Fluorination effect in the volatility of imidazolium-based ionic liquids

Fluorination effect in the volatility of imidazolium-based ionic liquids

Accepted Manuscript Fluorination effect in the volatility of imidazolium-based ionic liquids A.S.M.C. Rodrigues, A.M. Fernandes, J. Dévemy, M. Costa ...

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Accepted Manuscript Fluorination effect in the volatility of imidazolium-based ionic liquids

A.S.M.C. Rodrigues, A.M. Fernandes, J. Dévemy, M. Costa Gomes, L.M.N.B.F. Santos PII: DOI: Reference:

S0167-7322(18)36712-6 https://doi.org/10.1016/j.molliq.2019.03.024 MOLLIQ 10565

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

20 December 2018 3 March 2019 5 March 2019

Please cite this article as: A.S.M.C. Rodrigues, A.M. Fernandes, J. Dévemy, et al., Fluorination effect in the volatility of imidazolium-based ionic liquids, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.03.024

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ACCEPTED MANUSCRIPT Fluorination Effect in the Volatility of Imidazolium-based Ionic Liquids A. S. M. C. Rodriguesa, A. M. Fernandesb

c

, M. Costa Gomesd* and L. M. N. B. F.

Santosa* CIQUP, Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal.

b

QOPNA Unit, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal.

c

Institut de Chimie de Clermont-Ferrand, CNRS & Univ Clermont Auvergne, 63000 Clermont-Ferrand, France.

d

Laboratoire de Chimie de l’ENS Lyon, CNRS & Univ Lyon, 46 allée Italie, 69634 Lyon, France.

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Abstract

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The volatility and cohesive energy of fluorinated ionic liquids, FILs, was evaluated and used to explored and understand the effect of fluorination of the ILs cations and anions on the cohesive interaction and structuration. For this, the vapour pressures at different temperatures using a Knudsen effusion apparatus coupled with a quartz crystal microbalance were measured for three fluorinated imidazolium-based ionic liquids. The enthalpies and entropies of vaporization were derived from the temperature dependence of the vapour pressures. The degree of fluorination of the ions leads to an increase of the IL volatility following the order: [C8mim][NTf2] < [C8mim][BETI] < [C8H4F13mim][NTf2] < [C8H4F13mim][BETI]. The Gibbs energy of vaporization follows the opposite trend proving that the fluorination effect on the volatility of the studied ionic liquids is ruled by the increase of the entropy of vaporization that overlaps the increase of the cohesive energy of the liquids. The binding energies of gas-phase FILs ions were measured using electrospray tandem mass spectrometry (ESI-MS-MS). The energy required for extracting the fluorinated cation from the [(cation)2(anion)]+ aggregates in these fluorinated ionic liquids is significantly higher than in their non-fluorinated counterparts. The measured binding energies trends are in agreement with those found for the enthalpies of vaporization.

*

Corresponding authors: [email protected] and [email protected]

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ACCEPTED MANUSCRIPT Introduction Ionic liquids (ILs) are nanostructured liquids constituted only by ions, with melting points close to or below room temperature. Their negligible vapour pressure at room temperature, high chemical and thermal stability, a wide range of solubility and miscibility with polar and non-polar compounds and broad electrochemical windows, make them suitable for many applications. Ionic liquids can also be tuned for particular applications by means of their functionalization.1 Successful modifications of existing ionic liquids have improved their uses

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as supports for synthesis, as catalysts for different chemical reactions,2 as more performant gas absorbents3,4 in analytical chemistry5–7 and in other industrial applications.8 The literature on the effect of the inclusion of fluorinated moieties on the properties of different families of ionic liquids has been recently reviewed,9 that highlighted the possibility of using these ionic liquids as an alternative to the perfluorinated compounds (PFC) currently used in industrial extraction processes and in the gas separation processes. The combination of the

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ILs properties with the presence of fluorinated moieties, enhances their ability as physical absorbents for CO2 and H2S10–13 leading to their consideration as alternative gas absorbents,

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with less environmental impact than conventional organic absorbents. Fluorinated ionic liquids, FILs, can exist in a large variety of molecular structures, all leading to nanostructured fluids characterised by polar and non-polar domains permeated by a

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three-dimensional network of ionic channels formed by anions and cations.14 Different authors have concluded, following both experimental

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and computational studies16,17 that

fluorous domains can be formed when perfluorinated and hydrogenated alkyl chains co-exist

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in the fluorinated ionic liquid. This segregation was already observed for organic solvents that tend to mix poorly with fluorous solvents due to the lack of affinity between

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perfluorinated chains and their hydrogenated counterparts.18 The physical-chemical characterization of partially fluorinated ionic liquids is crucial to a better understanding of

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structure – properties relationships. In this work, the volatility of three fluorinated imidazolium-based ionic liquids 1(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)-3-methylimidazolium bis[trifluoromethylsulfonyl]imide,

[C8H4F13mim][NTf2],

1-octyl-3-methylimidazolium

bis[pentafluoroethylsulfonyl]imide,

[C8mim][BETI],

1-(3,3,4,4,5,5,6,6,7,7,8,8,8-

tridecafluorooctyl)-3-methylimidazolium

and

bis[pentafluoroethylsulfonyl]imide,

[C8H4F13mim][BETI], schematically represented in Figure 1, were studied experimentally by measuring their vapour pressures and their cation-anion interaction energies. Molecular simulation was used to explore the microscopic structure of the ILs with different levels fluorination.

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ACCEPTED MANUSCRIPT From the experimental results, the enthalpies and entropies of vaporisation were derived

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and compared with the equivalent non-fluorinated ionic liquids.

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Figure 1. Schematic representation of the fluorinated ionic liquids studied. (I): [C8H4F13mim][BETI]; (II): [C8H4F13mim][NTf2]; (III): [C8mim][BETI].

Experimental section

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Synthesis and characterization of the ionic liquids The synthesis and purification of the samples of fluorinated ionic liquids used herein were

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previously described.11 The purity of each IL was checked again before the vapour pressure measurements by NMR spectroscopy (1H and

13

C) recorded on a Bruker Avance 300

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[operating at 300.13 MHz (1H), 75.47 MHz (13C)] spectrometer using TMS as internal reference and with CDCl3 as a solvent. For detailed NMR analysis, see Supporting Information. The raw samples of the ionic liquids present a yellowish colour due to some

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traces of impurities not detected by NMR. The ILs samples were previously distilled under high vacuum at the same temperature of the effusion measurements using a micro distillation apparatus that have been developed in our laboratory. The distillation vanished the yellowish colour of the samples. The NMR analysis of the fresh and distilled samples show no evidence of decomposition. Details about this procedure are described in supporting information as well the NMR results of the distilled ILs. The samples were degassed in situ under high vacuum (<0.001Pa) and moderated temperature (350 K) prior the vapour pressure measurements.

Vapour pressure measurements

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ACCEPTED MANUSCRIPT Vapour pressures measurements as a function of the temperature of the ionic liquids were measured using a Knudsen effusion methodology coupled with a quartz crystal microbalance, KEQCM. The methodology use in the KEQCM measurements is already described in the literature. 19,37

Molecular simulation The [C8C1im]+ and [C8H4F13mim]+ cations and the [NTf2]− and [BETI]− anions were represented by the CL&P all-atom nonpolarisable force field.10,20 Molecular dynamic

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simulations were performed using the LAMMPS21 software package with the Verlet integrator.22 Short-range forces (Lennard-Jones parameters) between unlike atoms were cut-off at 12 Å of interatomic separation and long-range electrostatic forces calculated with the particle-particle particle-mesh (PPPM) 23 method.

Configurations of cubic boxes of 500 ion pairs of the pure ionic liquids were equilibrated during 2 ns trajectories (starting from random configurations generated by the Packmol

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utility24) at 373 K and 1 bar, maintained by the Nosé-Hoover thermostat and barostat.25 Because the fixed-charge force fields used for the ionic liquids lead to slow dynamics when

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compared to experiments, scaled ionic charges (± 0.8e) were used.26,27 The equilibrated systems were used in production runs of 10 ns. Structural analyses of the four pure ionic liquids were made through the calculation of structure factor functions (S(q)) in cubic

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simulation boxes having an average side size of 72 Å.

Electrospray ionisation tandem mass spectrometry (ESI-MS-MS)

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Electrospray ionization tandem mass spectra (ESI-MS-MS) were acquired with a Micromass Quattro LC triple quadrupole mass spectrometer. The procedure and methodology

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described in the literature.28 Details about the operating conditions of the mass spectrometer are presented in the supporting information. ESI-MS-MS spectra were acquired by selecting

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the precursor ion with the first quadrupole, performing collisions with argon at variable energies (ELab) in the second quadrupole and analysing the fragment ions thus produced with the third quadrupole.

Results and discussion Vaporisation equilibrium The vapour pressures results for each FILs are presented in Table 1. Each data point is the average of three independent experimental series.

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Table 1 Experimental vapor pressures (p,T) results for the ILs studied

T/K

p / Pa

Δp/ Pa

468.04

0.0357

−0.0004

473.04

0.0518

0.0003

478.04

0.0734

0.0003

483.03

0.1035

0.0007

488.03

0.1438

0.0000

493.03

0.1994

−0.0003

498.02

0.2743

−0.0011

p / Pa

Δp/ Pa

0.0460

− 0.0001

0.0669

0.0000

0.0965

0.0004

0.1377

0.0005

483.03

0.1943

− 0.0002

488.02

0.2731

− 0.0003

493.01

0.3783

− 0.0035

498.01

0.5334

0.0035

503.01

0.7441

0.0134

T/K

p / Pa

Δp / Pa

473.04

0.0457

− 0.0001

478.04

0.0647

0.0001

483.04

0.0906

0.0002

488.04

0.1255

− 0.0002

493.03

0.1729

− 0.0006

498.03

0.2388

0.0006

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[C8H4F13mim][NTf2]

463.04 468.04 473.03

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478.03

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[C8H4F13mim][BETI] T / K

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[C8mim][BETI]

∆p = p − pcalc, where pcalc is calculated from the Clausius-Clayperon equation with the coefficients given in Table 2. Standard uncertainties, u, are u(T) =  0.02 K, u(p) =  (0.001 + 0.05·p) Pa, at the 95 % confidence level.

Figure 2 depicts the graphical representations of ln(p/Pa) = f [(1/T)/K-1] for the FILs studied herein together with literature data of some ILs from [CnC1mim][NTf2] series used for comparison. The results were fitted to the integrated form of the Clausius-Clayperon equation (1).

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ACCEPTED MANUSCRIPT (1) Where p is the pressure in Pascal (Pa) and T is the temperature in Kelvin (K). The fitted coefficients from equation (1), a and b, are listed in Table 2, together with the mean temperature, , and the pressure at the mean temperature p().

Table 2 Coefficients of the Clausius-Clapeyron, equation (1), for the studied ILs at the mean temperature, , and pressure, p(), of the experiments

[C8mim][BETI]

[C8H4F13mim][BETI]

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[C8H4F13mim][NTf2] a±σ

30.45 ± 0.12

29.78 ± 0.07

31.69 ± 0.08

b±σ/K

15809 ± 56

15548 ± 34

16097 ± 37

0.99994

0.99998

0.99997

/ K

482.56

485.21

482.20

p() / Pa

0.099

0.104

0.184

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r

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Figure 2. Graphical representation of the vapour pressure results, ln(p/Pa) = f [(1/T)/K ], for the studied FILs: this work:  [C8H4F13mim][BETI],  [C8H4F13mim][NTf2];  [C8mim][BETI]; and literature data:  [C8mim][NTf2]; (long dashed line for [C7mim][NTf2] and dotted line for 29 [C6mim][NTf2]).

The standard (p0 = 105 Pa) molar enthalpies and entropies of vaporization at the average temperature, ,

and

were calculated by equations (2) and (3),

respectively: (2) (3)

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ACCEPTED MANUSCRIPT where b is the coefficient from equation (1) and R is the gas constant (R = 8.3144598 Jmol−1K−1).30 The standard (p0 = 105 Pa) molar enthalpies,

at the reference temperature, T=

K,

were derived by equation (4): (4) where

is the standard molar heat capacity difference between the liquid and the . The standard molar entropies of vaporization,

, at T=

K, were calculated using equation (5):

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gas phase:

where p0 = 105 Pa. The standard molar Gibbs energies of vaporization,

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were determined using equation (6):

The difference between the liquid and gas heat capacities,

(5) , at T=

K,

(6)

, used in equations (4)

The derived values of

and ,

for the [C8C1im][NTf2].29,31,32

and

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presented previously for

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and (5), were estimated as (143  10) J.K-1.mol-1 for the FILs based in the correlation

for the FILs studied, at various temperatures,

are presented in Table 3. Figures 3 and 4 depict the standard molar thermodynamic

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properties (Gibbs energy, enthalpies and entropies, respectively) of vaporization corrected to T = 460 K, for all ILs considered in this study. This temperature has been used as reference

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in previous studies since as it is close to the average temperature of the ILs vapour pressure measurements and has the additional advantage of reducing the weight of the

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contribution in the uncertainty of the derived thermodynamic values.29

Effect of fluorination of the cation and anion in ILs volatility The vapour pressures of the ILs varies between 0.02 and 0.75 Pa in the temperature range covered. The results of Gibbs energy presented in figure 2 show that the volatility of ILs are significantly affected by the degree of fluorination of the ions following the order: [C8mim][NTf2] < [C8mim][BETI] < [C8H4F13mim][NTf2] < [C8H4F13mim][BETI. The enthalpies of vaporization,

, follows the opposite trend of the Gibbs free energy of vaporization, and

incr as s with th incr as of th fluorination l CF2-“

oi t in th

l of th i idazoliu

cation cor

alk l chain l ads to a s all but significant incr as

of th

Th “coh si

interactions resulting in a higher cohesive energy of the FILs ionic pairs. It is clear that the

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ACCEPTED MANUSCRIPT trend of the intermolecular interaction in the liquid is in opposite direction of the increase in the FILs volatility. 0

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Table 3 Standard (p = 10 Pa) molar enthalpies, , entropies, of vaporization for the ILs studied

/

/K

, and Gibbs energies,

/

/

kJ·mol-1

kJ·mol-1

J·K-1·mol-1

482.56

55.5 ± 0.7

131.4 ± 0.5

157.3 ± 1.0

298.15

90.4 ± 0.8

157.8 ± 1.9

226.2 ± 4.9

460.00

59.1 ± 0.7

134.6 ± 0.5

164.1 ± 1.1

485.21

55.5 ± 0.4

129.3 ± 0.3

152.0 ± 0.6

298.15

89.9± 0.5

156.0 ± 1.9

221.6 ± 4.9

460.00

59.5 ± 0.4

132.9 ± 0.4

159.6 ± 0.8

482.20

52.9 ± 0.4

133.8 ± 0.3

167.7 ± 0.6

298.15

89.6 ± 0.5

160.1 ± 1.9

236.4 ± 4.8

460.00

56.8 ± 0.4

137.0 ± 0.4

174.4 ± 0.8

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[C8H4F13mim][NTf2]

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[C8mim][BETI]

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[C8H4F13mim][BETI]

0

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Figure 3 Sequential representation of the standard (p = 10 Pa) molar Gibbs energy of vaporization (at T=460 K) of the studied FILs together with some members of the [CnC1im][NTf2] ILs series 29 previously reported in literature . Dashed lines are only guidelines for visualization and has no physical meaning.

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ACCEPTED MANUSCRIPT The schematically diagram in figure 5 presents the differential analysis of the enthalpy, and entropy,

,

, of vaporization of the studied ILs with the increase of fluorination level. It

was found that the increase in the fluorination level of the anion (from [NTF2] to [BETI]) have a minor effect in the enthalpy of vaporization when the cation is not fluorinated (0.9 kJ·mol -1) contrary to what is observed when fluorinated moieties in the cation are presented (increase of 2.4 kJ·mol-1). The associative effect of the fluorinated moieties could be related with the higher F···F interaction ability of the anion BETI with the alkyl fluorinated chain of

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imidazolium cation. In the liquids it is expected a lower F···F interaction ability between the fluorinated region of two anions (BETI···BETI) due to the electrostatic repulsion.

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II

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I

0

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Figure 4 Standard (p = 10 Pa) molar enthalpies (I) and entropies (II) of vaporization n (at T=460 K) of the studied FILs together with some members of the [CnC1im][NTf2] 29 ILs series previously reported in literature.

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Concerning the entropic contribution, the significant increase in the entropy of vaporization with the degree of fluorination of the ionic liquid points towards several contributions. There is an increase in the entropy of gas phase, Sgas, from the fluorine molar mass contribution to translational, Strans, and rotational, Srot, entropies. The weight of these contributions are, however, small since these effects are also presented in liquid phase so there is an

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liquid/gas compensation.

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Figure 5 Sch atic diagra of th ariations of th nthalp ∆∆H (in kJ·mol ), and entropy, -1 -1 ∆∆S (in J·K ·mol ) of vaporization, at T=460K, between the studied ILs.

The contribution of conformational entropy has a greater impact on the liquid than on the gas

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phase. The decrease of the absolute entropy of the liquid is the major contribution to the increase of the entropy of vaporization. Thus, in the liquid phase, it is expected that the conformational entropy of the fluorinated chain to be significantly lower than in the alkanes due to their higher energy barriers of the internal rotation and translational dynamics. In fact, the same trend in the entropy of vaporization was observed for long fluoroalkanes and fluoroalcohols,33 which present very analogous carbon backbone conformation to these fluorinated and hydrogenated ionic liquids, as shown in the optimized structures presented in Figure 6. The higher enthalpies of vaporization suggests stronger interactions for the FILs which may contribute to more organization in bulk and, hence, to a decrease in entropy in the liquid phase. Moreover, the predictable decrease of entropy, Sliq, for the FILs can be

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ACCEPTED MANUSCRIPT related with their higher viscosity relative to their hydrogenated analogous,13,34–36 which is related with their higher barrier to flow. The summation of these contributions, both in gas and liquid, are in conform with a significant increase in the entropy of vaporization of fluorinated ionic liquids. Hence, the fluorination effect on the volatility of ionic liquids is ruled by the increase of the entropy of vaporization,

, that overlaps the increase of cohesive interaction reflected in the change

of the enthalpy of vaporization. Nevertheless, the effect of the fluorination in the ionic liquids

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is however tricky to rationalize at molecular level, as it can be dependent whether if it is locat d in th polar or nonpolar r gion as w ll as on th a ailabilit of F∙∙∙F int raction

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location between cations and anions.

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Figure 6 Optimised geometries of [C8H4F13mim][BETI] and [C8mim][NTf2] determined at the M062x/6-31+G(d,p) level of theory.

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Electrospray ionisation tadem mass spectrometry (ESI-MS-MS) Electrospray ionisation mass spectrometry data was used to gather additional insights into interpreting

the

thermodynamic

properties

of

vaporisation

and

complement

the

co putational r sults r garding th FIL’s int raction n rgi s El ctrospra ionisation

ass

spectrometry (ESI-MS-MS) was used to measure the energy required to separate a cation or an anion from the neutral ionic liquid molecule in the gas phase according to the reactions expressed in equation (7) and (8). Cation separation anion

anion

(7)

Anion separation

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ACCEPTED MANUSCRIPT cation

anion

(8)

The experimental energies of separation, are expressed by Ecm,1/2 that is derived from experimental Elab,1/2 according equation (9): (9) Where

and

are the masses of the neutral target and precursor ion, respectively. The

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mass spectral data, represented as Ecm,1/2, for ionic liquids are listed in Table 4 and plotted in Figure 7 together with the results of enthalpies of vaporization.

Table 4 Experimental energies of separation, Ecm,1/2 for the studied ionic liquids

[(cation)2anion]+ [cation(anion)2]-

Ionic Liquid

Ecm,1/2/ eV 0.53

[C8mim][BETI]

0.38

0.40

0.52

0.56

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[C8H4F13mim][BETI] [C8mim][NTf2]

0.58

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[C8H4F13mim][NTf2]

Ecm,1/2/ eV

0.40

0.45

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Average standard deviation of Ecm,1/2 ± 0.01 eV

High level of fluorination

Figure 7 Ecm,1/2 in the positive (), equation (7) and negative (), equation (8) ion mode (left YY axis) and enthalpies of vaporization (right YY axis) () for the studied ILs. The [C6mim][NTf2], [C7mim][NTf2], [C8mim][NTf2] results are from the literature. 28,29

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ACCEPTED MANUSCRIPT The magnitude of the separation energy, Ecm,1/2, is a good approximation to the cation or anion interaction energy of the ionic pair. The analysis of the data reveals a significant differentiation between the FILs and the analogous imidazolium-based ILs. The energy required for extracting the fluorinated cation from the [(cation)2(anion)]+ aggregates in FILs is significantly higher than the respective analogous [C8C1im][NTf2] ionic liquid. Moreover, the dissociation energy seems to be less dependent on the anion which is consistent with the enthalpies of vaporization differentiation (0.9 kJ·mol-1) from NTF2 to BETI in the case of non

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fluorinated cations. The fluorination of the alkyl side chain in the cation leads to a higher cohesive energy between the ion pairs.

Quantum chemical calculation of the ionic liquids pairs were performed to derived the cationanion interaction energies, Δint??m. The Δint??m results of [C8H4F13mim][NTf2], [C8mim][NTf2], [C8H4F13mim][BETI] and [C8mim][BETI] are: −375 kJ·mol-1, −348 kJ·mol-1, −367 kJ·mol-1 and −347 kJ·mol-1 respectively. The observed differentiation of the interaction energies between

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the fluorinated and the nonfluorinated cation (20-30 k ∙ ol-1) is consistent with the trend observed in experimental energies of separation, Ecm,1/2, and with the higher enthalpy of

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vaporization when fluorinated moieties in the cation are present.

Molecular simulation analysis

The differential analysis of the molecular structure and ILs structuration between the

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different levels of ILs fluorination was explored by molecular simulation. In Figure 8 are represented static cation-anion structure factors of the ionic liquids, calculated by molecular

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simulation. In Figure 8; (a) terminal carbons of the alkyl side-chain of the cation; (b) terminal carbons of the alkyl side-chain of the cation versus other carbons of the alkyl side-chain of the cation (C3 to C7); (c) fluorinated carbons of the anions. The structuration observed at

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large distances (only the lower values of q are represented) is more pronounced in the more fluorinated ionic liquids, in agreement with the observed lower absolute entropies in liquid

S(q)

1

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phase (corresponding to higher entropies of vaporization).

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(a)

7

q /nm−1

9

(b)

S(q)

1

3

5

7

q /nm−1

9

(c)

S(q)

1

3

5

7

9

q /nm−1

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ACCEPTED MANUSCRIPT Figure 8 Site-site static structure factors of the ionic liquids studied: [C8mim][BETI]; [C8mim][NTf2]; [C8H4F13mim][BETI]; [C8H4F13mim][NTf2]. (a) terminal carbons of the alkyl side-chain of the cation; (b) terminal carbons of the alkyl side-chain of the cation versus other carbons of the alkyl side-chain of the cation (C3 to C7); (c) fluorinated carbons of the anions.

Conclusions The volatility and cohesive energy of fluorinated ionic liquids, FILs, was evaluated and used to explored and understand the effect of fluorination of the ILs cations and anions on the cohesive interaction and structuration. The enthalpies and entropies of vaporization increase

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with the degree of fluorination of the ions to follow the order: [C8mim][NTf2] < [C8mim][BETI] < [C8H4F13mim][NTf2] < [C8H4F13mim][BETI]. The mass spectrometry results follow the same trend of enthalpies of vaporization. The energy required for extracting the fluorinated cation from the [(cation)2(anion)]+ aggregates in these fluorinated ionic liquids is significantly higher than in [CnC1im][NTf2] ionic liquid. It was found that the fluorination of the ionic liquids increases their volatility and this is ruled by the increase of the entropy of vaporization which

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overlaps the contribution of the increase of the cohesive energy. The structuration observed at large distances (only the lower values of q are represented) using molecular simulation is in agreement with the observed lower entropy in liquid phase of the fluorinated ionic liquids

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that was derived from the volatility results.

Acknowledgements

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We thank Fundação para a Ciência e Tecnologia (FCT), Lisbon, Portugal, and the European Social Fund (ESF) for financial support to CIQUP, University of Porto (Projects: PEstC/QUI/UI0081/2013, FCUP-CIQ-UP-NORTE-07-0124-FEDER-000065).

A.S.M.C.R also

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thank FCT for the award of the Research Grants SFRH/BD/81261/2011. M.C.G. acknowledges the financial support of the project IDEXLYON of the University of Lyon

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Highlights

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The volatility of fluorinated ionic liquids was measured using a Knudsen effusion apparatus. The fluorination of the ionic liquids increases their volatility. The volatility of fluorinated ionic liquids is ruled by the increase of the entropy of vaporization.

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