54 1163.53 1171.48 1180.11
0.0497 0.1001 0.1492 0.2049 0.2941 0.3827 0.4388
1053.69 1087.79 1108.60 1125.13 1142.77 1154.47 1160.16
T ) 298.15 K 149 0.5730 261 0.6430 350 0.7102 414 0.7636 487 0.8500 517 0.8992 543 0.9259 529 T ) 318.15 K 228 0.5011 382 0.5806 496 0.7075 572 0.6486 640 0.7797 661 0.8578 651 0.9306
1184.33 1188.35 1191.74 1193.96 1197.32 1198.97 1199.93
499 455 391 354 251 193 142
1165.60 1170.96 1177.86 1175.12 1181.05 1184.06 1186.72
609 569 441 480 349 235 88
xIL[b2mpy][BF4] + (1 - xIL)Methanol 0.0532 0.1033 0.1509 0.2096 0.3049 0.4071 0.5097
884.03 945.97 989.58 1030.56 1077.52 1112.27 1138.03
0.0344 836.06 0.0806 902.54 0.1493 972.68 0.2235 1024.96 0.3310 1074.96 0.4150 1102.20
T ) 298.15 K -492 0.5520 -653 0.6388 -716 0.7058 -751 0.7993 -698 0.8561 -588 0.9464 -504 T ) 318.15 K -415 0.4736 -648 0.5358 -885 0.6959 -1025 0.7856 -1008 0.8660 -925
1146.32 1161.71 1171.44 1182.91 1189.09 1197.68
-423 -360 -280 -162 -114 -34
1117.21 1130.75 1156.94 1168.14 1176.85
-838 -753 -467 -314 -210
xIL[b2mpy][BF4] + (1 - xIL)Ethanol 0.0360 853.45 0.0547 882.67 0.0948 932.65 0.1250 963.65 0.2676 1058.83 0.0527 838.96 0.1009 889.70 0.1518 933.02 0.2108 975.07 0.3129 1030.54 0.4397 1079.70
T ) 298.15 K -229 0.3860 -338 0.5402 -415 0.6246 -488 0.7398 -557 0.8245 T ) 318.15 K -410 0.5814 -604 0.6499 -697 0.7104 -836 0.7827 -925 0.8665 -857 0.9135
1104.33 1143.01 1158.49 1175.45 1185.45
-472 -333 -264 -179 -93
1119.42 1134.27 1145.79 1158.72 1171.06 1177.48
-720 -576 -441 -377 -178 -95
xIL[b2mpy][BF4] + (1 - xIL)Propan-1-ol 0.5277 1102.19 0.6373 1132.22 0.7152 1150.44 0.0191 804.05 0.0362 821.60 0.2698 987.52 0.3386 1020.39 0.4200 1053.64 0.5385 1093.16
T ) 298.15 K -291 0.7807 -227 0.8554 -171 T ) 318.15 K -103 0.6120 -206 0.7555 -609 0.7829 -641 0.8144 -669 0.8865 -638 0.9385
1164.18 1178.40
-125 -77
1113.67 1147.08 1152.29 1157.94 1170.83 1179.17
-594 -470 -387 -279 -169 -58
xIL[b2mpy][BF4] + (1 - xIL)Butan-1-ol 0.0038 794.43 0.5577 1084.46 0.6128 1101.18
T ) 318.15 K -36 0.7145 1128.50 -553 0.8334 1156.16 -537 0.8943 1168.58
-433 -288 -167
cally, the effect of temperature on the system [b2mpy][BF4] + alkanol is also similar to that on all the other isomers of the IL, E /dT)p > 0. However, the order indicated and in all cases (dHm
changes at the temperature of 318.15 K, and [b3mpy][BF4] can now be found in the intermediate position, between the other two, so the order now followed is [b4mpy][BF4] > [b3mpy][BF4] > [b2mpy][BF4]. This is due to a relative increase E , which results in a lower energy exchange when the in the Hm -CH3 group is closer to the nitrogen. This same arrangement E values, which we will can be observed in the change in Vm discuss below. Regarding the VEm, which are represented at two temperatures in Figure 6, a clear contraction effect is observed in all cases, which becomes less pronounced as the alkanol chain length increases; nevertheless, the contraction increases with temperature. In the inset graphs, VEm(x)0.5) values are also represented relative to alkanol chain length for the three isomers of IL [bXmpy][BF4] (X ) 2, 3, 4) at each temperature. The influence of temperature in these systems presents some important aspects. Hence, the change in VEm with temperature of the binary systems containing [b3mpy][BF4] and [b4mpy][BF4] is less than that corresponding to systems of [b2mpy][BF4], for which the degree of contraction is almost double that of the former ones. This is E (x ) 0.5) for the shown by an inversion in the change of Vm different ILs when passing from 298.15 to 318.15 K. This can be justified by the better spatial arrangement of the [b2mpy][BF4] that permits a better packing, as the increase in temperature makes the molecule dilate and also because of the E mentioned previously. lower values of Hm 4. Methodology of the COSMO-RS Model in Mixtures [b2mpy][BF4] + H2O or + CWH2W+1(OH) 4.1. Calculation Procedure. The molecular geometry of all compounds (common solvents and ionic liquids) was optimized at the B3LYP/6-31++G** computational level in the ideal gasphase using the quantum chemical Gaussian03 package.22 As a molecular model to simulate the pure ionic liquid, ion-paired structures including both counterions were optimized and vibrational frequency calculations were performed in each case to confirm the presence of an energy minimum. Then, the standard procedure was applied for COSMO-RS calculations, which consisted of two steps: First, Gaussian03 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. Subsequently, COSMO files were used as an input in COSMOtherm23 statistical thermodynamic code to calculate the excess enthalpy of the mixtures studied at 298.15 and 318.15 K. According to our chosen quantum method, the functional and the basic 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. 4.2. COSMO-RS Description of Pure Compounds. One advantage of the COSMO-RS methodology is that it provides the charge distribution (σ, sigma) of the specific polarity on the molecular surface, easily visualized by the σ-profile histogram. The σ-profile is used in the statistical thermodynamic procedure by the COSMO-RS to obtain the interaction energy between pairs of segments on the surface. Consequently, the σ-profile determines estimation of the electrostatic interactions and of the hydrogen bond of the COSMO-RS model. In fact, qualitative analysis of the σ-profile of the components of a mixture can be used to predict its behavior. Figure 7a presents the σ-profile of the ionic liquid studied, [b2mpy][BF4]. A peak is observed at 0.011 e/Å2 corresponding to the [BF4]- anion
2684 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 E E E Table 6. Coefficients hij, Wij, and k, and Standard Deviations s(Ym ), Obtained in the Correlation, Respectively, of Hm or Vm , Using Eq 1
xIL[b2mpy][BF4] + (1 - xIL)water
(1 - xIL)methanol
(1 - xIL)ethanol
(1 - xIL)propan-1-ol
(1 - xIL)butan-1-ol
E E Ym ) Hm /J · mol- 1
kh (at 298.15 K) kh (at 318.15 K) h00 · 104 h01 · 104 h10 · 104 h11 · 104 h20 · 104 h21 · 104 E s(Hm ) at 298.15 K E s(Hm ) at 318.15 K
0.639 0.942 -3363.4 12.1 6587.2 -22.9 -3329.3 11.8 1 32
0.219 0.221 -1240.1 4.9 2434.9 -9.9 -3423.0 14.0 57 137
0.298 0.300 -1290.0 5.0 3654.5 -12.7 -5006.3 18.2 60 86
0.318 0.320 34023.1 -106.2 -86905.7 272.7 53657.1 -166.9 16 83
0.449 0 79617.7 0 -191649.2 0 136783.4 85
E E Ym ) 109Vm /(m3 · mol- 1)
kV (at 298.15 K) kV (at 318.15 K) V00 · 104 V01 · 104 V10 · 104 V11 · 104 V20 · 104 V21 · 104 E s(Vm ) at 298.15 K E s(Vm ) at 318.15 K
1.112 0.480 367.2 -0.9 -1387.8 4.3 270.2 -0.7 6 11
0.207 0.209 54.4 -0.5 -24.2 0.2 1421.1 -5.0 12 17
0.297 0.301 272.7 -1.1 -151.6 0.5 1578.6 -5.3 15 33
0.381 0.385 3458.2 -11.1 -8875.9 28.2 7251.4 -23.3 1 32
0.470 0 6219.4 0 -19243.8 0 94.183 8
Figure 4. Plots of experimental values and correlation curves for excess molar properties at T ) 298.15 K (solid symbols) and T ) 318.15 K (open symbols) E for the binaries [b2mpy][BF4] (IL) + water. The inset represents the excess equimolar properties as a function of temperature and the IL isomers. (a) Hm Vs E xIL. (b) Vm Vs xIL. Labels indicate the X-value in [bXmpy][BF4].
and, since this peak is in the region of high polarity (above the cut off σH-Bond > 0.0085 e/Å2), this group can be considered as a hydrogen bond acceptor. On the negative side, unresolved peaks of low intensity are observed at values below the cut off σH-Bond < -0.0085 e/Å2. These signals are associated with the hydrogen atoms of the pyridine ring and could contribute to hydrogen bonds becoming donors. The distribution of charge
densities around zero (-0.0085 e/Å2 < σ < 0.0085 e/Å2) corresponds to the nonpolar alkylic groups of the cation and to the carbon atoms of pyridine. The COSMO-RS methodology also provides the σ-potential of a compound. The σ-potential describes the likeliness of the solvent studied to interact with compounds with a charge density [pX(σ)] with polarity σ. Figure 7b presents the σ-potential of [b2mpy][BF4]. As can be
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2685
E Figure 5. Plots of experimental values and correlation curves Hm for binary mixtures [b2mpy][BF4] + alkanols: 2, methanol; 9, ethanol; [, propan-1-ol; E (at xIL ) 0.5) as a function of X in [bXmpy][BF4]. (a) 298.15 K; (b) 318.15 K. Labels indicate number of carbon +, butan-1-ol. The inset represents the Hm atoms, V, in alkanols.
E Figure 6. Plots of experimental values and correlation curves Vm for binary mixtures [b2mpy][BF4] + alkanols: 2, methanol; 9, ethanol; [, propan-1-ol; E b, butan-1-ol. The inset represents the Vm (at xIL ) 0.5) as a function of X in [bXmpy][BF4]: (a) 298.15 K; (b) 318.15 K. Labels indicate the number of carbon atoms, V, in alkanols.
observed, the ionic liquid will present attractive interactions with acid groups and repellent ones with basic groups. Further, the hydrocarbon groups of the cation present slightly attractive interactions with apolar compounds. Regarding the second component of the mixtures studied, Figure 7c presents the σ-profile of the water, methanol, and ethanol series. On the basis of their σ-profile, these compounds present functional groups that are clearly more polarized than those of ionic liquids, detecting signals corresponding to strong donator segments (peaks at approximately -0.016 e/Å2) and acceptor ones (peaks at 0.018 e/Å2) of hydrogen bonds, associated with the hydroxyl amphoteric group. In the case of alcohols, signals are observed corresponding to alkylic groups in the nonpolar region of the σ-profile, that increase in intensity with the chain length. The σ-potential of this seriesswater, methanol, and ethanolsshows that these compounds will present strongly attractive interactions
with both the acid and the basic groups in its mixtures (Figure 7d), although the water showed a greater capacity as a hydrogen bond donor and the alcohols as acceptors. One additional difference is that the alcohols show slightly favorable interactions with compounds of a clearly apolar nature, while in the case of water such interaction is clearly repellent. 4.3. COSMO-RS Description of the HEm in the Mixtures [b2mpy][BF4] + H2O or + CWH2W+1(OH). Taking into account the aforementioned calculations and considerations, the COSMOE RS methodology was used to estimate the Hm of the binary mixtures of [b2mpy][BF4] with solvents of interest, such as E water and alkanols. Figure 8 shows the Hm calculated by this method and the experimental data for the mixtures mentioned, at temperatures of 298.15 and 318.15 K. As can be observed, E the model gives a reasonable estimation of the Hm , including
2686 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009
Figure 7. σ-profiles and σ-potentials of pure compounds: (a and b) σ-profile and σ-potential, respectively, for [b2mpy][BF4]; (c and d) σ-profile and σ-potential, respectively, for water, methanol, and ethanol.
the effect of the alkyl chain length of the alkanol and of temperature on the excess enthalpies of these mixtures. E in the COSMO-RS model arise from The estimations of Hm summing the three contributions associated with interactions: polar misfit, HEm(misfit); hydrogen bonds, HEm(H-bond); and Van E (VdW). In other words, der Waals forces, Hm HEm ) HEm(H-bond) + HEm(misfit) + HEm(VdW) E Hm
(7)
values Therefore, the method can be used to analyze the in terms of the different intermolecular interactions between the components of mixtures with IL. Hence, Figure 9 shows the E (H-bond), individual contributions represented by eq 7, Hm E E E (misfit), and Hm (VdW), at the Hm Hm of equimolar mixtures of [b2mpy][BF4] with the hydroxylic compounds chosen here at 298.15 and 318.15 K. It can be seen that the dominant interaction is that of the hydrogen bond, which contributes to the endothermicity of the mixtures in all cases. The electrostatic interactions (misfit) are repellent, and their contribution increases with the length of the alkanol chain. In the case of the mixture E (misfit) term has a slightly [b2mpy][BF4] + water, the Hm
negative value. However, the Van der Waals interactions make little contribution to the excess values of these mixtures, with the endothermicity also increasing with the chain length of the alcohol. As can be observed in Figure 9b, temperature mainly affects the enthalpic term corresponding to the hydrogen bond, E (H-bond), and its value increases significantly with shorter Hm alkanol chain lengths, with the extreme case being that of water. It is noteworthy that the value of each summand in eq 7 is obtained from the contribution of each component of the mixture, according to the expressions E E (H-bond)]+xsolvent[Hsolvent (H-bond)] HEm(H-bond) ) xIL[HIL (8) E E (misfit)]+xsolvent[Hsolvent (misfit)] (9) HEm(misfit) ) xIL[HIL E E (VdW)]+xsolvent[Hsolvent (VdW)] (10) HEm(VdW) ) xIL[HIL
where solvent ) water or alkanol, and the contribution of each E of each type of interaction (H-bond, component i to the Hm misfit, and VdW) is defined as
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2687
HEi (interaction) ) Hi,mixture(interaction)-Hi,pure(interaction) (11) Therefore, the COSMO-RS method can determine the contribution made by each component to the final values of the E . As an example, Figure 10 presents enthalpy of the mixture Hm E E and Halkanol for the H-bond, misfit, and VdW the values of HIL interactions of the mixtures considered in this work [b2mpy][BF4] + methanol and [b2mpy][BF4] + butan-1-ol at 298.15 K. A first result of interest is that the alkanol component, E E , makes a major contribution to the total Hm . Moreover, Halkanol E in all the interaction types, the Halkanol increases the endothermic behavior of the final mixture. In other words, more favorable interactions are produced between the molecules of the alkanol (autoassociative effect) than between these and the IL molecules. E are negative for the three types By contrast, the values of HIL of simulated interactions; in other words, the IL species in a fluid interacts with a greater attraction (or less repulsion) with
the alkanol than with itself. Figure 10b shows that, as the chain length of the second compound increases, most of the endothermicity of the IL + alkanol mixture must be attributed to the significant increase in repellent electrostatic interactions (misfit) between the alkanol and the IL in the mixture. On the other E (H-bond) (endothermic) hand, a smaller contribution of Halkanol can be observed as the size of the alkanol increases. Lastly, according to the results shown in Figure 10a, the higher values E as the temperature increases, from 298.15 to 318.15 K, of Hm E (H-bond). must mainly be attributed to the increase in Halkanol 5. Discussion The COSMO-RS methodology can be used to study the behavior of mixtures of IL + water or IL + alkanols in terms of excess enthalpy, HEm. Hence, endothermic effects are observed for all the mixtures [b2mpy][BF4] + water or + alkanol, analogously to results published in the literature for the mixtures
E Figure 8. Plot of experimental values (solid symbols) and those obtained by COSMO-RS (open symbols) of excess enthalpies Hm vs xIL for binaries [b2mpy][BF4] + solvent at the temperatures: (a) T ) 298.15 K; (b) T ) 318.15 K. (b) water; (2) methanol; (9) ethanol; ([) propan-1-ol; (+), butan-1-ol.
E Figure 9. COSMO-RS prediction of equimolar Hm for binary mixtures [b2mpy][BF4] + solvent at (a) T ) 298.15 K and (b) T ) 318.15 K, in terms of their E E E particular contributions: (gray bars) Hm (H-bond); (black bars) Hm (misfit); (white bars) Hm (VdW).
2688 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009
E E Figure 10. Predictions obtained by COSMO-RS for the contributions of individual compounds, HIL and Halcohol to the total excess enthalpy of an equimolar E mixture Hm for binary mixtures [b2mpy][BF4] + solvent in terms of H-bond, misfit, and VdW. (a) [b2mpy][BF4] at 298.15 K (gray bars) and at 318.15 K (bars with checkerboard pattern), and methanol at 298.15 (bars with slanted lines) and at 318.15 K (white bars). (b) [b2mpy][BF4] (gray bars) and butanol (white bars) at 318.15 K.
E Figure 11. Predictions obtained by COSMO-RS of equimolar Hm for binary mixtures of different ionic liquids with water, methanol, and acetone at T ) E E E 298.15 K, in terms of their particular contributions: (gray bar) Hm (H-bond); (black bar)Hm (misfit); (white bar) Hm (VdW).
[bmim][BF4] + water24 and [bmim][PF6] or [hxmim][Tf2N] + alkanol.25,26 On the basis of the COSMO-RS simulation shown in Figure 11, these results must mainly be associated with the rupture of hydrogen bonds between water molecules or the alkanol. On the other hand, the interaction between very different molecules produces negative (exothermic) effects, such as the interactions of the -OH group of the alkanol with the anion, which are stronger than the ones with the cation of the pyridine. In the literature, the case of [mmim][CH3SO4] + water27 is described as an exothermic mixture due to the high basicity of this anion, which promotes strong interactions of ionic-water liquid hydrogen bonds, as can be deduced from the COSMORS data (Figure 11). Mixtures of IL with strongly basic solvents, such as acetone, are also predicted by COSMO-RS to show exothermic behavior (Figure 11), this time attributable to effective interactions between the cation of the ionic liquid and the keto group (hydrogen bond acceptor) of the solvent. As described in section 5, endothermic effects increase as the temperature of the mixtures [b2mpy][BF4] + water or alkanol E has also been observed rises. This effect of temperature in Hm
in other mixtures of ionic liquids with water and alcohols,24,25 being interpreted by COSMO-RS in terms of greater losses of hydrogen bonds from the water or alkanol in the mixture. The summary of the application is an acceptable qualitative predicE , although quantitatively there are tion of the method for Hm differences which, if possible, must be reduced in future applications. E magnitude not The interpretation of the excess volume Vm simulated by COSMO-RS for the mixtures is not so simple, since the final values are the result of the combined effects. E , such as those obtained in the mixture Positive values of Vm [b2mpy][BF4] + water, have been reported for the mixture E [bmim][BF4] + water,24 also observing an increase in Vm with temperature. The COSMO-RS results indicate that in these cases the positive/expansion effects prevail due to the rupture of hydrogen bonds between the water or alkanol molecules as they dissolve in the mixture. In contrast, the mixtures [b2mpy][BF4] + alkanol present negative VEm, a behavior that was also observed in the mixtures [omim][BF4] + butanol,28 [mmim][CH3SO4] + water or methanol,27 and [bmim][PF6] + butanone.29 The
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2689
E Figure 12. Prediction obtained by COSMO-RS of equimolar Hm for binary mixtures of [b2mpy][BF4] + different solvents at T ) 298.15 K, in terms of their E E E different contributions: (gray bars) Hm (H-bond); (black bars) Hm (misfit); (white bars) Hm (VdW).
COSMO-RS analysis shows that negative contraction effects are dominant in these systems, owing to favorable bonding interactions between the anion (or cation) of IL and acid -OH groups (or basic -CdO) of the solvent. Moreover, in all the E it was observed that the contraction mixtures with negative Vm effects increased with a rise in temperature, which associates the COSMO-RS with a more exothermic enthalpy HEIL(H-bond) as the temperature rises. When further developed, the COSMO-RS analysis can be used in practice to design mixtures with specific excess properties. For example, Figure 12 shows the contributions E of mixtures of H-bond, misfit, and VdW made to the Hm [b2mpy][BF4] with differently functionalized alcohols. The endothermicity of the mixture can be observed to increase by simply adding -CH2- groups to the alcohol chain, consequently increasing the repellent electrostatic interactions between this and the ionic liquid. On the other hand, the behavior of the mixtures of IL + alkanol can even become inverted; in other E can be obtained, increasing the words, negative values of Hm acidity of alkanol by introducing halogenated or saturated groups (multiple bonds or aromatic rings). It is noteworthy that by E of -10660 J mol-1 can following this approach values of Hm be reached in the case of the mixture [b2mpy][BF4] + hexafluoropropanol. The COSMO-RS analysis followed here can be of great value to design applications based on ionic liquids, since they permit to rationalization of the selection of the cation-ion pair for obtaining the type of interactions required to achieve a given behavior as a solvent. Finally, if we evaluate the experimental results in relation to the position of the CH3 group in the pyridine ring, for mixtures with water, the [b4mpy][BF4] is the most exothermic and also the most expansive. In relation to mixtures with alkanols, [b4mpy][BF4] is the least exothermic from the viewpoint of energetics, with the packing being the worst and therefore with a lower degree of contraction than those of the other two isomers, [b2mpy][BF4] and [b3mpy][BF4], which is only to be expected given the more linear shape of the molecule [b4mpy][BF4]. In any case, the rise in temperature favors the
packing of the final solution of IL + alkanols, increasing the contraction effects. Basically, [b2mpy][BF4] can be distinguished from the other two isomers by its greater density, its greater viscosity, and its higher UCST′s. Nevertheless, it does not E E values or the lowest Vm , and the present either the highest Hm COSMO-RS results do not indicate an exceptional behavior either. All this means that the resulting behavior is in fact a subtle combination of several different interactions. Acknowledgment The authors gratefully acknowledge the financial support received from the Ministerio de Educacio´n y Ciencia (Spain) for the Project CTQ200-12027. Literature Cited (1) Ortega, J.; Vreekamp, R.; Marrero, E.; Penco, E. Thermodynamic properties of 1-butyl-3-methylpyridinium tetrafluoroborate and its mixtures with water and alkanols. J. Chem. Eng. Data 2007, 52, 2269–2276. (2) Ortega, J.; Vreekamp, R.; Penco, E.; Marrero, E. Mixing thermodynamicpropertiesof1-butyl-4-methylpyridiniumtetrafluoroborate[b4mpy][BF4] with water and with an alkan-1-ol (methanol to pentanol). J. Chem. Thermodyn. 2008, 40, 1087–1094. (3) Range, J.; Mu¨ller, A.; Botin-Weber, U.; Stock, F.; Stolte, S.; Arning, J.; Sto¨rmann, R.; Jastorff, B. Lipophilicity parameters for ionic liquid cations and their correlation to in vitro cytotoxicity. Ecotoxicol. EnViron. Saf. 2007, 67, 430–438. (4) Klamt, A. COSMO-RS: From Quantum Chemistry to Fluid Phase Thermodynamics and Drug Design, 1st ed.; Elsevier: Amsterdam, 2005. (5) Klamt, A.; Eckert, F. COSMO-RS: a novel and efficient method for the a priori prediction of thermophysical data of liquids. Fluid Phase Equilib. 2000, 172, 43–72. (6) Ortega, J.; Marrero, E.; Palomar, J. Description of Thermodynamic Behavior of the Systems Formed by Alkyl Ethanoates with 1-Chloroalkanes Using the COSMO-RS Methodology Contributing with New Experimental Information. Ind. Eng. Chem. Res. 2008, 47, 3253–3264. (7) Diedenhofen, M.; Eckert, F.; Klamt, A. Prediction of Infinite Dilution Activity Coefficients of Organic Compounds in Ionic Liquids Using COSMO-RS. J. Chem. Eng. Data 2003, 3, 475–479. (8) Marsh, K. N.; Boxall, J. A.; Lichtenthaler, R. Room temperature ionic liquids and their mixturessa review. Fluid Phase Equilib. 2004, 1, 93–98.
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ReceiVed for reView June 25, 2008 ReVised manuscript receiVed November 27, 2008 Accepted December 8, 2008 IE8009878
