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How Does the Addition of a Third Ion Affect the Molecular Interactions and the Thermodynamic Properties of Acetate-Based Ionic Liquids? Ines Otero, Luiz Fernando Lepre, Alain Dequidt, Pascale Husson, and Margarida Costa Gomes J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b06452 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 11, 2017

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The Journal of Physical Chemistry

How Does the Addition of a Third Ion Affect the Molecular Interactions and the Thermodynamic Properties of Acetate-Based Ionic Liquids? I. Oteroa, L. F. Lepreb, A. Dequidta, P. Hussona, M. F. Costa Gomesa*

a

Institut de Chimie de Clermont-Ferrand, CNRS and Université Clermont Auvergne, BP 80026, Aubière Cedex F-63171, France. b

Laboratorio de Espectroscopia Molecular, Instituto de Quimica, Departamento de Quimica Fundamental, Universidade de Sao Paulo, CP 26077, Sao Paulo 05513-970, Brazil.

Abstract The effect of the addition of a third ion to the ionic liquid 1-butyl-3-methylimidazolium acetate, [C4C1Im][OAc], was studied through the measurement of the enthalpy of mixing and of the excess molar volume of its mixtures with 1-butyl-3-methylimidazolium trifluoroacetate, [C4C1Im][CF3CO2], 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C4C1Im][NTf2], and tetrabutylphosphonium acetate, [P4444][OAc]. Negative enthalpies of mixing (∆mixH < 0) and positive excess molar volumes (VE > 0) were observed in all cases. The IR and NMR study of the pure ionic liquids and their mixtures shows that the presence of a third ion with a weaker affinity with the common counter-ion contributes to the reinforcement of the more favorable hydrogen-bond network, herein always between the imidazolium cation and the acetate anion. Both the radial and the spatial distribution functions calculated by molecular simulation confirm this behavior. The remarkable enhancement of the viscosities of the [C4C1Im][OAc] + [P4444][OAc] mixtures could be discussed in light of the calculated friction coefficients.

*

[email protected]

ACS Paragon Plus Environment

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Introduction Ionic liquids are fluids composed exclusively of ions and so their physicochemical properties are determined by the nature of the cation and anion that constitute them. Different interactions between the ions (Coulombic interactions, dispersion forces and hydrogen bonding)1,2 lead to specific liquid structures and properties. The possibility of selecting the cation and anion that constitute an ionic liquid for obtaining adequate properties for a task specific application is a main advantage of these fluids for different applications. However, this synthetic flexibility can also be seen as a drawback since the number of possible cation and anion combinations is huge and renders the selection of the best ionic liquid for a given application difficult. Furthermore, the effort to establish structure-property relationships makes more costly and slow the application of new ionic liquids outside academic laboratories.3 In addition to changing the cation or the anion, mixing other molecular solvents has been investigated as a possible method to modify the ionic liquid properties.4 However, even if some properties may be improved, this usually leads to a mixture with increased volatility. In order to maintain the advantageous features of ionic liquids, mixtures of ionic liquids started to be considered as alternative for fine tuning their properties.5,6 Some ionic liquid mixtures have already been studied for certain applications in the fields of electrochemistry, carbon dioxide absorption, organic chemistry, extraction/separation, biomass processing, energetic materials, oil and gas industry and gas chromatography as highlighted by Chatel et al.5 The physicochemical behavior of ionic liquid mixtures does not show a specific, single trend. Whereas several studies have indicated that their properties can be described by a simple mixing rule or that they show an almost ideal behavior3,7,8, others revealed mixing gaps9,10 or higher deviations from ideality.9,11 The chemistry of ionic liquid mixtures cannot always be deduced from the individual components in the mixture thus studies on their physicochemical properties are needed for a better understanding of these multi-ionic fluids. In this study, three mixtures containing an acetate based ionic liquid are investigated with the aim of better understanding the mixing behavior of ionic liquids as a tool for the fine tuning of their properties. Two of the systems share a common cation and have different anions, whereas the third one has different cations and the same anion. Thus,

binary

mixtures

of

the

ionic

liquid

1-butyl-3-methylimidazolium


acetate 2

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[C4C1Im][OAc]

were

[C4C1Im][CF3CO2],

prepared

with



trifluoroacetate

bis(trifluoromethylsulfonyl)imide

[C4C1Im][NTf2] and tetrabutylphosphonium acetate [P4444][OAc]. The acetate anion, which contains a hydrophilic carboxylate head group, is polar, small and has a strong H-bond accepting ability.12 The fluorination of its methyl group, in the trifluoroacetate anion, decreases the electron density on the carboxylic group leading to a lower Lewis basicity.13-15 On the other hand, the NTf2- anion is large, highly flexible and weakly coordinating.12 Regarding the cations, C4C1Im+ contains an acidic hydrogen atom at position 2 in the imidazolium ring,16 which allows hydrogen-bond interactions with anions while the P4444+ cation, where the central phosphonium atom is bonded to alkyl chains, is devoid of hydrogen-bond donor. The effect of all these different ions on the mixing behavior of the different systems is analyzed through the measurement of the physicochemical properties of the mixtures (density and viscosity), calorimetric investigations (enthalpy of mixing) and of their molecular interactions (IR and NMR). Molecular dynamics simulations have been run to provide structural and dynamical insights on the mixtures containing the P4444+ cation.

Experimental Materials The ionic liquids 1-butyl-3-methylimidazolium acetate [C4C1Im][OAc] (> 98%) and 1-butyl3-methylimidazolium trifluoroacetate [C4C1Im][CF3CO2] (> 97%) were supplied by Iolitec. The

ionic

[C4C1Im][NTf2]

liquid was

1-butyl-3-methylimidazolium synthesized

in

a

bis(trifluoromethylsulfonyl)imide

previous

work.17

The

ionic

liquid

tetrabutylphosphonium acetate [P4444][OAc] was synthesized and kindly provided by QUILL (Queen's University Ionic Liquid Laboratories) with a purity greater than 99%.18 The chemical structures of all the ionic liquids studied in this work are presented in Fig. 1. The water content of the ionic liquids, after being dried for several hours under primary vacuum, was determined by coulometric Karl Fisher titration (Mettler Toledo DL32): 1600-1800 ppm, 1046 ppm, 343 ppm and less than 50 ppm for [C4C1Im][OAc], [P4444][OAc], [C4C1Im][CF3CO2] and [C4C1Im][NTf2], respectively. Mixtures of ionic liquids were prepared gravimetrically in glass vials using an Explorer EX225D/AD Ohaus balance (±0.01mg) under an Argon atmosphere. The vials


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were sealed and stirred at temperatures above ambient to ensure a good mixing. The prepared mixtures were stored in a glove box with a reduced water content atmosphere. The mixtures of [C4C1Im][OAc] with [P4444][OAc] showed partial immiscibility at certain temperatures and compositions. The miscibility gap was checked visually by immersing the prepared mixtures, under moderate stirring, in a liquid thermostat maintained at 311.15 or 312.15 K during at least 12 hours. The miscibility of the [C4C1Im][OAc] and [P4444][OAc] ionic liquids was visually confirmed at 313.15 K. The highest composition evaluated of P4444+ which resulted to be miscible at 313.15 K was x = 0.6500. The next composition considered (x = 0.7417) showed partial immiscibility at 313.15 K. The uncertainty on the mole fraction composition of the prepared mixtures is estimated to be ±0.0001. Hereinafter, in the nomenclature used for the mixtures ([C4C1Im][OAc](1-x)[NTf2]x, [C4C1Im][OAc](1-x)[CF3CO2]x and [C4C1Im](1-x)[P4444]x[OAc]), x stands for mole fraction composition. Density Measurements Densities were measured at atmospheric pressure and in the temperature range of 293.15 to 353.15 K using a U-shaped vibrating-tube densimeter (Anton Paar, model DMA 5000M). The calibration of the equipment was verified with air and tridistilled water before and after each measurement. The estimated uncertainties of this equipment are 0.0001 gcm-3 and 0.01 K for density and temperature, respectively. Viscosity Measurements Viscosity measurements were performed in a falling-ball based microviscosimeter (Anton Paar, Lovis 2000 M/ME) at atmospheric pressure and in the temperature range 293.15 to 353.15 K. The temperature was controlled to within 0.005 K and measured with accuracy better than 0.02 K. A capillary tube of 1.8 mm diameter, previously calibrated as function of temperature and angle of measurement with reference oils, was used for the measurements. The overall uncertainty on the viscosity was estimated as 2 %. Isothermal Titration Calorimetry (ITC) An isothermal titration nano-calorimeter (TA Instruments) equipped with a Thermal Activity Monitor TAM III thermostat (TA Instruments) was used for measuring the excess molar enthalpies (HE, or mixing enthalpies, ∆Hmix) at atmospheric pressure and at 313.15 K. The temperature of the thermostat is controlled to within 10−5 K.


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Two cells of 1 ml made of Hastelloy were used; one of them containing the sample (measuring cell) and the other filled with a similar amount of the same sample (reference cell). During the titration experiments, small volumes (8 or 10 µl) of a pure ionic liquid or of a mixture of ionic liquids of a known composition were injected (from a 250 µL gastight Hamilton syringe) into the measuring cell. To ensure an effective mixing, the liquid in the cell was stirred at 90 or 120 rpm. A motor driven pump (Thermometric 3810 Syringe Pump) was used for the automatic injections (10 to 20 per experiment), each one during 60 or 120 s. Between them, an interval of 45-60 min was stablished to allow the baseline recovering. After each injection, a peak is recorded and corresponds to the heat effect of the mixing process. The area of the peak, Qi, is proportional to the heat involved and it is a value required for the excess molar enthalpy calculation. The integration of the peaks recorded during the titration experiments was done using the TAM Assistant software. The consistency of the results was checked by measuring the mixing enthalpies at several mole fractions along the whole composition range. The heat effects determined experimentally, Qi, can be related to the partial molar excess enthalpy, HiE . For example, QIL2 corresponds to the heat effect recorded when a small amount of ionic liquid 2 (IL2), ∆nIL2, is injected into ionic liquid 1 (IL1) (or into a binary mixture – IL1+IL2 – of known composition) and is related to the partial molar E excess enthalpy of IL2 in the mixture, HIL2 :

(

)

∂ n + n ∆ H  IL1 IL2 mix IL1+IL2 Q E  HIL2 = ≈ IL2   ∂nIL2  p,T,nIL1 ∆nIL2

(1)

where nIL1 and nIL2 indicate the amounts of IL1 and IL2, respectively; ∆mixHIL1+IL2 is the enthalpy of mixing of the two ionic liquids. ∆nIL2 is the quantity of IL2 per injection calculated from the injected volumes and the experimental densities. The enthalpy of mixing can be represented, as a function of the mole fraction composition of the mixtures, by a Redlich-Kister as function of the mole fraction composition

(

)

n

(

∆ mix HIL1+IL2 = 1− x IL2 x IL2 ∑ Ai 1− 2x IL2

)

i

(2)

i=0

where xIL2 denotes the mole fraction of IL2 in the mixture.


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E The partial molar excess enthalpy of IL2 in the mixture, HIL2 , can be obtained from

the derivative with respected to composition of eq (2) and the fitting of the Redlich-Kister parameters (Ai). A more detailed description of this treatment can be found elsewhere.19,20 Finally, the enthalpy of mixing can be directly calculated with eq (2) and the partial molar excess enthalpies at infinite dilution through n

( )

E,∞ E HIL1 = lim H IL1 = ∑ −1 Ai xIL1→0

i =0

i

(3)

n

E,∞ E HIL2 = lim H IL2 = ∑ Ai xIL2 →0

(4)

i=0

Spectroscopic Measurements Infrared (IR) spectra were recorded at room temperature using a Nicolet 380 FT-IR spectrometer equipped with single reflection diamond ATR from Specac and a DTGS detector. The IR spectra were averaged on 32 scans, with a resolution of 4 cm-1. NMR analyses were performed on a Bruker 400 MHz Avance spectrometer equipped with a 5 mm pulsed-field z-gradient QNP probe or on a Bruker 500 MHz Avance III HD equipped with a 5 mm pulsed-field z-gradient TXI probe. The samples were introduced into 5mm NMR tubes, where a coaxial tube containing the solvent DMSO-d6 was also inserted. Thus, the sample was not diluted in the solvent. The chemical shift of DMSO-d6 was used as reference at 2.5 ppm and 39.51 ppm for the proton (1H) and the carbon (13C) spectra, respectively. Molecular Simulations The [C4C1Im](1-x)[P4444]x[OAc] mixture was studied using molecular dynamics simulations. The simulations used the CL&P force field21 with electric charges reduced by a factor 0.8. Reducing the charges makes the simulated dynamics faster and in better agreement with the experiments22. Three simulations at compositions x = 0, 0.5 and 1 were performed with simulation boxes containing 500 ion pairs in total. The systems were first equilibrated in the NPT ensemble for 0.1 ns at 1 atm and 400 K and the following acquisition runs lasted 10 ns. Note that the temperature in the simulation is higher than in the experiment, because the dynamics is faster at higher temperature. This allows for reaching the diffusion regime faster.


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The structure was analyzed using radial distribution functions g(r) and the apparent friction between the various species was estimated. The friction ζAB between species A and B is defined here by writing that the friction force on an ion i of species A in the diffusive regime is = ∑ 〈 〉 −

(5)

where (t) is the mean instantaneous velocity of ions of species B at time t in the liquid at rest. This definition corresponds to the Maxwell-Stefan equations of diffusion, which are a generalization of Fick’s law to arbitrary mixtures23. The ζAB frictions can be computed from the ionic diffusion coefficients and from the cross-species diffusion coefficients24. The higher ζAB, the more A and B tend to diffuse together on average. The physical meaning of ζAB is how much the diffusion of A drags B on average or how A is hold back by B (and conversely, because ζAB = ζBA).

Results and Discussion The densities of the ionic liquids [C4C1Im][CF3CO2], [C4C1Im][NTf2] and [P4444][OAc] and the

mixtures

[C4C1Im][OAc](1-x)[CF3CO2]x,

[C4C1Im][OAc](1-x)[NTf2]x

and

[C4C1Im](1−x)[P4444]x[OAc] are represented as function of temperature in Fig. 2. The reported densities of the [C4C1Im][OAc] sample are the ones recently measured and reported by our group.11 The experimental values are collected in Table S1, Table S2 and Table S3 in the supplementary information (SI). The temperature dependence of the density for each ionic liquid or mixture at a fixed composition was fitted to the polynomial:

ρ = A0 + A1T

(6)

The coefficients A0 and A1 corresponding to eq (6), together with the absolute average deviations (AAD), which are in all cases better than 0.01%, are collected in Table S4 (SI). The density values of the pure ionic liquids used in this work follow the order [C4C1Im][NTf2] > [C4C1Im][CF3CO2] > [C4C1Im][OAc] > [P4444][OAc].

This

trend

is

in

agreement with the molecular mass of the anions for the ionic liquids containing the cation C4C1Im+, as it was previously found for other imidazolium-based ionic liquids.25 In the case of ionic liquids with the same anion, a lower density is related with a higher molecular volume and with the increased importance of dispersive interactions (longer


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alkyl chains).

25

Page 8 of 30

In this work the same behavior was found, proving that the volume of the

cation P4444+ is much larger than that of the cation C4C1Im+. A comparison with previously published literature data is depicted in Fig. S1 and in Fig. S3 for [C4C1Im][CF3CO2] and [C4C1Im][NTf2], respectively. The agreement between our density experimental values and those from literature is within 1% and 0.20% for [C4C1Im][CF3CO2]

and

[C4C1Im][NTf2],

respectively.

Regarding

the

ionic 11

[C4C1Im][OAc], the comparison with literature data done by Lepre et al.

liquid

revealed

deviations (up to 0.4%) that are lower when comparisons are done with recently published data26 (0.05%). For the ionic liquid [P4444][OAc] only Tao et al.27 reported a liquid density value at 298.15 K, a temperature below the ionic liquid melting temperature of 54.5 °C.28 Kanakubo et al.29 and Almeida et al.30 have recently published experimental density data for the mixture [C4C1Im][OAc](1−x)[NTf2]x, the results obtained in this work showing a similar behavior as those reported by these authors as can be seen in Fig. S5a. For the pure ionic liquids, the results reported herein deviate in average 0.02 %29,31 and 0.06 %30 for [C4C1Im][NTf2], and −0.15 %30 and −0.04 %29 for [C4C1Im][OAc]. The excess molar volumes, VE, were determined from the density values according to eq 7.

VE =

(x

MIL1 + xIL2MIL2

IL1

ρmix

)−x

IL1

MIL1

ρIL1

xIL 2MIL2

−

(7)

ρIL 2

where ρmix and ρIL are the densities of the mixture and of the pure ionic liquids, respectively; xIL, is the mole fraction of each ionic liquid in the mixture and MIL is the molecular weight. The excess molar volumes are represented in Fig. 2b, Fig. 2d and Fig. 2f for some selected temperatures and listed in Table S1, Table S2 and Table S3. As can be observed in Fig. 2b, Fig. 2d and Fig. 2f, the VE for all mixtures are positive in the range of temperature investigated and in the whole composition range, indicating a volumetric expansion of the systems. However, these VE values are significantly higher for [C4C1Im][OAc](1−x)[NTf2]x

than

for

[C4C1Im][OAc](1−x)[CF3CO2]x

or

[C4C1Im](1−x)[P4444]x[OAc]. For example, at x ≈ 0.5 and 333.15 K the excess molar volume is

0.1

cm3mol−1

for

[C4C1Im][OAc](1−x)[CF3CO2]x 3

−1

[C4C1Im](1−x)[P4444]x[OAc] whereas it is of 0.6 cm mol

and

0.2

cm3mol−1

for

for [C4C1Im][OAc](1−x)[NTf2]x, as


8

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expected in light of the differences in size between the ions7. Compared to [C4C1Im][OAc], the [C4C1Im][OAc](1−x)[CF3CO2]x mixture is the most similar, as the acetate anion loses basicity by the addition of the CF3- group but the size differences are not enormous. On the other hand, for the system [C4C1Im](1−x)[P4444]x[OAc], the cation introduced, P4444+, does not contain an acidic proton for the favorable interaction with the acetate anion. In its place, the P core is surrounded by alkyl chains that may be more difficult to compact. Finally, for the system showing the highest excess molar volumes, [C4C1Im][OAc](1−x)[NTf2]x, the NTf2- anion is large and weakly coordinating compared to the acetate, which may explain the expansion of the structure. Another difference between the VE curves for [C4C1Im][OAc](1−x)[CF3CO2]x and [C4C1Im][OAc](1−x)[NTf2]x is the position of the maximum value. In the case of [C4C1Im][OAc](1−x)[CF3CO2]x, this maximum is displaced towards compositions richer in the CF3CO2− anion (x ≈ 0.66), whereas for [C4C1Im][OAc](1−x)[NTf2]x the maximum is around equimolar composition. A similar behavior was reported by Almeida et al.30 for the mixture [C4C1Im][OAc](1−x)[NTf2]x. However, a different trend was published by Kanakubo et al.29 for [C4C1Im][OAc](1−x)[NTf2]x. These authors showed curves with the maximum displaced to mole fraction compositions of NTf2− of 0.36-0.41 at temperatures 298-333 K. The VE values reported for both authors are in the same order as those calculated in this work; for example, at 298.15 K and xNTf2 ≈ 0.5 these values are 0.575 cm3mol−1 (this work), 0.49 cm3mol−1 (Almeida et al.30) and 0.67 cm3mol−1 (Kanakubo et al.29). Differences in the composition (xNTf2 = 0.4999, xNTf2 = 0.4984 and xNTf2 = 0.525, respectively), as well as the deviations previously noted for the density values of the pure ionic liquids, can probably account for the small differences in VE. Several studies of VE of ionic liquid mixtures have been reviewed by Chatel et al.5. Some

examples

of

high

[C2C1Im][EtSO4]x[NTf2](1−x)

32

VE E

values

were 3

with V = +0.76 cm mol

−1

reported

for

the

mixtures

or [C4C1Im][NTf2]x[BF4](1−x)30 with

VE = +0.37 cm3mol−1. Lower deviations from ideality were obtained for the mixtures [C4C1im][BF4]x[PF6](1−x)

with

VE = +0.07 cm3mol−1

or

[C4C1im][PF6]x[NTf2](1−x)

with

VE = +0.12 cm3mol−1. (all data at 298.15 K and x = 0.5; [EtSO4]: ethylsulphate, [BF4]: tetrafluoroborate, [PF6]: hexafluorophosphate). In general, larger positive excess molar volumes are found for mixtures containing ions with different sizes.5 However, as correctly affirmed by Clough et al.3 other differences between the ions in terms of polarity


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Page 10 of 30

or hydrogen bond character, dipole moments, charge arms and flexibility can also explain relatively large values for the excess molar volumes. The viscosities, η, determined experimentally for the pure ionic liquids [C4C1Im][CF3CO2]

[C4C1Im][NTf2]

and

[P4444][OAc],

and

the

mixtures

[C4C1Im][OAc](1−x)[CF3CO2]x, [C4C1Im][OAc](1−x)[NTf2]x and [C4C1Im](1−x)[P4444]x[OAc] are plotted as function of temperature in Fig. 3a, Fig. 3c and Fig.3e together with the values for the ionic liquid [C4C1Im][OAc] reported in the literature.11 All these values are summarized in Table S1, Table S2 and Table S3 (SI). The temperature dependences of the viscosity of the pure ionic liquids and of the mixtures were correlated with the VogelFulcher-Tammam (VFT) formulation. The VFT parameters are collected in Table S5 (SI). The viscosity of the pure ionic liquids follows the order [P4444][OAc] > [C4C1Im][OAc] >> [C4C1Im][CF3CO2] > [C4C1Im][NTf2]. As expected, the acetate based ionic liquids are more viscous than the ones based on the fluorinated anions.4 At 303.15 K,

the

ionic

liquid

[C4C1Im][OAc]

is

significantly

more

viscous

than

[C4C1Im][CF3CO2] and [C4C1Im][NTf2], respectively (255 mPa s, 58 mPa s, 41 mPa s). The difference in viscosity is lower between [C4C1Im][CF3CO2] and [C4C1Im][NTf2], as the former is 30 % more viscous than the latter. Deviations between the experimental data and those from the literature are depicted in Fig. S2 and Fig. S4 (SI) for the pure ionic liquids [C4C1Im][CF3CO2] and [C4C1Im][NTf2], respectively. The deviations obtained are within −27% and +1% for [C4C1Im][CF3CO2] and −7.5% and +6% for [C4C1Im][NTf2]. For [C4C1Im][OAc], the viscosity values from Lepre et al.11 used in this work are 40%4 and 7%26 lower and 25%33 higher than those from the literature.11 These large deviations can be attributed, on one hand, to the presence of variable quantities of water in this ionic liquid and, on the other hand, to the possibility of a proton exchange between the cation and the anion with the formation of volatile acetic acid.4,34,35 Only one value for the viscosity of the ionic liquid [P4444][OAc] is reported at 298.15 K (267 mPa s)27, a temperature below the melting point of this ionic liquid. The viscosities of the mixture [C4C1Im][OAc](1−x)[NTf2]x reported by Almeida et al.30 and Kanakubo et al.29 at 298.15 K are presented in Fig. S5 (SI) together with those obtained in this work. The differences between the literature values are higher as the mole fraction of [C4C1Im][NTf2] decreases (higher viscosity values), the viscosities


10

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measured herein laying between the two sets reported by other authors.29,30 At 298.15 K, the deviations of the viscosity values in this work for [C4C1Im][OAc] are 21% higher and 19% lower than those reported by Almeida et al.30 and Kanakubo et al.29, respectively. The addition of [C4C1Im][CF3CO2] or [C4C1Im][NTf2] to [C4C1Im][OAc] considerably reduces its viscosity. At 303.15 K mixtures with a low content on CF3CO2− or NTf2−, like [C4C1Im][OAc]0.7487[CF3CO2]0.2513

(171

mPa

s) and

[C4C1Im][OAc]0.7497[NTf2]0.2503

(161 mPa s), are 33% and 37% less viscous than pure [C4C1Im][OAc] (255 mPa s). The viscosity reduction is larger than the average of the viscosities of the pure ionic liquids as seen in Fig. 3b and Fig. 3d (the values are listed in Tables S1 and S2 in the SI). The deviations for both mixtures are comparable at the same temperature but are significantly higher at lower temperatures. The addition of [P4444][OAc] to [C4C1Im][OAc] leads to an increase in viscosity, as can be seen in Fig. 3e. This behavior was predictable since [P4444][OAc] is already more viscous than [C4C1Im][OAc]. However, an unexpected trend was obtained: the mixtures of these two ionic liquids show a viscosity higher than that of pure [P4444][OAc], the more viscous of the two ionic liquids. Even for the mixture with a low content on P4444+, [C4C1Im]0.7510[P4444]0.2490[OAc] (78 mPa s), at 333.15 K, the viscosity is 61% higher than that of pure [C4C1Im][OAc] (48 mPa s) and 16% higher than that of pure [P4444][OAc] (67 mPa s). Unusual behavior of the viscosities have been reported before by Annat et al.9

for

mixtures

of

partially

miscible

ionic

liquids

([P6,6,6,14][NTf2],

trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide, + [C3C1pyr][NTf2], Nmethyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)amide) but, even if the viscosities of the mixtures were considerably lower than that of the average of the viscosities of the pure ionic liquids, their values were still between those of the pure ionic liquids. A deeper investigation on the non-ideality behavior of the ionic liquid mixtures studied

in

this

work,

[C4C1Im][OAc](1−x)[CF3CO2]x,

[C4C1Im][OAc](1−x)[NTf2]x

and

[C4C1Im](1−x)[P4444]x[OAc], was performed by measuring the enthalpies of mixing. The results obtained are depicted in Fig 4a. The experimental stoichiometric data and heat effects are reported in Table S6, Table S7 and Table S8 (SI). The experimental data are plotted with the best adjusted curve in Fig. S6 (SI). Even if the information available in the literature for the enthalpies of mixing of ionic liquids are scarce,19 three previously investigated

binary

mixtures

containing

the


same

cation,

C4C1Im+

11


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

([C1C4Im][NTf2](1−x)[PF6](x)36, [C1C4Im][OAc](1−x)[C(CN)3]x11,

Page 12 of 30

[C1C4Im][BF4](1−x)[PF6](x)

8,19

)

are included in Fig. 4 for comparison. The three mixtures investigated in this work showed an exothermic mixing behavior, the mixing enthalpies varying in the order (lower mixing enthalpies corresponding

to

more

exothermic

mixing):

[C4C1Im](1−x)[P4444]x[OAc] < [C4C1Im][OAc](1−x)[NTf2]x < [C4C1Im][OAc](1−x)[CF3CO2]x.

As

an example, at a composition of x = 0.5 the mixing enthalpies are −2.88 kJ mol−1, −1.52 kJ mol−1 and −0.65 kJ mol−1 for [C4C1Im](1−x)[P4444]x[OAc], [C4C1Im][OAc](1−x)[NTf2]x and [C4C1Im][OAc](1−x)[CF3CO2]x, respectively. For [C4C1Im](1−x)[P4444]x[OAc] the mixing enthalpies are more negative than those obtained experimentally for other mixtures reported in the literature11,19. Large excess enthalpies of mixing have been estimated using COSMO-RS for partial immiscible ionic liquid mixtures, but the values reported are positive (endothermic mixing process).37 Only positive mixing enthalpies were reported in the literature19 for ionic liquid mixtures with a common anion, with exception of the mixture

[C4C1C1Im][BF4],

1-butyl-2,3-dimethylimidazolium 38

[C2C1Im][BF4], 1-ethyl-3-methylimidazolium tetrafluoroborate

tetrafluoroborate

+

for which the mixing

enthalpy changes from negative to positive with the increase of [C4C1C1Im][BF4] in the mixture38. The partial excess enthalpies at infinite dilution are summarized in Fig. 4b and the values are collected in Table S9 (SI). For the three mixtures studied in this work, the excess enthalpy at infinite dilution of [C4C1Im][OAc] is always more negative than that of the other ionic liquid in the mixture – [C4C1Im][CF3CO2], [P4444][OAc] or [C4C1Im][NTf2] – indicating that the addition of a limiting amount of [C4C1Im][OAc] in the other ionic liquid is enthalpically more favorable than if the addition is done the other way around. The highest (by almost 100%) negative partial excess enthalpy at infinite dilution of [C4C1Im][OAc] was obtained for its mixture with [P4444][OAc]. This is in agreement with the displacement of the minimum of the curves of enthalpy of mixing (Fig. 4a) towards higher

compositions

of

[P4444][OAc]

and

[C4C1Im][CF3CO2].

For

the

mixture

[C4C1Im][OAc](1−x)[NTf2]x the minimum is around equimolar composition, leading to a smaller difference between the partial excess enthalpy at infinite dilution of [C4C1Im][OAc] and [C4C1Im][NTf2] in this mixture. The effect of the addition of a third ion to [C4C1Im][OAc] was studied by infrared spectroscopy in the mixtures [C4C1Im][OAc](1−x)[CF3CO2]x and [C4C1Im][OAc](1−x)[NTf2]x,


12

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as a function of composition. Fig. 5 a and c show that the band at ca. 1377 cm−1, assigned to the symmetric stretching mode of the acetate anion COO group, is shifted to higher wavenumbers by the addition of [C4C1Im][CF3CO2] or [C4C1Im][NTf2]. In agreement with previous work, where the [C1C4Im][OAc](1−x)[C(CN)3]x mixtures were studied,11 these results suggest that the association of the C4C1Im+ cation with the OAc− anion strengthens when the amount of the CF3CO2− or NTf2− anions increases. This result is further supported by observing the high wavenumber region of the IR spectra (Fig. 5 b and d). The vibrational bands above 3000 cm−1, assigned to the C-H modes of the imidazolium ring, are known to be good indicators of the hydrogen bonding strength between imidazolium cations and anions.

11,39,40

The C2-H stretching band contribution of

[C4C1Im][OAc] is more red-shifted (at ca. 3027 cm−1) than that of [C4C1Im][CF3CO2] 10,11,41

and [C4C1Im][NTf2] (at ca. 3100 cm−1). This suggests that the hydrogen bond

strength between the anion and the C2-H of the cation in the pure ionic liquids follows the order [C4C1Im][OAc] > [C4C1Im][CF3CO2] > [C4C1Im][NTf2]. Despite the position of the C2-H stretching band is quite similar for [C4C1Im][CF3CO2] and [C4C1Im][OAc], the shapes of the spectra are considerably different. In the case of [C4C1Im][CF3CO2] three bands can be easily distinguished, whereas in the case of [C4C1Im][OAc] the C2-H stretching band is broader and overlapped with the contributions of C4,5-H stretching. For the mixtures, especially for [C4C1Im][OAc](1−x)[NTf2]x, it can be observed that the intensity at ca. 3027 cm−1 is higher when the mixtures are richer in OAc−, showing that the H-bond networks in [C4C1Im][OAc](1−x)[NTf2]x and [C4C1Im][OAc](1−x)[CF3CO2]x are influenced by the concentration of each anion in the mixture. Aiming to better determine the relative contribution of the three imidazolium ring H atoms to the hydrogen bond network, the 1H and the pure ionic

liquids

13

C NMR spectra were measured for

and for two mixtures: [C4C1Im][OAc](1−x)[CF3CO2]x and

[C4C1Im](1−x)[P4444]x[OAc]. A similar study was recently reported by Mathews et al.42 for the mixture [C4C1Im][OAc](1−x)[NTf2]x, whose data were used herein for comparison. The spectra and the chemical shifts obtained at positions 2, 4 and 5 of the C4C1Im+ cation (Fig. 1 for atom labelling) are available in Fig. S7 and Fig. S8 and Table S10 and Table S11 (ESI). The chemical shifts are plotted as function of composition for the three mixtures in Fig. 6 and Fig. S9. Both the inductive effects of the imidazolium ring and the interactions with the anions have been known to contribute to the shielding of the nuclei, especially for the


13


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Page 14 of 30

most acidic imidazolium hydrogen H(C2).42 However, the C4C1Im+ structure is kept constant, making it possible to disregard the inductive effects on the hydrogens of the imidazolium ring. Considering the different anions and despite of the predominance of specific interactions such as preferential hydrogen bonding, we can consider the following sequence of increasing electron shielding effects: OAc- > CF3CO2- > NTf2-. However, the infrared analysis suggests that H-bonding is a key for understanding the interactions between the ions. Therefore, the chemical shift changes are considered as mainly due to H-bond. In the

1

H NMR spectra, the peaks corresponding to the imidazolium ring

hydrogens can be used to study differences in cation-anion interaction strength and also H-bonding. Higher 1H chemical shifts are related with higher C-H bond distances43 and therefore stronger H-bond interactions44. The δH(C2) peaks of the pure ionic liquids studied in this work appear in the order [C4C1Im][OAc] (10.38 ppm) > [C4C1Im][CF3CO2] (9.29 ppm) > [C4C1Im][NTf2] (7.97 ppm)42 showing that, as expected, the OAc− anion establishes stronger H-bond with the hydrogen in the position 2 of the imidazolium ring. This is in agreement with the IR spectroscopy results in the range of 2800-3200 cm-1, discussed above. The 1H chemical shifts at the positions 4 and 5 of the imidazolium ring are also higher for the pure [C4C1Im][OAc], suggesting a stronger H-bond network involving the three imidazolium ring hydrogens.11,12 Regarding the chemical shifts obtained for the mixtures (Fig. 6), two different situations must be distinguished. First, the two pure ionic liquids in the mixture contain the C4C1Im+ cation ([C4C1Im][OAc](1−x)[CF3CO2]x and [C4C1Im][OAc](1−x)[NTf2]x) so that we have an excess of C4C1Im+ cation for each OAc− anion in mixtures with high x (x standing for CF3CO2− or NTf2−). Second, only one of the pure ionic liquids in the mixture contain the C4C1Im+ cation (system [C4C1Im](1−x)[P4444]x[OAc]) so that we have an excess of OAc− anion for each C4C1Im+ cation in mixtures rich in P4444+. In the first case – [C4C1Im][OAc](1−x)[CF3CO2]x and [C4C1Im][OAc](1−x)[NTf2]x – an increase of the concentration of the anions CF3CO2− or NTf2− reduces the chemical shift values

(upfield

shifts)

in

a

nonlinear

way,

especially

in

the

case

of

[C4C1Im][OAc](1−x)[NTf2]x. In both mixtures the experimental δH(C2) chemical shifts are higher than the average, suggesting a preferential H-bond interaction of the OAc− anion with the hydrogen in position 2 in the C4C1Im+ cation.39,43,44 This behavior is more evident in [C4C1Im][OAc](1−x)[NTf2]x than in [C4C1Im][OAc](1−x)[CF3CO2]x, as could be expected in


14

Page 15 of 30

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light of the structure and the associative character of NTf2− anion in relation to the acetate OAc− anion. Concerning the CF3CO2− anion and its similarities of structure and associative character with the OAc− anion, the chemical shifts deviation from the average is lower. It is interesting to note that these observations are in agreement with the mixing enthalpies

discussed

above,

[C4C1Im][OAc](1−x)[NTf2]x

where

deviations

(∆mixH = −1.52

kJ

from

ideality

mol-1)

in

were

higher

comparison

for to

-1

[C4C1Im][OAc](1−x)[CF3CO2]x (∆mixH = −0.65 kJ mol ). The chemical shifts obtained for δH(C4) and δH(C5) atoms are only slightly below the dashed line, revealing the leading role played H(C2) atom in the hydrogen network of these imidazolium-based ionic liquids. These conclusions were further supported by

13

C NMR (Fig. S9), where the same trend

1

as the one obtained with H are observed, as expected42. In the second case – for the mixtures of [C4C1Im][OAc] and [P4444][OAc] – it can be seen that the addition of P4444+ leads to an increase on the δH(C2), δH(C4) and δH(C5) chemical shifts of the imidazolium cation. These results suggest also that preferential Hbond interactions are established between C2-H hydrogens and the OAc− anion upon mixing [C4C1Im][OAc] with [P4444][OAc]. These observations can probably explain the highly negative excess enthalpy measured for the [C4C1Im](1−x)[P4444]x[OAc] mixtures (Fig. 4) and suggest the formation of stronger H-bonds between the imidazolium cations and acetate anions in the mixtures in comparison to that of the pure [C4C1Im][OAc]. These stronger H-bonds could also explain the viscosity behavior of [C4C1Im](1−x)[P4444]x[OAc] mixtures (Fig. 3). The main differences in phosphonium and imidazolium ionic liquids lie on their relative volumes and charge distribution. A comparison between the longer alkyl chains [P6 6 6 14]Cl and [C10C1Im][PF6] done by Blesic et al. revealed that phosphonium and imidazolium ionic liquids have distinct shapes of the polar network and the non-polar regions.45 While the former has a string-like nature of the polar network, the later has a more globular nano-segregation between the polar and nonpolar domains. Despite of the fact that Blesic et al.45 studied ionic liquids with different anions and with longer alkyl chains size, the picture of a more localized polar network in the phosphonium-based ionic liquid suggests that in [C4C1Im](1−x)[P4444]x[OAc] mixtures a polar domain may be formed by C4C1Im+ and OAc− establishing strong H-bonds and being surrounded by the P4444+ cations. In this context, the work of Bowron et al.12, where it was shown that both O atoms of acetate COO group stablish hydrogen bonds with the three H atoms of the imidazolium


15


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Page 16 of 30

ring in pure [C2C1Im][OAc] (Fig. 1). With that picture in mind, the present IR and NMR results suggest that OAc- anion stablishes hydrogen bonds with C4C1Im+ preferentially in the position C2 of the imidazolium ring upon the addition of weaker bases like CF3CO2- or NTf2-, or weaker acids like P4444+. The simulations of the [C4C1Im](1−x)[P4444]x[OAc] mixtures yield both structural and dynamical insights. The most interesting radial distribution functions g(r) are displayed in Fig. 7. They show that the interactions of OAc− with C4C1Im+ at site 2 are even more probable in the mixture than in the pure ionic liquid. The same holds for the interactions with sites 4 and 5 of the C4C1Im+ cation (Fig. S10). On the other hand, the interactions of OAc− with P4444+ are less probable in the mixture than in the pure ionic liquid. This result is thus consistent with the spectroscopic and calorimetric analysis. The acetate oxygens tend to turn away from the P4444+ to go forming H-bonds with C4C1Im+. The spatial distribution functions (Fig. S10 of the supplementary information) show the same trend. The friction coefficients for [C4C1Im](1−x)[P4444]x[OAc] at three different compositions are listed in Table 1. They show that in the pure [C4C1Im][OAc], the friction between ions is lower (3.9 kJ nm ns mol-1) than in the pure [P4444][OAc] (14.6 kJ nm ns mol-1). This correlates with the experimental viscosities. In the mixture, the friction between OAc− and C4C1Im+ is reinforced while the friction between OAc− and P4444+ is weakened. This supports the result of the structure analysis, namely that the interactions between OAc− and C4C1Im+ are favored in the mixture in detriment of the interactions of OAc− with P4444+. Therefore, preferred hydrogen-bond interactions are correlated with having stronger friction. The preferential interactions of OAc− with C4C1Im+ seem to have a greater impact on viscosity than the decreased interactions with P4444+.

Conclusions The addition of a third ion significantly affects the molecular interactions and the thermodynamic and dynamic properties of acetate-based ionic liquids. We have studied in detail two types of mixtures – one having a common cation, C4C1Im+, and different anions, OAc-, CF3CO2− and NTf2−, and another having a common anion, OAc−, and different cations, C4C1Im+ and P4444+. In both cases, the ions mixed exothermally leading to expanded ionic fluids, compared with the ionic liquids used to prepare them. These behaviors could be explained by a detailed analysis of the IR and NMR spectra of the


16

Page 17 of 30

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starting ionic liquids and the mixtures with different compositions. In both cases, the addition of the ion that shows a weaker affinity by the common counter-ion in the mixtures, contributes to the reinforcement of the more favorable hydrogen-bond network, herein always between the imidazolium cation and the acetate anion. Molecular simulation provided further insights on the behaviors of the ionic liquid mixtures sharing a common anion. Both the radial and the spatial distribution functions show that the interactions between the acetate anion and the imidazolium cation are favored compared to those between the anion and the phosphonium cation. The first are reinforced in the mixture while the second are weakened. The same trend is found in the analysis of the friction coefficients calculated for the same mixtures, leading to a remarkable enhancement of their viscosities that are higher than those of both pure ionic liquids.

Supplementary Information The supplementary information includes tables of the experimental results on density, viscosity and excess molar volume of the studied ionic liquids and their mixtures (Tables S1 to S3) with graphic representation on their comparison with literature data (Figures S1 to S5). The parameters found for the fitting of the density and viscosity data are listed in Tables S4 and S5, respectively. The stoichiometric data and heat effects of the calorimetry experiments are listed in Tables S6 to S8 and are represented in Figure S6. Table S9 lists the partial excess enthalpies and the parameters of their fitting to a Redlich-Kister equation. NMR data are represented in Figures S7 to S9 and listed in tables S10 and S11. Figure S10 shows the spatial distribution functions of some of the mixtures studied and can be manipulated when viewed with Acrobat Reader.

Acknowledgment L.F.L. thanks São Paulo Research Foundation FAPESP for fellowship supports 2013/23234-7 and 2015/07145-0. The authors thank Dr. M. Traikia for his help with the NMR analyses and Mrs L. Pison for her assistance in the calorimetric and viscosity measurements. The authors would like to thank Prof. Seddon and his group for providing the sample of one of the ionic liquids studied.


17


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Page 18 of 30

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Figure 1. Chemical structures of the cations and anions which constitute the ionic liquids studied in this work. a) 1-butyl-3-methylimidazolium, C4C1Im+, b) tetrabutylphosphonium, P4444+ c) acetate, OAc-, d) trifluoroacetate, CF3CO2-, and e) bis(trifluoromethylsulfonyl)imide, NTf2 . The number labels in the cation C4C1Im+ indicate the atomic sites as they are considered in this work


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Figure 2. Density, ρ, and excess molar volume, VE, of the mixtures [C4C1Im][OAc](1−x)[CF3CO2]x (a and b), [C4C1Im][OAc](1−x)[NTf2]x (c and d) and [C4C1Im](1−x)[P4444]x[OAc] (e and f) as function of temperature and composition. Symbols indicate experimental values and lines correspond to fitting curves: linear correlation (a, c and e), second order Redlich-Kister polynomial (b and d) and zero order Redlich-Kister polynomial (f). In a, c and e: , [C4C1Im][OAc]; , [C4C1Im][CF3CO2]; , [C4C1Im][NTf2]; , [P4444][OAc]; +, xCF3CO2:0.7492 and xNTf2:0.7488; , xCF3CO2:0.5001, xNTf2:0.4999 and xP4444:0.4982; , xCF3CO2:0.2513, xNTf2:0.2503 and xP4444:0.2490. In b, d and f: , T=293.15 K (only in d); , T=303.15 K (in b and d); , T=313.15K (in b and d); T=333.15 K; , T=343.15 K (only in f); , T=353.15K.


,

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n

Figure 3. Viscosity, η, and viscosity deviations from linear mixing,

∆η = η mix

−

∑xη

i i

, of the

i =1

mixtures [C4C1Im][OAc](1−x)[CF3CO2]x (a and b), [C4C1Im][OAc](1−x)[NTf2]x (c and d) and [C4C1Im](1−x)[P4444]x[OAc] (e and f) as function of temperature and composition. Symbols indicate experimental values and lines correspond to fitting curves: Vogel-FulcherTammann equation (a, c and e) and second order Redlich-Kister polynomial (b and d) and zero order Redlich-Kister polynomial (f). In a, c and e: , [C4C1Im][OAc]; , [C4C1Im][CF3CO2]; , [C4C1Im][NTf2]; , [P4444][OAc]; +, xCF3CO2:0.7492 and xNTf2:0.7488; , xCF3CO2:0.5001, xNTf2:0.4999 and xP4444:0.4982; , xCF3CO2:0.2513, xNTf2:0.2503 and xP4444:0.2490. In b, d and f: , T=293.15 K (only in d); , T=303.15 K (in b and d); , T=313.15K (in b and d); , T=333.15 K; , T=343.15 K (only in f); , T=353.15K.


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Figure 4. (a) Enthalpy of mixing, ∆mixH, and (b) partial molar excess enthalpy at infinite dilution, HILE,∞, of the mixtures studied in this work and also of other mixtures of ionic liquids with the same cation [C4C1Im][An1](1-x)[An2]x from the literature. In (a) the symbols are used just to better identify the curves: , [C4C1Im][OAc](1−x)[CF3CO2]x; , [C4C1Im][OAc](1−x)[NTf2]x; [C4C1Im](1−x)[P4444]x[OAc]; , [C4C1Im][NTf2](1−x)[PF6]x;36 , [C4C1Im][OAc](1−x)[CCN3]x;11 [C4C1Im][BF4](1−x)[PF6]x;8,19. In (b), the symbols correspond to the same ionic liquid mixtures indicated in (a) and the bars are placed in pairs, being the first column the partial excess enthalpy of the ionic liquid containing the [An1] and the second one of the ionic liquid with [An2]. For the bars: in red [C4C1Im][OAc], in blue [C4C1Im][CF3CO2], in green [C4C1Im][NTf2], in yellow [P4444]x[OAc] and, for the mixtures from the literature, in black [C4C1Im][An1] and in white [C4C1Im][An2].


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Figure 5. Infrared spectra in the range of COO symmetric stretching (a and c), νs(COO),and in the range of C-H vibrational modes of the imidazolium ring (b and d) obtained for ( )[C4C1Im][OAc], ( )[C4C1Im][CF3CO2] and ( )[C4C1Im][NTf2], and for the mixtures [C4C1Im][OAc](1−x)[CF3CO2]x and [C4C1Im][OAc](1−x)[NTf2]x at the compositions: ( )[CF3CO2]0.8004, [NTf2]0.7488 ; ( )[CF3CO2]0.5001,[NTf2]0.4999; ( )[CF3CO2]0.2513, [NTf2]0.2503.


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Figure 6. 1H NMR chemical shifts as function of composition for () [C4C1Im][OAc](1−x)[CF3CO2] at 302.15 K, () [C4C1Im][OAc](1−x)[NTf2]x at 353.15 K and () [C4C1Im](1−x)[P4444]x[OAc] at 343.15 K. Left, middle and right columns stand for δH(2), δH(5) and δH(4), respectively. See Fig. 1 for atom labelling. Dashed lines indicate the average between the chemical shifts of the pure mixture components. The data for [C4C1Im][OAc](1−x)[NTf2]x were obtained from the literature42.


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Figure 7. Radial distribution functions of the acetate O around the H in position 2 of C4C1Im+ (left peaks, blue) and around the P of P4444+ (right peaks, red). The dashed lines correspond to the pure ionic liquids, whereas the full lines correspond to the mixture. The height of the curves is the extra probability of finding an O atom at that distance, relative to a homogeneous distribution. In the mixture, this extra probability increases in the vicinity of C4C1Im+ but decreases in the vicinity of P4444+.


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Table 1. Apparent friction coefficient between species in the [C4C1Im](1−x)[P4444]x[OAc] mixture. The values are in kJ.nm.ns.mol-1 units. x 1.0 0.5 0.0

OAc-–C4C1Im+ 10.7 3.9

cross friction, ζAB OAc-–P4444+ 14.6 10.4

C4C1Im+–P4444+ -0.1


OAc7.8 1.2 0.4

self friction, ζAA C4C1Im+ P4444+ 13.1 -1.5 16.4 0.3

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Experimental enthalpies of mixing of the ionic liquids studied

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Radial and spatial distribution functions of OAc- around C4C1Im + and P4444+


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