Polarity of Some [NR1R2R3R4]+[Tf2N]â Ionic Liquids in Ethanol

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Polarity of Some [NR1R2R3R4]+[Tf2N]− Ionic Liquids in Ethanol: Preferential Solvation versus Solvent−Solvent Interactions Maria-Luísa C. J. Moita,*,† Â ngela F. S. Santos,‡ Joaõ F. C. C. Silva,† and Isabel M. S. Lampreia‡ †

Departamento de Química e Bioquímica, Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016, Lisboa, Portugal ‡ Departamento de Química e Bioquímica, Centro de Ciências Moleculares e Materiais, Faculdade de Ciências, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016, Lisboa, Portugal S Supporting Information *

ABSTRACT: The polarity of the ionic liquids [N4111][Tf2N], [N4441][Tf2N], and [choline][Tf2N], as well as their binary mixtures with ethanol has been investigated using solvatochromic dyes and expressed in terms of the Reichardt parameter, ENT , and the Kamlet−Taft parameters (π*, α, and β), at 298.15 K. The synergetic behavior revealed essentially by the ethanol + [N4441][Tf2N] system for the acidity of the solvent shows the potential importance of this solvent media for some chemical applications. Even though the preferential solvation model of Bosch and co-workers has been applied with some statistical success to the whole composition range of EtOH + [N4111][Tf2N] and EtOH + [N4441][Tf2N] binary mixtures, the interpretation of the results is somehow difficult due to the competition of molecular interactions in the evaluation of transition energies for the pairs: dye−solvent (dye−cation and/or dye−anion, dye− EtOH, dye−complex entity) and cation−anion, depending on the composition range. Different patterns of solvent polarity behavior are shown along the entire ionic liquid (IL) composition.

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INTRODUCTION Although molecular solvents have been the usual media for physical and chemical processes in solution,1 in the past decade, room temperature ionic liquids (RTILs) have shown to be very useful because of their very particular properties, such as nonflammability, thermal stability, very low volatility at ambient conditions, and high solubility in organic and inorganic compounds with a wide range of polarity. These characteristics make ionic liquids (ILs) promising compounds for use in industry and with a recent emphasis in the pharmaceutical industry.2−8 When compared with molecular solvents, ILs often present higher dipolarity/polarizability characteristics which make them valuable “effective polarity” solvents.9−11 In this perspective it will be crucial to obtain information regarding the relative polarity of ILs and to study the change in physicochemical properties produced by their addition to molecular solvents. It has been already identified that, when an IL is mixed with an other solvent such as water or an alcohol, the polarity properties of the system can be tunable depending on the combination of the cation and anion and on the careful choice of the cosolvent.12−17 The solvent polarity is defined as “its overall solvation capability” depending on the action of all possible, specific (hydrogen bonds, electron pair donor/electron pair acceptor), and nonspecific (van-der-Waals and Coulombic interactions) intermolecular solute−solvent forces.1,18 The inadequate way of characterizing the solvent polarity, merely based on macro© 2012 American Chemical Society

scopic physical parameters, gave rise to the appearance of solvatochromic parameters. These parameters are based on molecular probes, the spectroscopic properties of which are strongly solvent-dependent and serve as appropriate model processes for the study of other solvent effects.19−24 The purpose of this work is to measure the polarity of the ILs, butyl-trimethylammonium bis(trifluoromethansulfonyl) imide, [N 4111 ][Tf 2 N], tributylmethylammonium bis(trifluoromethansulfonyl) imide, [N4441][Tf2N], and 2-(hydroxyethyl)-(trimethylammonium) bis(trifluoromethansulfonyl) imide, [choline][Tf2N], and their binary mixtures with ethanol. Figure 1 shows the molecular structures of the anion and the three cations of the ILs used. The whole composition range was covered for the binary ethanol + [N4111][Tf2N] and ethanol + [N4441][Tf2N] mixtures. However, for the ethanol + [choline][Tf2N] mixture only the range 0 < xcholine < 0.3 was studied due to solubility problems for higher compositions. Three molecular probes (Figure 2) were used to determine some solvatochromic parameters, SP, at 298.15 K. The Reichardt’s dye (probe 1) is a hydrogen-bond acceptor molecule (HBA) as well as an electron pair-donor (EPD), via the phenolate oxygen. Therefore, this dye measures the solvent Received: May 22, 2012 Accepted: August 1, 2012 Published: August 23, 2012 2702

dx.doi.org/10.1021/je300563k | J. Chem. Eng. Data 2012, 57, 2702−2709

Journal of Chemical & Engineering Data

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respectively. The π* parameter should also measure the electrostatic contributions in the case of ILs. The preferential solvation phenomena in binary mixtures containing ILs is much more difficult to interpret than in the case of two molecular solvents. The probe molecule can be preferentially solvated either by the molecular solvent, by the anion, by the cation, or by an entity (or more than one) resulting from the interaction between IL and solvent molecules (cation−solvent and/or anion−solvent). As a consequence, too much work has to be made to clarify the nature of these interactions.

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Figure 1. Molecular structure of the cations (a) [N4111], (b) [N4441], and (c) [choline] and the anion (d) of the ILs used in this work.

EXPERIMENTAL SECTION Materials and Methods. The ethanol (EtOH) used was from Panreac (purity >99.9 mass %). It was stored in a special container for calibrants with molecular sieves (3 Å) purchased from Aldrich. The three ILs ([N4111][Tf2N], [N4441][Tf2N], and [choline][Tf2N]) were delivered by Iolitec (Ionic Liquid Tecnhologies) (purity >99 mass %) and used without additional purification. The IL vials were all opened under an atmosphere of argon 6 (purity >99.9999 mass %, obtained from Praxair). The vials were always sealed after being used and stored in a desiccator containing a dry desiccant. The water content of the first two ILs ([N4111][Tf2N] and [N4441][Tf2N]) was checked before and after the preparation of the binary mixtures, using a Karl Fischer coulometric titrator (Metrohm 831 KF). Initially, the water content was 184 ppm for the former and 45 ppm for the latter IL. At the end of the process of solution preparation there was a maximum increase of 3 % in the water content. The halide content of ILs given by the suppliers was less than 100 ppm. All solutions were prepared by mass in special designed volumetric containers to avoid evaporation and contamination by contact with air. The total volume of solution was prepared attending to the minimization of the vapor phase. Buoyancy corrections were done. Uncertainties in the calculated mole fractions were found to be less than ± 0.00005. The three molecular dyes used were: 2,6-diphenyl-4-(2,4,6triphenylpyridinium-1-yl) phenolate (Reichardt’s betaine, RB), probe 1, from Fluka, 98.5 mass %; 4-amino-nitrobenzene, probe 2, from Aldrich, 99 mass %; and 4-(dimethylamino)-nitrobenzene, probe 3, from Tokyo Chemical Industry Co.-GR, 99 mass %. UV−vis spectroscopic measurements were performed with a double-beam Nicolet Evolution 300 spectrometer (VISON pro software) and a pair of matched quartz analytical cuvettes of 1 cm optical length sealed with a Teflon cap. The temperature was kept constant at 298.15 ± 0.10 K. The filling of the cells was carefully done under an atmosphere of argon. The amount of each dye used was evaluated to obtain absorbance values ranging from 0.5 to 1. The concentration of the dye was changed until the peak wavelength was kept constant to avoid the formation of aggregates. The wavelength values of maximum absorption for dyes were determined following the procedure described before.25 The arithmetic average of at least four wavelength values has been used, for each solution, with a standard deviation less than 0.8 nm. The solvatochromic parameters were then determined from the wavenumber of maximum absorbance of each dye, ν̃i(i = 1, 2, or 3), expressed in kiloKaiser (1 kK = 1000 cm−1). Data

Figure 2. Molecular structure of the solvatochromic probes used: Reichardt’s dye (1), 4-amino-nitrobenzene (2), and 4-(dimethylamino)-nitrobenzene (3).

HBD acidity in addition to the nonspecific dye−solvent interactions. This betaine dye was used to define the ET(30) parameter and the normalized one (ENT ) to give a dimensionless value of 1 for water and 0 for tetramethylsilane, TMS. 4-Aminonitrobenzene (probe 2) is a hydrogen-bond donor molecule (HBD) and an electron pair-acceptor (EPA) as well. The enhanced solvatochromic band shift of this probe relative to its homomorph, the 4-(dimethylamino)-nitrobenzene (probe 3), allows the determination of the Kamlet−Taft β parameter, standardized to give a value of 1 for hexamethylphosphoramide, HMPA. This parameter reflects the solvent HBA basicity by means of specific solute−solvent interactions. The Kamlet−Taft α-parameter values were estimated bearing in mind the enhanced solvatochromic shift band of Reichardt’s dye (probe 1) relative to 4-(dimethylamino)-nitrobenzene (probe 3), standardized to give a value of 1 for methanol. This parameter accounts for the HBD acidity of the solvent by means of specific dye−solvent interactions. The 4-(dimethylamino)-nitrobenzene (probe 3) is a nonhydrogen bond donor aromatic molecule (non-HBD) of the type A-C6H4-D, having an electron-acceptor group, A = −NO2, and an electron-donor group, D = −N(CH3)2. This probe was also used to evaluate the Kamlet−Taft π* parameter, standardized to give a value of 0 for cyclohexane and 1 for dimethyl sulfoxide. This parameter accounts for nonspecific dipolarity/polarizability solute−solvent interactions. In summary, Reichardt’s parameter, ENT , is a measure of the dipolarity/polarizability and HBD acidity of the solvent, and the Kamlet−Taft parameters α, β, and π* are measures of the HBD acidity, HBA basicity, and dipolarity/polarizability, 2703



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Table 1. Regression Coefficients and Statistical Parameters of Equation 2 Corresponding to the Application of the PS Model, at 298.15 K EtOH (1) + [N4111][Tf2N] (2) A/kK RB 4-amino-nitrobenzene 4-(dimethylamino)-nitrobenzene

18.14 26.848 25.843

RB 4-amino-nitrobenzene 4-(dimethylamino)-nitrobenzene

B/kK

C/kK

E

N

σ/kK

R2

F

4.86 47.600 28.237

21 19 21

0.20 0.03 0.02

0.9771 0.9917 0.9931

170.6 417.6 575.8

D

3570.70 −0.31 988.77 53.616 −20.193 107.338 −6.608 −26.873 7.942 EtOH (1) + [N4441][Tf2N] (2)

A/kK

B/kK

C/kK

D

E

N

σ/kK

R2

F

18.05 26.875 25.867

28.33 23.659 −6.197

40.65 −26.161 −33.971

17.81 103.245 6.674

8.65 54.956 35.048

14 14 14

0.10 0.02 0.01

0.9893 0.9958 0.9980

208.5 536.9 1106

Table 2. Estimated Physicochemical Parameters for the EtOH (S1) + IL (S2) Binary Mixtures, Calculated by Using the PS Model Described by Equations 2 to 5, at 298.15 K EtOH (1) + [N4111][Tf2N] (2) ν̃1/kK RB 4-amino-nitrobenzene 4-(dimethylamino)-nitrobenzene

18.14 26.85 25.84

RB 4-amino-nitrobenzene 4-(dimethylamino)-nitrobenzene

ν̃2/kK

ν̃12/kK

21.75 18.08 27.35 26.43 25.01 24.89 EtOH (1) + [N4441][Tf2N] (2)

f12/2

4.86 47.60 28.24

0.005 0.44 3.56

ν̃2/kK

ν̃12/kK

f 2/1

f12/1

f12/2

18.05 26.88 25.87

19.64 27.10 24.94

22.74 26.39 24.90

17.81 103.24 6.67

8.65 54.96 35.05

0.49 0.53 5.25

Supporting Information (Figures S1 and S2) for the systems [N4111][Tf2N] and [N4441][Tf2N] with EtOH, respectively.

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RESULTS AND DISCUSSION Preferential Solvation Model. The general solvent− exchange equilibrium model proposed by Skwierczynski and Connors26 and further extended by Bosch and co-workers27,28 has been successfully applied to a wide range of solvent systems and more recently to a few mixtures of ILs with molecular solvents.13,16,29,30 This preferential solvation (PS) model describes the transition energy of a probe molecule as the average of the energies of all chemical entities that compose its cybotactic region, according to their mole fractions within this microsphere. However, in binary mixtures in which one of the components is a IL, the study of this phenomena is a much more complex task because the probe molecule can be preferentially solvated either by the molecular solvent, the IL anion, the IL cation, or by an entity (or more than one) resulting from the interactions of IL−molecular solvent. The PS model mentioned above has been applied to the wavenumber values of maximum absorbance for the dyes (1 to 3) in the two binary systems [N4111][Tf2N] and [N4441][Tf2N] in EtOH. The exchange process model with the formation of a new entity designed as S12 on the cybotactic solute region, resulting from the interaction between S1 (EtOH) and S2 (IL), may be written as

I(S1)2 + S2 ↔ I(S12)2 + S1

f12/1

ν̃1/kK

values for the three binary EtOH (1) + IL (2) systems are available in the Supporting Information (Tables S1 to S3).

I(S1)2 + 2S2 ↔ I(S2)2 + 2S1

f 2/1 988.77 107.34 7.94

ν̃ = A +

B(x 2)2 + C(1 − x 2)x 2 (1 − x 2)2 + D(x 2)2 + E(1 − x 2)x 2

(2)

In this equation x2 represents the mole fraction of the IL solvent in the bulk, and A, B, C, D, and E are the regression coefficients that minimize the square residuals of the ν̃ values. These coefficients and the statistical regression parameters such as the standard deviation of the fit, σ, the number of data points, N, the coefficient of determination, R2, and the Fisher statistics, F, were calculated using a nonlinear regression with the computer program LAB fit V7.2.48. These results are presented in Table 1. According to the model described above, the physicochemical significance of each regression coefficient is: A = ν̃1, B = f 2/1(ν̃2 − ν̃1), C = f12/1(ν̃12 − ν̃1), D = f 2/1, and E = f12/1. The equilibrium constants for those processes, f 2/1, f12/1, and f12/2, were defined as: f2/1 = f12/1 =

x 2s/x1s (x 2/x1)2

(3)

s x12 /x1s (x 2/x1)

(4)

and (1a)

f12/2 =

(1b)

27,28

f12/1 f2/1

(5)

xsi

where and xi represent the mole fractions of component i (1 and 2 stand for EtOH and IL, respectively) in the cybotactic region of the probe, and in the solvent bulk, respectively. The

According to this simple model, the wavenumber value of maximum absorbance of a dye (ν̃) in the mixture as a function of the mole fraction x2 is given by eq 2 and is illustrated in the 2704



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Table 3. Determined Solvatochromic Parameters and Literature Data for Pure Solvents, Measured at 298.15 K solvent [N4111][Tf2N] [N1124][Tf2N] [N4441][Tf2N] [N122(2‑OMe)][Tf2N] EtOH

ET(30)/kcal·mol−1

ENT

π*

α

62.08 49.3a 59.00b 55.78 46.7c

0.968 0.574a 0.868b 0.774 0.494c

0.903 0.970a 0.89b 0.923

1.273 0.471a 1.16b 0.853

51.76 51.9e

0.65

0.972d 0.655 0.54e,f,g

0.824 0.86e,f,g

a f

Reference 32. bReference 33. cReference 34. Reference 37. gReference 38.

d

Table 4. Solvatochromic Parameters of EtOH (1) + [N4111][Tf2N] (2) Binary Mixtures, Measured at 298.15 K

(6)

E T(30)solvent − E T(30)TMS E (30)solvent − 30.7 = T E T(30)water − E T(30)TMS 32.4 (7)

ν3̃ − νC̃ 6H12 − νC̃ 6H12 νDMSO ̃

α1 − 3 =

=

ν3̃ − 28.18 −3.52

(8)

ΔΔ(1 − 3)solvent ΔΔ(1 − 3)MeOH

ΔΔ(1 − 3)solvent 5.47 1.318ν3̃ − 47.7 + ν1̃ = 5.47 =

β2 − 3 =

(9)

−ΔΔ(2 − 3)solvent −ΔΔ(2 − 3)HMPA

−ΔΔ(2 − 3)solvent 2.759 0.9841ν3̃ + 3.49 − ν2̃ = 2.759

0.263d 0.757 0.75e,f,g

probes other than the one we used. The differences observed between our ENT and α parameter values and literature results for the [N4111][Tf2N] IL cannot be attributed to the water content, because if this had been the case, we would have obtained a higher value for the π* parameter. We note that the solvatochromic parameters for water1,27 are ENT = 1.00, α = 1.17, and π* = 1.09. However, recent data33,35 for [N1124][Tf2N] and [N122(2‑OMe)][Tf2N] ILs corroborate our results. The evaluated SP parameters, for the binary mixtures of EtOH (1) + LI (2), at different mole fractions, x2, are presented in Tables 4 to 6. The corresponding plots are presented in

E T(30)/kcal·mol−1 = hcν1̃ NA = 2.8591· 10−3(ν1̃ /cm−1)

π* =

0.27b 0.325

Reference 35. ([N122(2‑OMe)] is diethylmethyl(2-methoxyethyl)ammonium). eReference 36.

subscript 12 refers to entity S12, described before. While the f 2/1 and f12/1 parameters quantify the solvating ability of S2 and S12 relative to S1, the f12/2 parameter quantifies the solvating ability of S12 relative to S2. Results obtained from the application of the PS model to the two binary systems are given in Table 2. The RB and the 4amino-nitrobenzene probe are preferentially solvated, over the whole composition range, by the IL [N4111][Tf2N] or [N4441][Tf2N], relative to ethanol and relative to the entity formed, S12, as f 2/1 > 1 and f12/2 < 2. Then, both probe dyes tend to be solvated by the S12 complex, rather than by pure ethanol as f12/1 > 2. However, the 4-(dimethylamino)nitrobenzene probe seems to be preferentially solvated by the S12 entity rather than by any of the two IL as f 2/1 > 1, f12/1 > 2, and f12/2 > 2. Solvatochromic Parameters. The following eqs 6 to 10 were used to calculate the solvent parameters:1,2,25,31

E TN =

β 0.259

x2

ENT

π*

α

β

0 0.01000 0.01998 0.02998 0.03910 0.04994 0.05995 0.09993 0.12839 0.15646 0.20034 0.24988 0.29995 0.34955 0.40037 0.50015 0.59036 0.69967 0.80092 0.89900 1

0.650 0.695 0.717 0.804 0.860 0.835 0.924 0.918 0.950 0.970 0.950 0.968 0.972 0.969 0.955 0.967 0.966 0.965 0.981 0.990 0.968

0.655 0.738 0.768 0.793 0.806 0.814 0.838 0.868 0.877 0.895 0.895 0.912 0.91 0.923 0.917 0.926 0.919 0.921 0.912 0.903 0.903

0.824 0.848 0.867 1.025 1.13 1.073 1.236 1.199 1.257 1.283 1.243 1.265 1.275 1.257 1.235 1.251 1.255 1.251 1.29 1.317 1.273

0.757 0.711 0.687 0.671 0.656 0.623 0.621 0.548 0.528 0.492 0.452 0.411 0.393 0.386 0.357 0.326 0.305 0.305 0.285 0.259

Figure 3. It is very important to stress that all of these “solvent polarity parameters” are estimates of the alleged “fundamental properties”39 since they represent the propensity of the solvent to solvate a particular molecule (probe), according to their nature. Looking at Figure 3, it is a general observation that the addition of [N4111][Tf2N], [N4441][Tf2N], or [choline][Tf2N] to pure ethanol causes an abrupt increase in acidity and dipolarity/polarizability and in parameter ENT , a measure of the solvent HBD acidity and dipolarity/polarizability, as well. This dual characteristic is clearly demonstrated in the case of the

=

(10)

The experimental solvatochromic parameters, SP, are compared with literature values32−38 for the pure solvents used in this work and are presented in Table 3. Our results for ethanol are in good agreement with those found in literature, and the small observed deviations, in the case of the π* parameter values, can be attributed to the use of different 2705



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should be easier to occur in the case of the [N4111][Tf2N] IL as it seems to present lower stereochemical interference. A synergistic behavior is clearly observed in the case of the [N4441][Tf2N] + EtOH system, when we observe both ENT (Figure 3a) and α (Figure 3c) versus x2 plots. This fact validates the existence of a complex S12 with a higher polarity than that of the pure components. Both IL cations account primarily for the differences found in the acidity parameter plots as the anion is the same in both IL; even more, it is generally accepted that the ILs acidity is predominantly attributed to cations.32,33,40,41 It is perceptible that, in the range of greatest IL concentration, the addition of EtOH molecules to [N4111][Tf2N] seems not to change significantly the solvent acidity (measured by ENT and α). The addition of alcohol molecules to [N4441][Tf2N] causes a slight increase in the same solvent parameters, reaching a maximum value at x2 ≅ 0.19. The [N4111][Tf2N] IL is apparently more structured than [N4441][Tf2N] and probably contains more voids that can accommodate the EtOH molecules without destroying its structure. The disruption of the [N4441][Tf2N] structure by EtOH molecules seems to be easier to occur. Simultaneously, there is the possibility of the occurrence of IL-EtOH interactions leading to an entity S12 with higher polarity expressed by the estimated parameter ν̃12 = 22.74 kK relative to those estimated for the pure solvents ν̃1 = 18.05 kK and ν̃1 = 19.64 kK (using RB as a probe molecule, Table 2). The analysis of the π* versus x2 plots (Figure 3b) shows that for both ILs, [N4111][Tf2N] and [N4441][Tf2N], the profile of the curves is very similar and the values only differ significantly for x2 > 0.6. The π* parameter value is slightly higher for the [N4441][Tf2N] than for [N4111][Tf2N] pure ILs (Table 3 and Figure 3b). This can be mainly attributed to the increase of the alkyl chains length42 and consequently the increase of the cation volume43 on the former, giving rise to a more polarizable solvent. Considering the solvent basicity, β, we can observe in Figure 3d a similar behavior for the binary [N4111][Tf2N] + EtOH and

Table 5. Solvatochromic Parameters of EtOH (1) + [N4441][Tf2N] (2) Binary Mixtures, Measured at 298.15 K x2

ENT

π*

α

β

0 0.01492 0.02976 0.04996 0.06998 0.09241 0.18822 0.28131 0.39909 0.60048 0.79881 0.87650 0.93638 1

0.650 0.695 0.718 0.773 0.817 0.840 0.891 0.893 0.873 0.857 0.820 0.806 0.797 0.774

0.655 0.756 0.804 0.832 0.859 0.865 0.901 0.917 0.923 0.928 0.930 0.923 0.919 0.923

0.824 0.831 0.837 0.928 0.996 1.038 1.114 1.105 1.059 1.021 0.944 0.920 0.905 0.853

0.757 0.697 0.670 0.649 0.621 0.597 0.529 0.487 0.445 0.395 0.363 0.351 0.350 0.325

Table 6. Solvatochromic Parameters of EtOH (1) + [Choline][Tf2N] (2) Binary Mixtures, Measured at 298.15 K x2

ENT

π*

α

β

0 0.03986 0.09983 0.19902 0.29490

0.650 0.786 0.827 0.858 0.873

0.655 0.816 0.883 0.939 0.976

0.824 0.969 0.996 1.014 1.013

0.757 0.633 0.547 0.434 0.348

mixture EtOH + [N4441][Tf2N] as there is a very good biparametric correlation between ENT values and the corresponding π* and α values (Table 7). The acidity of pure [N4111][Tf2N] is much higher than that of [N4441][Tf2N]. It means that the dipolar electronic ground state of the RB probe is much more stabilized in the former than in the latter IL. This stabilization is probably due to EPA−EPD interactions between the IL cation (EPA) and the phenolate anion (EPD) that

Figure 3. Solvatochromic parameters ENT , π*, α, and β versus x2, at 298.15 K, for the binary systems EtOH (1) + [N4111][Tf2N] (2) (●), EtOH (1) + [N4441][Tf2N] (2) (■), and EtOH (1) + [choline][Tf2N] (2) (△). Dashed lines represent polynomial adjustments of the 6th order. 2706



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Table 7. Biparametric Linear Regression between the Solvent Parameters ETN and (π*, α) According to ENT = A + Bπ* + Cαa A

B

C

EtOH + [N4111][Tf2N]

−0.0145

0.4038

EtOH + [N4441][Tf2N]

−0.01578

N

0.4857 21 (π*, α) Colinearity: r2 = 0.88 0.40939 0.48269 14 (π*, α) Nonlinearity: r2 = 0.33

R2

σ

0.99999

0.0004

1.00000

0.0000

F 839451 1.3·1030

a

The adjustment parameters of the two regressions are: the regression coefficients (A, B, C), the number of data points (N), coefficients of determination (R2), standard deviation of the fit (σ), and the Fisher statistics (F). The noncolinearity between π* and α parameters is checked using the r2 parameter (uniparametric coefficient of determination).

Figure 4. Mixing solvatochromic parameters ΔENT , Δπ*, Δα, and Δβ for EtOH (1) + [N4111][Tf2N] (2) (●) and EtOH (1) + [N4441][Tf2N] (2) (■) versus x2. Dashed lines stand for Redlich−Kister adjustments and are drawn to guide the eye only. Table 8 shows the adjustment coefficients as well as some statistical information about each fitting.

Table 8. Redlich−Kister Coefficients, Coefficients of Determination (R2), Standard Deviation of the Fit (σ), and the Fisher Statistics (F), for the Mixing Solvatochromic Parameters, ΔSP, versus the Mole Fraction of Solvent 2 According to the Following Equation: ΔSP = x1x2∑kj=0Aj(x1 − x2)j EtOH (1) + [N4111][Tf2N] (2) ΔENT Δπ* Δα Δβ

ΔENT Δπ* Δα Δβ

A0

A1

A2

0.633 0.590 0.829 −0.5985

0.772 0.754 0.774 −0.4387

0.292 0.054 0.256 1.3642

A3

A4

−0.638 −1.903 1.702 −0.4409 EtOH (1)

A5

2.203 2.609 1.458 4.212 3.871 5.2340 −0.04188 + [N4441][Tf2N] (2)

A0

A1

A2

A3

A4

A5

0.6120 0.586 0.799 −0.470

0.3665 0.748 0.888 −0.190

0.6156 −0.225 1.036 −0.515

1.4231 −1.658

0.6081 2.073

−0.7026 3.832

−0.996

A6

R2

σ

F

−5.8825

0.9907 0.9932 0.9767 0.9988

0.02 0.01 0.04 0.005

265.49 364.05 134.43 1567.8

A6

R2

σ

F

0.9988 0.9917 0.9787 0.9894

0.006 0.02 0.03 0.01

1150.99 159.56 168.25 234.28

Mixing Solvatochromic Parameters for [N4111][Tf2N] and [N4441][Tf2N] ILs in Ethanol. The mixing solvatochromic parameters, ΔSP, were calculated according to the following equation:

[N4441][Tf2N] + EtOH mixtures. It has been claimed that the solvent basicity parameter can rather account for the nature of the anion3,40,44 although some effects from cation can occur. In fact, the basicity of [N4111][Tf2N] pure IL is slightly lower than that of [N4441][Tf2N] (Table 3 and Figure 3d). This fact suggests stronger Coulombic interactions between ions in the [N4111][Tf2N] pure IL case, leading to weaker interactions between the HBA anion and the 4-amino-nitrobenzene molecule (HBD, probe 2). The [Tf2N] anion is considered as a “soft anion”,44 and so it does not easily polarize the cation. The addition of [choline][Tf2N] to pure ethanol causes a gentle decrease of the solvent basicity up to a mole fraction of EtOH around 0.3.

ΔSP = SP − (x1SP1 + x 2SP2)

(11)

where SP, SP1, and SP2 are the solvatochromic parameters of the solvent mixture, the component 1 (EtOH) and the component 2 (IL), respectively. Plots of these mixing parameters as a function of IL mole fractions are depicted in Figure 4. Redlich−Kister adjustments to the experimental 2707



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+ [N4441][Tf2N] systems or by an entity formed between IL and EtOH molecules responsible for the synergism observed for the ENT and α parameters as a function of x2, depending on the composition range. The application of the preferential solvation model to our experimental results emphasized the formation of a new entity S12 with an acidic capability higher than that of this two pure components in the case of the binary [N4441][Tf2N] + ethanol mixture.

results were made using least-squares plotting polynomials, according to eq 12. k

ΔSP = x1x 2 ∑ Aj (x1 − x 2) j j=0

(12)

The regression parameters together with the standard deviation of the fits, the coefficients of determination, and the Fisher statistics are shown in Table 8. The analysis of these mixing properties as a function of the mole fraction provides valuable complementary information about the preferential solvation of the probes and about the characteristics of the mixtures themselves. ΔENT , Δα, and Δπ* parameters for both [N4111][Tf2N] and [N4441][Tf2N] ILs in ethanol are all positive indicating that the probes are preferentially solvated by the more polar component, the IL, or by a new entity S12 with a higher polarity. These conclusions validate the suppositions previously advanced and based on the preferential solvation model. In Figure 4a,c, the mixing solvent acidity parameters, ΔENT and Δα, attain maximum values at x2 ≅ 0.13 and at x2 ≅ 0.19, respectively, for both systems. The stabilization of the electronic ground state of RB (via phenolate anion−IL cation interaction) is higher and reaches its maximum value at lower mole fractions for [N4111][Tf2N] relative to the [N4441][Tf2N] IL. Beyond those compositions the ILs will probably start to form aggregates, earlier in the case of [N4111][Tf2N] as compared with [N4441][Tf2N] as the former has a lower molar volume.43 Concerning the π* behavior it appears that there are no significant differences between the binary EtOH + [N4111][Tf2N] and EtOH + [N4441][Tf2N] systems. With the addition of the molecular solvent to these ILs, Δπ* values reach a maximum at x2 ≅ 0.09, where the amount of EtOH molecules turns significant. For compositions higher than x2 ≅ 0.09, the stabilization of the 4-(dimethylamino)-nitrobenzene (probe 3) is probably made by a highly dipolar and/or polarizable entity resulting from the EtOH−IL interactions. However, the mixing Δβ values for both [N4111][Tf2N] and [N4441][Tf2N] ILs in ethanol are all negative, reaching minimum values at x2 ≅ 0.29 in the former and x2 ≅ 0.19 in the latter binary system. While the addition of the IL to EtOH causes a gradual reduction of solvent basicity, the Δβ versus x2 curve crossing at x2 ≅ 0.2 suggests that the ethanol molecules (HBA) would initially control the solvation process of the probe molecule and for x2 > 0.2 the new entity S12 would preferentially solvate the 4-amino-nitrobenzene until x2 ≅ 0.8. Beyond this latter composition the preferential solvation of the probe would rather be controlled by the [Tf2N] anion.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 to S3 and Figures S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +351217500088. E-mail: [email protected]. Funding

Financial support from the Portuguese Foundation for Science and Technology (FCT) is gratefully acknowledged, part of it own projects PEst-OE/QUI/UI0536/2011 and PEst-OE/ QUI/UI0612/2011. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.L.C.J.M. and I.M.S.L. thank the University of Lisbon for granting a sabbatical leave of absence in the academic year 2009/2010.

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REFERENCES

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CONCLUSION It was observed that the addition of the ILs [N4111][Tf2N], [N4441][Tf2N], and [choline][Tf2N] to pure ethanol resulted in a pronounced increasing of the acidity and dipolarity/ polarizability of the solvent mixture. However, the addition of EtOH to the pure ILs [N4111][Tf2N] and [N4441][Tf2N] does not bring any significant alteration of the solvent polarity. These observations led us to expect different regimes of solvent aggregation depending on the composition of the binary solvent mixture. By analyzing the mixing solvatochromic properties, ΔSP, it can be concluded that the probes were, in general, preferentially solvated by the IL in both ethanol + [N4111][Tf2N] and ethanol 2708



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