Polarity issues in room temperature ionic liquids
Shashi Kant Shukla & Anil Kumar
Clean Technologies and Environmental Policy Focusing on Technology Research, Innovation, Demonstration, Insights and Policy Issues for Sustainable Technologies ISSN 1618-954X Clean Techn Environ Policy DOI 10.1007/s10098-014-0864-y
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Author's personal copy Clean Techn Environ Policy DOI 10.1007/s10098-014-0864-y
ORIGINAL PAPER
Polarity issues in room temperature ionic liquids Shashi Kant Shukla • Anil Kumar
Received: 7 March 2013 / Accepted: 6 October 2014 Ó Springer-Verlag Berlin Heidelberg 2014
Abstract The study on ionic liquids has occupied an important place in chemical processes because of several beneficial properties. In order to use these ionic liquids as solvent media in chemical processes, it is essential to know their solvent properties. In this review, an attempt has been made to critically evaluate and discuss the polarity parameters of ionic liquids. The experimental polarity data on ionic liquids are not yet available for numerous ionic liquids. This review will serve as a guideline to investigate polarity issues of these materials and emerge with a simple, but still sound theoretical or semi-empirical models to account for the variation in polarity in the ionic liquid solutions.
Introduction From the last few decades ionic liquids (ILs) have been widely explored in various synthetic (Wassercheid and Welton 2002; Reichardt 2003) and catalytic (Fischer et al. 1999; Earle et al. 1999) processes. The burgeoning interests about ILs are because of their negligible vapor pressure, wide liquidus range, high thermal stability, wide electrochemical window, recyclability, and so on (Fuller et al. 1997; Peter and Wilhelm 2000; Plechkova and Seddon 2008; Smiglak et al. 2007; MacFarlane et al. 2001). In 1914, Walden synthesized first room temperature ionic liquid (m.p. 12 °C) ethylammonium nitrate (Walden 1914) which was used as a substitute for LiCl–KCl electrolyte in thermal batteries. The ancient ILs (first generation of IL) S. K. Shukla A. Kumar (&) Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India e-mail:
[email protected]
were the eutectic mixture of aluminum chloride with ethyl pyridinium halides and dialkylimidazolium chloroaluminate (also known as chloroaluminate ILs). Their hygroscopic behavior poses a problem in their commercialization. Later, more efficient (non-chloroaluminate) ILs containing substituted imidazole, pyridine, pyrrolidine, alkylamine cations along with weakly coordinating anions BF4-, PF6-, and NTf2 (second generation of ILs) were used in various synthetic and catalytic applications. The third generation of IL is ‘task-specific’ IL that accomplishes a given task in order to meet the desired result, e.g. chiral IL for stereospecific and stereoselective reactions. Broadly, ILs are classified into aprotic (AIL) and protic (PIL) subgroups depending on the presence of alkyl group (-R) or a proton (H?) on the quaternized nitrogen. These two subgroups of ILs have different Coulombic as well as van der Waal forces between ions. The relative magnitudes of these forces determine the appearance and functioning of ILs. Behavior of a solute in a medium depends on the various possible specific and non-specific interactions offered by solvent. The overall interaction of a solvent toward a solute can be expressed in terms of ‘‘solvent polarity’’ (Reichardt 1994). Polarity of a solvent determines the rate, equilibrium as well as position, and intensity of spectral absorptions. Polarity of ILs cannot be defined simply in terms of the physical parameters such as relative permittivity (er), dipole moment (l), and refractive index (n) values due to its non-structured nature. However, polarity of IL and their binary mixtures can be successfully measured by solvatochromic method. It depends on the change in electronic absorption of spectrum due to change in solvent polarity. Empirical equations have been constructed for polarity measurement using regression analysis of the data for a given probe in different media. The important solvent
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NH2
N
O-
NO2
NO2
Reichardt's Dye
4-nitroaniline
N,N-diethyl-4-nitroaniline
N
Cl
N
Cl
O-
2,6-dichloro-4-(2,4,6-triphenylphenyl-pyridinium-1-yl) phenolate
Fig. 1 Structures of common solvatochromic probes
Cations O
N
N
R
R' R
R'
R
R
R'
R'
N
P
N
N
N
N
R
R
R
R
R
R
R
R
R
R = CH3, C2H5, C4H9, C6H13, C8H17, C10H21 and R' = H, CH3, C2H5, C4H9
Anions O
F
F F
B F F
F
F
C F
F
H3C
O
O
N S
P F
O
O
F
S
C F
F
C O
F
O F
F
S
F
C F
N
O C
O
N
C N
F
Fig. 2 Structures of different ILs
parameters are electronic transition energy (ET30), and Kamlet–Taft parameters, hydrogen bond donor (HBD) acidity (a), hydrogen bond acceptor (HBA) basicity (b), and polarity index (p*). Polarity of ILs in terms of static dielectric constant (es) was reported by Weingartner. He observed low es value (8.8–15.2) for AIL, indicating low polarity (Wakai et al. 2005), while high polarity values are indicated by the solvatochromic probes. Polarity measurements in different classes of AIL with a variety of anions have been reported by different workers (Crowhurst et al. 2003; Clarissa et al. 2008; Lee et al. 2008; Lee and Prausnitz 2010; Khupse and Kumar 2010). The structures of common solvatochromic dyes are shown in Fig. 1, and the structures of the
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representative classes of cation and anion in Fig. 2. In comparison with AILs, very few reports are available on the polarity of PILs (Shukla et al. 2012) due to their acidic nature that causes disappearance of charge transfer spectra of Reichardt’s dye (2,6-diphenyl-4-(2,4,6-triphenyl-pyridinium-1-yl)-phenolate). The functional probe for PILs are Wolfbies’ betaine (2,6-dichloro-4-(2,4,6-triphenyl-pyridinium-1-yl)-phenolate), and Nile red because of their low basicities. N,N-diethyl-4-nitroaniline and 4-nitroaniline were used for the measurement of Kamlet–Taft parameters (a, b, and p*). Sufficient data on the polarities of mixtures of ILs are available (Sarkar et al. 2008; Khupse and Kumar 2011). Binary mixtures of IL with hydroxylic solvents always
Author's personal copy Polarity issues in room temperature ionic liquids
result more polar intermediate solvent because of the formation of synergetic complex between them. Different results were also discussed using preferential solvation model (Herodes et al. 1999). Along with the polarity measurement at ambient conditions, temperature-dependent polarity i.e., thermosolvatochromism has been extensively studied for different classes of IL. Thermosolvatochromic data assist in developing the understanding about the nature and strength of the solute– solvent interactions.
Thermosolvatochromism of AILs First report on the qualitative discussion of polarity in ILs was presented by Welton and coworkers (Crowhurst et al. 2003). They conveyed their views on polarity and Kamlet– Taft parameters (a, b, and p*) in terms of the solute–solvent interactions by varying cation, anion, and also by changing the functional group on the alkyl side chain. They observed that EN T is an excellent measure of the overall solvation effect provided by the ILs, however, dipolarity/ polarizability (p*) reflects only the strength of the electrical field generated by the IL. HBD and HBA tendencies depend on the choice of cation and anion, respectively. A detail discussion about the polarities of imidazolium class of ILs with different possible anions was accounted by Reichardts (Reichardt 2005). El Seoud and coworkers have first studied the thermosolvatochromism of aqueous [BMIM][BF4] and compared the outcome with aqueous alcohols using four different probes (Clarissa et al. 2008). In both cases, a non-linear plot between the ET(30) and vw was observed due to preferential solvation of probe either by pure solvent or by their intermediate combination. The non-ideality in composition versus polarity relation was different for different probes for both aqueous IL and aqueous alcohol because of the dissimilar hydrophobicity of probes. Prausnitz and coworkers have reported the thermosolvatochromism for substituted pyridinium bis(trifluoromethylsulfonyl)imide between the temperature range 298–338 K (Lee et al. 2008). They noticed non-negligible contribution of alkyl side chain on polarity (EN T ). However, EN T decreases slightly with the rise in temperature from 298 to 338 K. HBD and HBA tendencies were noted to increase with the rise in temperature from 298 to 338 K. The dipolarity/polarizability (p*) was noted to decrease with temperature. In a parallel report on the thermosolvatochromism of methylpyrrolidinium cation with [NTf2] and [N(CN)2] anions, they observed that opposite to pyridinium cation where alkyl chain elongation has minimal effect on polarity, however, polarity can be tuned by increasing the alkyl chain length in pyrrolidinium cation (Lee and
Prausnitz 2010). These conflicting observations were attributed to the symmetrical/unsymmetrical cationic head for pyridinium and pyrrolidinium cations, respectively. Symmetrical nature of cation lowers the polarity due to close packing of ions. A very large polarity value for methyloctylpyrrolidinium bis(trifluoromethanesulfonyl)imide was noted which is also supported by large a value (1.00), similar to water (1.12) and methanol (1.05). Thus, a value governs polarity of pyrrolidinium-based ionic liquids similar to water and methanol. However, b and p* values remain unaffected with the increase in alkyl chain length. The presence of the electronegative groups such as disulphide and cyano on alkyl side chain alters the p*, while EN T , a, and b remain unaffected. An increase in temperature from 298 to 338 K decreases the EN T , a, and p*, while b increases marginally. A contrasting trend in the thermosolvatochromic behavior of pyridinium-, pyrrolidinium-, and phosphonium-based ILs incorporated with bis(trifluoromethanesulfonyl)imide ([NTf2]), tetrafluoroborate ([BF4]), alanate ([Ala]), and valinate ([Val]) anions was observed (Khupse and Kumar 2010). For pyridinium class of ILs with [BF4] anion, EN T deceases with the size of alkyl chain from -C4H9 to -C8H17; however, in the case of [NTf2] anion with same cation, EN T varies in the order [OP][NTf2] [ [BP][NTf2] [ [HP] [NTf2]. For pyrrolidinium-based ILs, EN T vary in the order [BMPyrr][NTf2] [ [HMPyrr][NTf2] [ [OMPyrr][NTf2]. The EN T for [TBP][Ala] was noted lower than that of [TBP][Val] (Fig. 3). Thermosolvatochromic response in both the pyridinium and pyrrolidinium classes of ILs was noted to decrease with rise in temperature. However, for [BP][NTf2], a constant polarity was observed within the temperature experimental range from 298 to 353 K. Contrasting behavior was observed for [TBP][Ala] and [TBP][Val], where EN T was found increasing with the rise in temperature from 298 to 353 K. This behavior was attributed to the increased electrolytic dissociation on increasing temperature. A reverse variation of a and b for both pyridinium and pyrrolidinium-based ILs was observed with the rise in temperature from 298 to 353 K. For [TBP][Ala] and [TBP][Val], both a and b increased with temperature. A large p* value for highly polar compound was noted which decreased with the rise in temperature for all classes of ILs. Unlike, AILs, insufficient data on polarities of PILs are available due to more acidic behavior. However, polarity (EN T ) of alkylimidazolium class of PILs with [HSO4], [HCOO], [CH3COO], and [CH3CH2COO] anions have been measured by using less basic probe 2,6-diphenyl-4(2,4,6-triphenyl-pyridinium-1-yl) phenolate (Shukla et al. 2012) (Fig. 4). Polarity in both the classes, viz. 1-methylimidazolium and 1-butylimidazolium with different anions varies
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Author's personal copy S. K. Shukla, A. Kumar Fig. 3 EN T –T plots for a pyridinium-based ionic liquids [BP][BF4] (open square), [OP][BF4] (open circle), [BP][NTf2] (open up-pointing triangle), [HP][NTf2] (closed down-pointing triangle), [OP][NTf2] (filled diamond), and b phosphonium-based ionic liquids [TBP][Ala] (open square), [TBP][Val] (open circle)
(b) 1.20
(a) 1.10
1.12
1.00
1.04
ET N
0.90
ET N
Fig. 4 EN T –T plots of a [HmIm][HSO4] (filled square), [HmIm][HCOO] (open circle), [HmIm][CH3COO] (filled up-pointing triangle), [HmIm][CH3CH2COO] (open down-pointing triangle), b [HbIm][HSO4] (filled square), [HbIm][HCOO] (open circle), [HbIm][CH3COO] (filled uppointing triangle), and [HbIm][CH3CH2COO] (open down-pointing triangle)
0.80 0.70
0.96 0.88
0.60
0.80
0.50 0.40
0.72 300
315
330
345
360
300
T/K inversely to the Coulombic forces between ions. By contrast, thermosolvatochromic response in PILs remains unaffected with the rise in temperature; while it increases/ decreases for AILs. Unlike AILs, where polarity varies accordingly, HBD tendency (a), polarity of PIL, varies according to the HBA tendency (b). Polarities of different ILs were further tested with the criteria of Fajan’s rule of polarization and later it was generalized that polarity values of different classes of IL varies inversely with the polarizability of anion. Highly polarizable anion develops more covalent character; thereby, reducing the polarity.
Solvatochromism in binary mixtures of ILs Polarity measurements in binary mixture of solvents cannot be compared on the similar grounds as in the case of pure solvent. In binary mixture, solvent composition around the probe always remains different than that of the bulk composition. This phenomenon is known as preferential solvation. Attempts have been made from time to time to develop plausible solvation models to find the composition of solvents around probe. Roses and coworkers developed
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315 330
345 360
T/K
solvation model using polarities of binary mixtures of formamide, N-methylformamide, and N,N-dimethylformamide in water, methanol, and 2-propanol, and predicted the composition of the solvation shell (Herodes et al. 1999). Along with the binary mixtures of molecular solvents, polarity measurement in binary mixture of ILs such as [BMIm][NTf2], [EMIm][NTf2] (Fletcher et al. 2003), and [BMIm][PF6] have also been reported. Strong HBD solvents such as alcohols and polyethylene glycols always exhibit synergetic behavior with ILs because of the formation of donor–acceptor complex between IL and these protic solvents. Similarly, a non-ideal behavior along with synergism was displayed by binary mixtures of [BP][BF4], [3-MBP][BF4], and [4-MBP][BF4] with methanol, water, and dichloromethane (Fig. 5). The results were analyzed with the choice of suitable solvation models constructed on the assumption of the twostep solvent-exchange process shown as IðS1Þ2 þ ðS2Þ ! IðS2Þ1 þ 2S1 IðS1Þ2 þ S2 ! IðS12Þ2 þS1 where S1 and S2 are pure solvents and S12 is a combination of solvents 1 and 2. I(S1)2, I(S2)2, and I(S12)2
Author's personal copy Polarity issues in room temperature ionic liquids Fig. 5 The plots of EN T –x2 for binary mixtures of [BP][BF4] (filled square), [3-MBP][BF4] (open circle), and [4MBP][BF4] (filled up-pointing triangle) in a water and b methanol
(a) 1.05
(b)
1.00
0.85
0.95
0.80
0.85 N
N
0.90
ET
0.90
ET
0.80 0.75
0.75 0.70
0.70 0.65
0.65 0.60 0.0
0.2
0.4
0.6
0.8
1.0
0.60 0.0
0.2
x2
indicate Reichardt’s dye solvated by solvents S1, S2, and S12, respectively.
Model 1
f 2=1
¼ xS2 =xS1 = x02 =x01
2
and
f 12=1
¼ xS12 =xS1 = x02 =x01
2
N N N ETN ¼ xS1 ET1 þ xS2 ET2 þ xS12 ET12 2 N 2 N ETN ¼ET1 1 x02 þET2 f2=1 x02 2 2 N þ ET12=1 f12=1 1 x02 x02 = 1 x02 þ f2=1 x02 þ f12=1 1 x02 x01
0.4
0.6
0.8
1.0
x2
suggests the higher solvation of probe by IL compared to dichloromethane and pseudo-solvent. However, in case of IL–methanol binary mixture where synergetic effect operates, because of formation of donor–acceptor complex between IL and water a very large value of f12/1 also support this view. Thus, probe is mainly solvated by the IL– methanol pseudo-solvent. As seen above, polarity, one of the most significant solvent properties, can alter the reaction rates via solute– solvent interactions. From the application point of view, rate constants of cycloaddition reaction have been correlated with polarity parameters using linear free-energy relationship (LFER) (Reichardt 1994). It is therefore possible for a process engineer to use the expression of the type,
N N N ET12 ¼ ET1 þ ET2 =2
ln k2 ¼ XYZ0 þ aa þ bb þ sp þ
where f2/1 and f12/1 are the preferential solvation parameters indicating the tendency of S2 and S12 for solvating the probe as compared to S1.
where XYZ0, a, b, and s are solvent-independent correlation coefficients. It depends on the nature of reaction and sensitivity of indicator molecule in polarity measurement. The analysis of kinetic data using the above equation confirms which polarity parameter is significant in optimizing the reaction rate of a given organic reaction. For example, the rate constants for the intermolecular Diels–Alder reaction between anthracene-9-carbinol and N-ethylmaleimide in pyridinum-based ionic liquids with [BF4] and [NTf2] anions are strongly correlated with the help of b and p* values (Tiwari and Kumar 2012).
Model 2
N ETN ¼ ET1 þ a x02 2 2 þ c 1 x02 x02 = 1 x02 þf2=1 x02 þf12=1 1 x02 x01 where a and c are the fitting parameters. Model 1 is used for the analysis of IL–water and IL– dichloromethane binary mixture, while model 2 is picked for the IL–methanol system where synergetic interaction comes into picture. In the case of IL–water, a higher value of f12/1 as compared to f2/1 indicates the preferential solvation by pseudo-solvent compared to water or IL. For ILdichloromethane system, higher value of f2/1 clearly
ð1Þ
Conclusions In conclusion, polarities of ionic liquids, which originate because of the ionic characteristic, have been widely studied using solvatochromic technique. Thermosolvatochromic response in both aprotic ionic liquids and protic ionic liquids was different because of the different extents
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of dye-ionic liquid interaction. For protic ionic liquid, it almost remains constant with the temperature change, while for aprotic ionic liquid, a decrease/increase in polarity was observed for different classes of ionic liquids. Polarities of aprotic ionic liquids largely depend on the hydrogen bond donor characteristic of cation, while for aprotic ionic liquid, it depends on the hydrogen bond acceptor tendency of anions. Addition of cosolvent modifies the probe-ionic liquid interactions due to the non-uniform solvation sphere around dye. Preferential solvation model suggests the presence of both ionic liquid and cosolvent along with their intermediate combination/ pseudo-solvent. The knowledge of different polarity parameters can be used in correlating the outcome of different organic reactions using linear free-energy relationship. Acknowledgments SKS is grateful to UGC, New Delhi, for awarding him a research fellowship. AK thanks DST, New Delhi, for a J. C. Bose National Fellowship (SR/S2/JCB-26/2009).
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The ET(30) polarity of binary mixtures of formamides with hydroxylic solvents. J Phys Org Chem 12:109–115 Khupse ND, Kumar A (2010) Contrasting thermosolvatochromic trends in pyridinium-, pyrrolidinium-, and phosphonium-based ionic liquids. J Phys Chem B 114:376–381 Khupse ND, Kumar A (2011) Delineating solute–solvent interactions in binary mixtures of ionic liquids in molecular solvents and preferential solvation approach. J Phys Chem B 115:711–718 Lee JM, Prausnitz JM (2010) Polarity and hydrogen-bond-donor strength for some ionic liquids: effect of alkyl chain length on the pyrrolidinium cation. Chem Phys Lett 492:55–59 Lee JM, Ruckes S, Prausnitz JM (2008) Solvent polarities and Kamlet–Taft parameters for ionic liquids containing a pyridinium cation. J Phys Chem B 112:1473–1476 MacFarlane DR, Golding J, Forsyth S, Forsyth M, Deacon GB (2001) Low viscosity ionic liquids based on organic salts of dicyanamide anion. Chem Commun 1430–1431 Peter W, Wilhelm K (2000) Ionic liquids-new solutions for transition metal catalysis. Angew Chem Int Ed 39:3772–3789 Plechkova NV, Seddon KR (2008) Applications of ionic liquids in the chemical industry. Chem Soc Rev 37:123–150 Reichardt C (1994) Solvatochromic dyes as solvent polarity indicators. Chem Rev 94:2319–2358 Reichardt C (2003) Solvents and solvent effects in organic chemistry, 3rd edn. Wiley-VCH, Weinheim Reichardt C (2005) Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes. Green Chem 7:339–351 Sarkar A, Trivedi S, Pandey S (2008) Unusual solvatochromism within 1-butyl-3-methylimidazolium hexafluorophosphate ? poly(ethylene glycol) mixtures. J Phys Chem B 112:9042–9049 Shukla SK, Khupse ND, Kumar A (2012) Do anions influence the polarity of protic ionic liquids? Phys Chem Chem Phys 14:2754–2761 Smiglak B, Metlen A, Rogers RD (2007) The second evolution of ionic liquids: from solvents and separations to advanced materials-energetic example from the ionic liquid cookbook. Acc Chem Res 40:1182–1192 Tiwari S, Kumar A (2012) Viscosity dependence of intra- and intermolecular Diels–Alder reactions. J Phys Chem A 116:1191–1198 Wakai C, Oleinikova A, Ott M, Weingartner H (2005) How polar are ionic liquids? Determination of the static dielectric constant of an imidazolium-based ionic liquid by microwave dielectric spectroscopy. J Phys Chem B 109:17028–17030 Walden P (1914) Molecular weights and electrical conductivity of several fused salts. Bull Acad Imper Sci 8:405–422 Wassercheid P, Welton T (2002) Ionic liquids in synthesis. WileyVCH, Stuttgart
