Influence of Nanosegregation on the Phase Behavior of Fluorinated

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The Influence of Nanosegregation on the Phase Behavior of Fluorinated Ionic Liquids Margarida L. Ferreira, María José Pastoriza-Gallego, João M. M. Araújo, Jose N Canongia Lopes, Luis Paulo N. Rebelo, Manuel Martínez Piñeiro, Karina Shimizu, and Ana B. B. Pereiro J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00516 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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The Journal of Physical Chemistry

The Influence of Nanosegregation on the Phase Behavior of Fluorinated Ionic Liquids Margarida L. Ferreira,†,‡ María J. Pastoriza-Gallego,§ João M. M. Araújo,†,‡ José N. Canongia Lopes,†, ǁ Luís Paulo N. Rebelo,†,‡ Manuel M. Piñeiro,§ Karina Shimizu, ǁ Ana B. Pereiro†,‡,* †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa,

Apartado 127, 2780-157, Oeiras, Portugal ‡

LAQV, REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia,

Universidade Nova de Lisboa, 2829-516 Caparica, Portugal §

Departamento de Física Aplicada, Facultade de Ciencias, Universidade de Vigo, E36310 Vigo,

Spain ǁ

Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, 1049 001

Lisboa, Portugal

ACS Paragon Plus Environment

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ABSTRACT: Fluorinated ionic liquids (FILs) have received increasing attention due to their physicochemical properties. They allow us to enlarge the tuneability power of traditional ionic liquids. With the aim to understand the thermodynamic behavior of these compounds, a study of solid-fluid transitions using differential scanning calorimetry, thermogravimetric analysis, rheology and molecular dynamics simulation has been performed. A comparison between different cations, anions and hydrogenated alkyl chains was carried out using ionic liquids with fluorinated alkyl chain lengths equal or longer than four carbon atoms. In this work, we provide evidences of the fluorinated domain influence on the thermodynamic behavior of these compounds. Moreover, the results suggest that the nanosegregation of the fluorous domains might be an interesting structural feature that modifies and/or enhances the rich phase behavior of the FILs increasing the probability of these compounds to adopt different conformations. This information is crucial to design the best FIL and can increase their potential on a wide range of applications.


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INTRODUCTION During the last two decades, ionic liquids (ILs) research has acquired a growing interest. These salts have been extensively studied due to their exceptional physicochemical properties1–3 which are highly versatile, enabling a broad range of applications in different fields.4–9 The capability of combining many different types of anions and cations provides to these compounds a huge potential as tailored solvents.1,3,10 The tuning of their thermophysical and thermodynamic properties can be carried out taking into account the final purpose.1 However, there are many compounds less explored so far, as for instance those of the fluorinated ionic liquids (FILs) family.11 The molecular structure of FILs is characterized by their carbon-fluorine bonds, the strongest and stable, single covalent bond in organic chemistry,12 increasing their rigidity and decreasing their polarity.13 In this work, the ILs studied include anions with fluorinated chain lengths equal or longer than four carbon atoms, distinguishing them from mere fluoro-containing ILs, such as those based on bis(trifluoromethylsulfonyl)imide, hexafluorophosphate or tetrafluoroborate anions.11,14,15 These ILs can present advantageous and different properties due to the addition of a new nanosegregated fluorinated domain, which can modify their mesomorphic properties and molecular geometry,12,14-17 increasing their tuneability power as compared to that of conventional ionic liquids. The impact of these compounds in biomedical applications has been pointed out due to their potential as gas carriers, artificial blood substitutes, liquid ventilation media, imaging agents and intravenous formulations.13,17–21 Other unique properties characterizing these compounds are their low surface tension, weak interactions with organic compounds, remarkable chemical and biological inertness,17 tunable toxicity and easy recovery and recyclability.19,20,22 These exceptional properties also enable their application as new alternative cleaner compounds substituting the usually harmful perfluorinated surfactants


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used in a wide range of industrial applications.19,20,22 Several fluorinated ionic liquids have been explored for other applications as in electrochemistry, where they are mostly used as electrolyte components in the production of energy storage devices,23,24 metal batteries as lithium,25,26 fuel27 and solar cells28. These compounds can be also useful in catalysis of several reactions.29,30 Recently, several studies have established the interplay between cations and anions and charged/non-charged moieties in ionic liquids, leading to segregation between polar and apolar domains.10,31-33 This creates high-charge and low-charge electronic density distribution domains in the liquid. It is expected that the incorporation of fluorinated chains may induce the formation of one polar and two apolar (fluorinated and hydrogenated) nanosegregated domains. In our previous work,17 the characterization of two aprotic FILs based on the perfluorobutanesulfonate anion strongly supports the formation of a third nanostructured domain in ionic liquids, demonstrating that these three mesoscopic regions may enable the formation of multiple types of crystals. Shen et al12 studied the self-assembly nanostructure of protic ionic liquids with perfluorinated anions, using small- and wide-angle X-ray scattering showing, in the liquid state, the segregation of the polar, hydrocarbon and fluorocarbon domains. Russina et al16 used room temperature ionic liquids with medium length fluorinated tails to verify the phenomenon of mutual self-assembly into domains containing polar, hydrophobic and fluorophilic moieties at the mesoscopic level, using NMR and SWAXS measurements. Hollóczki et al15 reported the influence of inserting fluorinated alkyl chains on the cations, using fluorinated aprotic ionicliquid mixtures. The formation of three stable microphases (polar, lipophilic and fluorous) was also observed and an aggregation behavior of the fluorinated chains was detected by domain analysis, producing a fluorous microphase in the liquid state. Castiglione et al14 also reported


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evidences of microsegregation of fluoro chains of the anions at a molecular level by using intermolecular anion-cation Nuclear Overhauser Enhancements (NOEs). The aim of this work is to understand the role of these three nanosegregated domains (polar, hydrogenated apolar and fluorinated apolar) in the rich solid-fluid phase transitions of the neoteric fluorinated ionic liquids. The characterization of phase transitions using calorimetric and rheological tools has been achieved not only with the aim to study their structural behavior, but also to understand and optimize the FILs performance and to increment their potential for future applications. To accomplish our purpose, comparisons between new four different FILs based on the imidazolium, pyrrolidinium, pyridinium and ammonium cations, conjugated with two different fluorinated anions, perfluorobutanesulfonate and bis(nonafluorobutylsulfonyl)imide, have been performed. Their structure and nomenclature are presented in Table 1. Furthermore, the results obtained for FILs studied in our previous work,17 1-hexyl-3-methylimidazolium perfluorobutanesulfonate and tetrabutylammonium perfluorobutanesulfonate, were used for comparison purposes, and therefore they are also incorporated in Table 1. Several structural variations such as the length of the hydrogenated chains of the cations, the type of cation and the type of anion were considered in this study. The structural and thermal characterization of these FILs was performed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and rheology. These techniques allowed identifying the solid-fluid transitions, decomposition temperatures and thermodynamic behavior of FILs. Additionally, molecular dynamics (MD) simulations have been carried out in order to study the complex structure and properties at an atomistic level, in order to corroborate and understand the results obtained by calorimetric and rheological analysis.


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EXPERIMENTAL SECTION Materials. 1-Ethyl-3-methylpyridinium perfluorobutanesulfonate, [C2C1py][C4F9SO3], 99% mass

fraction

purity,

[C4C1pyr][N(C4F9SO2)2],

1-butyl-n-methylpyrrolidinium >

98%

mass

fraction

bis(nonafluorobutylsulfonyl)imide,

purity,

1-butyl-n-methylpyrrolidinium

perfluorobutanesulfonate, [C4C1pyr][C4F9SO3], > 98% mass fraction purity, and 1-methyl-3octylimidazolium perfluorobutanesulfonate [C8C1Im][C4F9SO3], > 99% mass fraction purity, were supplied by IoLiTec GmbH. Their structures and nomenclature are represented in Table 1. These fluorinated ionic liquids were dried under vacuum (3 × 10-2 Torr) and stirred at 323.15 K for at least 2 days immediately before use, to remove water and volatile substances. The water content, determined by Karl Fischer titration, was lower than 100 ppm in every case. The purity of the final products was checked by 1H and

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F NMR. Each sample was taken from the

respective Schlenk flask with a syringe under nitrogen flow to avoid humidity and immediately placed in the measuring devices. Thermal Analysis. A TGA Q50 Thermogravimetric Analysis (TA instruments) equipment was used to measure the weight loss as a function of temperature providing information concerning thermal stability. The samples were continuously purged using 50 ml·min-1 of dry N2 gas. About 10 to 25 mg of fluorinated ionic liquid was crimped in an aluminum standard sample pan. Samples were heated up to 873.15 K at a rate of 1 K·min-1 until complete thermal degradation was achieved. The relative uncertainty of the temperature was ± 4 K. The TGA curves are illustrated in Figure S1of Supporting Information. A Differential Scanning Calorimeter (DSC) Q200 (TA Instrument) was used to measure the phase transitions of the FILs. Cooling was accomplished by a refrigerated cooling system capable of controlling the temperature down to 183.15 K. The sample was continuously purged


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using 50 ml·min-1 of dry N2 gas. About 5 to 10 mg of each FIL was sealed in an aluminum standard sample pan. Indium (m.p., T = 429.76 K) was used as standard for the DSC calibration. Samples were all cooled down to 183.15 K and tempered during 30 min. Afterwards, they were heated until different temperatures leaving a range of 20 K between the last transition and the end of the cycle. The cooling–heating cycles were repeated three times at different rates, 10 K·min-1, 5 K·min-1 and 1 K·min-1. This scan rates selection guarantees the best delineation and characterization of the several solid-fluid phase transitions. The standard uncertainty was estimated as ± 2 K. Moreover, the melting temperatures were obtained through peak temperature due to the characteristic peak obtained with these ionic liquids (very broad melting curves similar to the behavior of polymers).34 All DSC runs using the different scan rates are plotted in Figures S2-S5 of Supporting Information. 1-Hexyl-3-methylimidazolium perfluorobutanesulfonate, [C6C1Im][C4F9SO3]17 and tetrabutylammonium perfluorobutanesulfonate, [N4444][C4F9SO3]17 were used for comparison. The DSC runs for these two compounds are also shown in Figures S6 and S7 of Supporting Information. Universal Analysis 2000 v. 4.5A software (TA instruments) was used to integrate the calorimetric peaks to determine the solid-solid transition (Ts-s) and melting temperatures (Tm) in the case of DCS data (see Table 2). Regarding TGA analysis, the onset (Tonset), start (Tstart) and decomposition (Tdec) temperatures (corresponding to the temperatures where the baseline slope changed during heating, the weight loss was less than 1% and the weight loss was 50%, respectively) were also determined using the same software and the experimental data are shown in Table S1 of Supporting Information. Rheological Analysis. The experimental device used was a Physica MCR 101 rheometer (Anton Paar, Graz, Austria), equipped with a plate-plate geometry with a plate diameter of 25


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mm. The upper plate went down to a gap of 1 mm from the lower plate and covered the whole sample for all tests. Temperature was controlled with a Peltier P-PTD 200 system, placed at the lower plate, with a diameter of 56 mm without groove. Different types of experiments were carried out to investigate the FILs rheological behavior. Linear viscoelastic measurements constitute the first group of experiments. The second group of experiments are non-linear tests. All these experiments were developed from torques of 0.1 µN·m in the temperature range of 269.15 to 432.15 K. Oscillatory and rotational experiments were carried out with the objective of analyzing both the relatively large deformations and small-amplitude oscillatory shear. More details about the experimental setup and operating conditions can be found in our previous papers.17,36,37 Molecular Dynamics (MD) Simulations. FILs were modeled using the previously reported CL&P atomistic force field.38-40 This method is an extension of AMBER/OPLS force fields,41 which are specifically fitted to enclose whole ionic liquids series in a systematic way. The Molecular Dynamics (MD) simulations were carried out by using the DL_POLY 2.20 package.42 The [C2C1py][C4F9SO3], [C4C1pyr][C4F9SO3] and [C4C1pyr][N(C4F9SO2)2] FILs were modeled in simulation boxes with 800 ion pairs for each pure compound, starting from low-density configurations that were equilibrated under isothermal-isobaric ensemble conditions (N-p-T) where T = 343 K and p = 0.1 MPa, with Nosé-Hoover thermostats and barostats with relaxation time constants of 0.5 and 2.0 ps, respectively. Several consecutive runs were accomplished at the temperature studied (equilibration around 10 ns; production runs of at least 6 ns; configuration dumps every 1 ps), until the density of each system reached constant and consistent values indicating that the equilibrium was attained. The MD simulation runs were accomplished in cubic boxes with at least 7 nm sides ([C2C1py][C4F9SO3] at 7.19 nm; [C4C1pyr][C4F9SO3] at 7.48


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nm and [C4C1pyr][N(C4F9SO2)2] at 8.46 nm) with a time step of 2 fs and a cutoff distance of 1.6 nm (with the Ewald summation approach applied to electrostatic interactions beyond that distance). In the case of [C6C1Im][C4F9SO3] and [N4444][C4F9SO3], the MD simulations were already executed as was previously described in reference 17 while [C8C1Im][C4F9SO3] was also modeled in other previous work.18 The combined results were used to characterize the structure and the aggregates formation of all FILs studied. Structural Analysis. The total static structure factors, S(q), were calculated using a previously described methodology.43 It was briefly obtained from:

S(q) = ∑∑Sij (q) i

(1)

j

R

Sij (q) =

ρo xi xjbi (q)bj (q)∫ 4πr2 gij (r) −1 0

sin(qr) sin(π R) dr qr πr R

2

  i i (q)   ∑xb i 

(2)

where Sij(q) is the partial static structure factor between atoms of type i and j (e.g. carbon, hydrogen or nitrogen) calculated from the corresponding Fourier transform of the partial radial distribution function gij(r); q is the scattering vector; ρo is the average atom number density; R is the cutoff used in the calculation of gij(r), i.e. half the side of the simulation box (in this case R = 3.5 nm); xi and xj are the atomic fractions of i and j; and bi(q) and bj(q) are the coherent bound neutron scattering lengths of the corresponding atom type, interpolated from recommended values in the International Tables for Crystallography.44 The term sin(πR)/( πr/R) of equation (2) is a Lorch-type window function used to reduce the effect of using a finite cutoff in the radial distribution function calculation.45 Dihedral Analysis. The MD simulations were also used to obtain specific data that were analyzed with the purpose of carefully describe the conformational and intermolecular behavior


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of the [N(C4F9SO2)2]¯ anion. In order to achieve this goal, conformer distributions profiles (CDP, population histograms as a function of the C1-S1-S2-C2 dihedral angle of [N(C4F9SO2)2]¯) were obtained at 343 K for the pure ionic liquid [C4C1pyr][N(C4F9SO2)2]. The geometry for the isolated single [N(C4F9SO2)2]¯ anion was fully optimized at density functional theory level according to Becke’s three-parameter hybrid method with LYP correlation (B3LYP)46 in vacuum using the 6-31G(d) basis set. DFT calculations were carried out using the Gaussian 03 program package.47

RESULTS AND DISCUSSION Thermal Analysis. Phase transitions and decomposition temperatures are critical properties as they provide information about crystalline structure formation,2 the liquid range of the compounds and their range of application.10,17 The melting (Tm), glass transition (Tg) and solidsolid transition (Ts-s) temperatures for the six FILs studied are summarized in Table 2, with their corresponding enthalpies. Their onset (Tonset), start (Tstart) and decomposition (Tdec) temperatures are shown in Table S1 of Supporting Information. The onset temperatures versus melting temperatures are plotted in Figure 1, and it is evident that the onset temperatures (above 580 K) are higher than the melting points (below 380 K), which has enabled the analysis herein discussed. The influence of the hydrogenated alkyl chain length, cation and anion on the thermal behavior was studied at different standpoints. Firstly, the length of the hydrogenated alkyl chain was studied by comparing two different FILs based on the imidazolium cation. Secondly, a comparison of four different cations (ammonium, pyridinium, pyrrolidinium and imidazolium)


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was also performed. All these FILs were based on the perfluorobutanesulfonate anion. Finally, the anion effect was studied for FILs based on the pyrrolidinium cation. In the first study, the [C6C1Im]+ and [C8C1Im]+ cations were studied and the results of melting points reveal a slight increase in the case of [C8C1Im]+. However, the opposite behavior was noticed for the onset temperatures. The main difference between these two cations is the size of the hydrogenated alkyl chain, suggesting a slight increase in the melting point and a decrease in the thermal stability when the alkyl chain is longer. It has been described48 that imidazolium cations combined with traditional fluoro-containing anions (in this case those based on bis(trifluoromethylsulfonyl)imide) usually tend to have a lower melting point when their hydrogenated alkyl chain increases. However, our results do not match with that statement. Verevkin et al. also described the behavior of [C6C1Im]+ and [C8C1Im]+ cations based on the chloride anion, and the results showed our tendency: [C8C1Im][Cl] has a slight increase in the melting point.49 Papaiconomou et al.50 also used imidazolium cations combined with perfluorobutanesulfonate anions. The melting points were higher for imidazolium cations with longer hydrogenated alkyl chains, thus supporting our results. Regarding the thermal stability of imidazolium FILs, a significant influence of the alkyl chain length has been demonstrated. Imidazolium ILs based on anions such as chloride, hexafluorophosphate, tetrafluoroborate and bis(trifluoromethylsulfonyl)imide exhibited a decrease in the thermal stability when the alkyl chain length increases.51 These results are in agreement with the onset temperatures obtained in this work. In the second case, FILs with cations of pyridinium, [C2C1py]+; pyrrolidinium, [C4C1pyr]+; ammonium, [N4444]+ and imidazolium, [C6C1Im]+, conjugated with the perfluorobutanesulfonate anion, [C4F9SO3]¯, were compared. The melting temperatures increase in the following order:


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[C2C1py]+ < [C6C1Im]+ < [N4444]+ < [C4C1pyr]+. Regarding decomposition temperatures, the behavior describes the following trend: [N4444]+ < [C6C1Im]+ < [C2C1py]+ < [C4C1pyr]+. It has been shown

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that pyrrolidinium ILs are more resistant to temperature, followed by

imidazolium, pyridinium and non-cyclic tetraalkyl ammonium, supporting the tendency obtained in this work. The anion effect on the thermal behavior was studied using the pyrrolidinium cation [C4C1pyr]+

combined

with

the

perfluorobutanesulfonate,

[C4F9SO3]¯,

or

the

bis(nonafluorobutylsulfonyl)imide, [N(C4F9SO2)2]¯, anions. Both melting and decomposition temperatures are higher for the [N(C4F9SO2)2]¯ anion which contains more fluorous atoms than the [C4F9SO3]¯ anion. Similarly, the same cation, [C4C1pyr]+, combined with other fluorocontaining anions, shows an increment of the melting temperature in the following order: bis(fluorosulfonyl)imide

Influence of Nanosegregation on the Phase Behavior of Fluorinated

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