Conductivity, Viscosity, Spectroscopic Properties of Organic Sulfonic Acid Solutions in Ionic Liquids

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Article Conductivity, Viscosity, Spectroscopic Properties of Organic Sulfonic Acid solutions in Ionic Liquids

Anh T. Tran, Jay Tomlin, Phuoc H. Lam, Brittany L. Stinger, Alexandra D. Miller, Dustin J. Walczyk, Omar Cruz, Timothy D. Vaden * and Lei Yu *

Department of Chemistry and Biochemistry, Rowan University, Glassboro, NJ 08028, USA; [email protected] (A.T.T.); [email protected] (J.T.); [email protected] (P.H.L.); [email protected] (B.L.S.); [email protected] (A.D.M.); [email protected] (D.J.W.); [email protected] (O.C.) * Correspondence: [email protected] (T.D.V.); [email protected] (L.Y.)

Received: 14 June 2019; Accepted: 25 September 2019; Published: 1 October 2019

Abstract: Sulfonic acids in ionic liquids (ILs) are used as catalysts, electrolytes, and solutions for metal extraction. The sulfonic acid ionization states and the solution acid/base properties are critical for these applications. Methane sulfonic acid (MSA) and camphor sulfonic acid (CSA) are dissolved in several IL solutions with and without bis(trifluoromethanesulfonyl)imine (HTFSI). The solutions demonstrated higher conductivities and lower viscosities. Through calorimetry and temperature-dependent conductivity analysis, we found that adding MSA to the IL solution may change both the ion migration activation energy and the number of “free” charge carriers. However, no significant acid ionization or proton transfer was observed in the IL solutions. Raman and IR spectroscopy with computational simulations suggest that the HTFSI forms dimers in the solutions with an N-H-N “bridged” structure, while MSA does not perturb this hydrogen ion solvation structure in the IL solutions. CSA has a lower solubility in the ILs and reduced the IL solution conductivity. However, in IL solutions containing 0.4 M or higher concentration of HTFSI, CSA addition increased the conductivity at low CSA concentrations and reduced it at high concentrations, which may indicate a synergistic effect.

Keywords: ionic liquid; sulfonic acid; solution properties

1. Introduction Investigations and reports about ionic liquids (ILs) have been burgeoning in the past two decades due to their unique chemical and physical properties as organic liquid substances. These properties include a combination of negligible vapor pressure, non-ﬂammability, good stability, good solvents for many organic and inorganic compounds, good ionic conductivity, and large electrochemical windows [1]. Because of these advantages, ILs have been broadly used in organic reactions as solvents and catalysts and in electrochemical devices as electrolytes. Many of the typical ILs are “neutral” in terms of their acid/base properties. Acidic ionic liquids (AILs) contain active protons from Brønsted acids present as cations, anions, or solutes in IL solutions. Their acidity provides active hydrogen ions (H+) for numerous chemical and electrochemical applications [2]. In this context, the activity, ionization, solvation, and transportation of the acidic protons (H+) in the ILs are critical fundamental knowledge to understand the structure-property relations in the ILs and to optimize their functions in diﬀerent areas of applications. In our previous reports, we investigated the properties of a series of acid solutions in ILs such as 3-butyl-1-methylimidazolium tetrafluoroborate (BMIBF4)[3], 3-butyl-1-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) [4,5], N-butyl-N-methylpyrrolidinium

ChemEngineering 2019, 3, 81; doi:10.3390/chemengineering3040081 www.mdpi.com/journal/chemengineering ChemEngineering 2019, 3, 81 2 of 14 bis(trifluoromethanesulfonyl)imide (PyrrTFSI) [6], and 3-ethyl-1-methylimidazolium acetate (EMIOAc) [7]. It has been concluded that the AIL properties strongly depend on the structures of ILs or + acids. When a very strong acid, such as HBF4 (in its aqueous solution), is dissolved in an IL, the H + ions will protonate water molecules in the solution and the non-volatile H3O ion will remain in the IL solution when water molecules are evaporated. On the other hand, when a weak acid (e.g., acetic acid) is dissolved in an IL, the acid does not dissociate significantly, and remains a neutral molecule in the solution. Bis(trifluoromethanesulfonyl)imine (HTFSI) is a typical strong acid in aqueous solutions, but a moderate one in non-aqueous solutions. In HTFSI/IL solutions, HTFSI molecules and TFSI ions can form dimers with an N-H-N “bridged” structure. In this paper, we investigate the properties of organic sulfonic acid (R-SO3H)/IL solution properties with a combination of various methods, including conductivity, viscosity, calorimetry, spectroscopy, and computational studies. Many organic sulfonic acids, R-SO3H, are relatively strong. For example, the ionization constant (pKa) of methane sulfonic acid (MSA) is 1.9, while toluenesulfonic acid has a pKa of 2.8 [8,9]. − − Sulfonic acids have applications not only as bulk materials or in their aqueous solutions, but also in IL solutions. As sources of active protons, sulfonic acids are less expensive and more stable compared with HTFSI. Moreover, the properties of the organic sulfonic acids can be adjusted by changing the structure of the R-group. Sulfonic acids in ILs have various applications. First, sulfonic acid-based AILs have been used as solvents and catalysts for Fischer esterification reactions and for alcohol, amine, and starch acetylation reactions [10–26]. When AILs are used in these reactions, they are “bi-functional”, as solvents and homogeneous catalysts. In many of the catalytic reactions, the active proton dissociated from the + –SO3H reacts and produces intermediators. For example, in the Fisher esterification reactions, the H catalyst adds to a carboxylic acid to make a reactive electrophilic intermediate, which eventually results in an H+ addition elimination. The sulfonic acid catalyst demonstrates better catalytic effects compared with sulfuric acid or other heterogeneous catalysts. Second, sulfonic acid AILs have also been used as electrolytes of electrochemical devices, such as fuel cells and flow batteries, although the long-term stability of the IL solutions in the battery/fuel cell conditions has not been fully understood [27–33]. The majority of the fuel cells use the proton exchange membrane (PEM), which can separate the cathode and anode compartments and allow the transportation of H+ ions thought the PEM. Nafion, a perfluorosulfonic acid membrane, is widely used in PEM fuel cells [34–38]. In recent years, IL-containing polymer membranes have been used as the PEM of fuel cells in order to improve the high temperature performance of the PEMs and fuel cells. ILs’ low volatilities and high thermal stabilities allow IL-based PEM works at temperatures as high as ~300 ◦C without observable IL evaporation and loss of electrochemical functionality. For this purpose, many cross-linked polymers with sulfonic acid functional groups have been synthesized and swollen with ILs. Extra sulfonic acids may also be added to further enhance the proton conductivity. Third, sulfonic acid-functionalized ILs are good solvents that can solubilize metal oxides and extract metals from raw materials [39,40]. Sulfonic acid AILs are milder compared with typical strong acids (HNO3,H2SO4) for metal extraction. HNO3 and H2SO4 are very corrosive, and their reactions with metal oxides produce noxious and corrosive gas products. The AILs exhibit large contact area with no vapor or gas produced. These applications, in general, are based on the strong acidic property of sulfonic acids and non-volatile liquid properties of the ILs. Therefore, the sulfonic acid dissociation, microenvironment of active protons, and the channels and mechanisms of proton transportation are very essential knowledge to know in order to understand their performance in applications and to design and develop optimistic systems for the applications. In dilute aqueous solutions, many sulfonic acids are very strong; most of the acids are ionized. However, the microenvironment, structure and interactions around the active proton in bulk ILs have not been made very clear to date. Molecular dynamics simulation indicates that the dissociation might be affected by the anions of the AILs, which can interact with the active H+ [41]. Any dissociation of the acids in the AILs should be accompanied by the solvation of the H+ ion by a Brønsted base, an anion in the IL systems. The H+ activity will be determined by the ChemEngineering 2019, 3, 81 3 of 14 relative strength of the acids and bases in the solution, based on the Brønsted-Lowry acid-base theory. In this paper, the anion in the solutions is TFSI anion which is the conjugate base of a strong acid HTFSI. The solution properties were characterized with and without the presence of extra HTFSI in the sulfonic acid/IL solutions. Conductivity and viscosity of the AIL solutions with different concentrations were measured at room temperature and elevated temperatures. The microenvironments/solvation clusters of H+ in the solutions are also characterized by Raman spectroscopy. The interactions at the molecular level are further supported by computational simulation with a “mini-clusters” method. The collective properties of the interactions are reflected by the solution enthalpy (∆Hsol) that is measured by solution calorimetry.

2. Materials and Methods

ILs PyrrTFSI, BMITFSI, and BMIBF4 were purchased from Ionic Liquid Technologies Inc (Heilbronn, Germany). The ILs were treated in a vacuum oven at 60 ◦C overnight prior to use. All the acids, including the MSA, camphor sulfonic acid (CSA), and HTFSI, were purchased from Sigma-Aldrich, Inc (St. Louis, MO, USA). MSA and CSA were treated in vacuum oven at room temperature overnight prior to use; HTFSI was used as received. An AC Mode TraceableTM conductivity meter (Traceable® Products Inc., Webster, TX, USA) at a constant frequency of 3 KHz was used to measure the conductivities of pure ILs and solutions. The meter has a pair of parallel Pt plate electrodes with a cell constant of unity. The cell was calibrated by standard solutions with conductivity of 1 and 10 mS/cm, respectively, prior to measurement. A hot water bath was used to control the temperature of the IL solutions. Temperature values were read by the built-in thermal couple of the conductivity probe. A Brookfield rotational viscometer was used to measure the viscosities of pure ILs and AIL 1 solutions at shear rates of 15–30 s− . The viscometer was equipped with a CPA-40Z cone-shaped spindle and a built-in temperature probe. The temperature of the samples (room temperature 70 C) − ◦ was controlled by circling water from a thermostat to the sample cup. The instrument was calibrated by mineral oil viscosity standards of 20 cP and 200 cP, respectively, prior to use. A standard “coffee-cup” calorimeter was used to measure the solution enthalpy (∆Hsol) when ILs and sulfonic acids were mixed in plastic containers that were isolated from the surroundings with aluminum foil and Styrofoam. The temperature of the system was monitored by a thermocouple connected to a computer. The specific heat capacity of the solutions was measured with a pre-heated copper slug. By measuring the dissolution of CaCl2 in H2O, we estimated that there was a ~28% total heat loss when the maximum temperature was reached within several minutes [42]. The measured ∆Hsol values were compensated for this 28% heat loss. A Horiba Jobin-Yvon LabRam Evolution Raman Microscope (Horiba, Edison, NJ, USA) was used to measure the Raman spectra. The excitation wavelengths were either 532 nm or 633 nm. Fluorescence backgrounds were subtracted using the Labspec 6 software. Density Functional Theory (DFT) calculations of IL solvation structures were performed with the GAMESS code incorporated with the ChemBio3D program (Perkin Elmer, Waltham, MA, USA) that can compute Raman intensities. IL structures used for inputs to DFT calculations were generated using molecular mechanics and molecular dynamics simulations in the ChemBio3D software package. Initial structures, chosen with random starting geometries, were subjected to a ~10 ps simulation with the MM2 force field followed by a geometry minimization using the same force field. These geometries of IL molecular cation-anion complexes with and without acid solutes were computed at the B3LYP/6-31+G* level. Raman spectra (frequencies and intensities) were computed at the same level of theory and simulated by scaling the 1 harmonic frequencies by 0.97 and convolving the frequencies and intensities with 10 cm− FWHM Lorentzian line shapes. ChemEngineering 2019, 3, 81 4 of 14

3. Results

3.1. MSA in PyrrTFSI

MSA is a low-volatility, stable, and relatively strong acid (pKa = 1.9) that has been demonstrated to − be a good catalyst for many organic reactions and a good electrolyte for electrochemical reactions [43–45]. MSA has good solubility in water and many organic solvents and has been found to be soluble or even miscible in several ILs. When MSA is added into PyrrTFSI, the conductivity of the IL solution increases from about 3 mS/cm, pure PyrrTFSI, to about 6 mS/cm, 6 M MSA, Figure1A. Similar results have also been observed when MSA is dissolved in ILs BMITFSI or BMIBF4, data not shown in this paper. In our previous reports, we also found the addition of weak organic acids to ILs increased the overall conductivity of the solutions, as well as the addition of polar organic solvents [7,46,47]. On the other hand, the addition of strong acid such as HTFSI will increase the conductivity at lower concentrations and then reduce the conductivity at higher concentrations [3,4]. The strong acid HBF4 will simply increase the conductivity of the IL solutions, because of the un-removable water carried into the solution [3]. MSA is relatively strong, compared with carboxylic acids, while it is signiﬁcantly weaker than HTFSI and HBF4. These results also indicate that the MSA may not ionize as much as the HTFSI in IL solutions, due to its relatively smaller Ka value. Further evidence of the low ionization of MSA is that when CH3SO3− ion is added, the solution conductivity decreases, Figure2. This result may indicate that CH3SO3− ion is not produced when MSA is dissolved in the ILs. The conductivity of the solutions increased when the solutions were heated to higher temperatures. The conductivity of the solutions was around ~10 mS/cm at a temperature of 78 ◦C. The conductivity changes as a function of the temperature (Arrhenius Plots) of MSA solution in PyrrTFSI with various concentrations was shown in Figure1B. As the Arrhenius plots are not linear, the results were ﬁtted with the Vogel-Fulcher-Tamman (VFT) equation of ionic conductivity (σ):

B T − T σ = σ0e − 0 (1) where σ0, B, and T0 are all constants dependent on the contents of the solution. Based on the free-volume theory [48,49], σ0 may be associated with the infinite conductivity (proportional to the concentration of charge carriers) of a system at an extreme temperature, B has a temperature unit and relates to the activation energy of charge carrier’s motion, and T0 is the virtual glassy transition temperature of the liquid. The values, as shown in Figure1C, were estimated by fitting the VFT equation with the data in Figure1B. It can be seen that the addition of MSA in the PyrrTFSI increased the σ0 value (number of charge carriers) and the B (apparent activation energy) slightly, while the T0 value has no significant change. The increase of “activation energy” of the charge carriers’ transportation may indicate the interactions between the ions and molecules in the solution are stronger compared with those in pure PyrrTFSI. This is also in agreement with the calorimetry results, which found dissolving MSA in PyrrTFSI to be an exothermic process. The solution enthalpy is ~ 0.4 0.1 kJ/mol. This also − ± indicates that the addition of the polar molecule MSA will cause the overall interactions between the charged ions and molecules to be associated more tightly through dipole interactions. The increase of the charge carriers’ concentration may be attributed to the “dilution” effects of the molecular solute MSA that has break the ion clusters in the pure IL and make more mobile charge carriers in the solution. Therefore, the increase of the solution conductivity may be due to the larger number of charge carriers. In our previous report, a weaker acid, acetic acid, was dissolved in EMIOAc [7]. It was found that the increase of conductivity was due to the same effect: the increase in the number of charge carriers. On the other hand, when a non-electrolyte polar molecular solvent propylene carbonate was dissolved in ILs, the increase of solution conductivity was attributed to a different mechanism: the reduction of the activation energy (B factor in the VFT equation) [47]. The viscosity of the solutions showed opposite trends of change from solution conductivity with changing concentration and temperature, as summarized by the Walden’s Rule. Increase of concentration or temperature reduces the viscosity ChemEngineering 2019, 3, 81 5 of 14

ChemEngineeringof the solutions, 2019 as, 3, shown x FOR PEER in Figures REVIEW1 D and3A. The Walden’s constant (conductivity X viscosity) 5 of is 14 ChemEngineeringrelatively stable 2019 over, 3, x FOR the concentrationPEER REVIEW range measured, as shown in Figure3B. 5 of 14 6.0 A 1.1 B 6.0 B 5.5 A 1.1 1.0 5.5 1.0 5.0 0.9 5.0 0.9 4.5 0.8 (mS/cm)

4.5 σ 0.8 0.7

4.0 (mS/cm) σ Log 0.7 0.5 M MSA in PyrrTFSI 4.0 0.6 1 M MSA in PyrrTFSI

3.5 Log 0.52 M MSA in PyrrTFSI Conductivity (mS/cm) 0.6 13 M MSA in PyrrTFSI 3.5 0.5 24 M MSA in PyrrTFSI Conductivity (mS/cm) 3.0 36 M MSA in PyrrTFSI room temperature 0.5 0.4 4 M MSA in PyrrTFSI 3.0 6 M MSA in PyrrTFSI 0123456room temperature 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.4 MSA Concentration (M) 1/T (K) 0123456 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 MSA Concentration (M) 1/T (K) 700 D C 60 700 D 600 C 60 Parameter σ 50 600 0 0.5M MSA in PyrrTFSI 500 Parameter B 1 M MSA in PyrrTFSI Parameter σ 50 0.5M MSA in PyrrTFSI Parameter T0 40 2 M MSA in PyrrTFSI 500 Parameter B0 1 M MSA in PyrrTFSI 400 3 M MSA in PyrrTFSI Parameter T 2 M MSA in PyrrTFSI 0 40 4 M MSA in PyrrTFSI 400 30 36 M MSA in PyrrTFSI 300 4 M MSA in PyrrTFSI 30

Fitted Values 6 M MSA in PyrrTFSI 300 (cP) Viscosity 20 200 Fitted Values

Viscosity (cP) Viscosity 20 200 10 100 10 100 0 0 0123456 20 30 40 50 60 70 80 0 o 0 MSA Concentration (M) Temperature ( C) 0123456 20 30 40 50 60 70 80 o MSA Concentration (M) Temperature ( C) Figure 1. (A) Conductivity vs. MSA concentration in PyrrTFSI; (B) Arrhenius plots of MSA/PyrrTFSI Figure 1. (A) Conductivity vs. MSA concentration in PyrrTFSI; (B)σ Arrhenius plots of MSA/PyrrTFSI Figuresolutions 1. (overA) Conductivity temperature vs. range MSA of concentration 23–80 °C; (C) inFitted PyrrTFSI; values ( Bof) Arrhenius0, B, and Tplots0 in the of MSA/PyrrTFSIVFT equation; solutions over temperature range of 23–80 ◦C; (C) Fitted values of σ , B, and T in the VFT equation; (D) Viscosity vs. temperature of the solutions with various MSA concentrations.σ0 0 solutions(D) Viscosity over vs. temperature temperature range of the of solutions23–80 °C; with (C) Fitted various values MSA of concentrations. 0, B, and T0 in the VFT equation; (D) Viscosity vs. temperature of the solutions with various MSA concentrations. 3.4 3.4 3.3 3.3 3.2 3.2 3.1 3.1 3.0 3.0 2.9

Conductivity (mS/cm) Conductivity 2.9 2.8 Conductivity (mS/cm) Conductivity 2.8 2.7 012345 2.7 BMICH SO Concentration (M) 0123453 3 BMICH SO Concentration (M) 3 3 FigureFigure 2.2. Conductivity ofof BMICHBMICH33SO3 solutionssolutions in in IL IL BMIBF BMIBF 44..

Figure 2. Conductivity of BMICH3SO3 solutions in IL BMIBF4.

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A 300300 A BB 7070 250250

6060 200200

5050 150150

Viscosity (cP) Viscosity 100 Viscosity (cP) Viscosity 4040 100

Walden Product (cP.mS/cm ) (cP.mS/cm Product Walden 50

Walden Product (cP.mS/cm ) (cP.mS/cm Product Walden 50 3030

00 01234560123456 01234560123456 MSAMSA ConcentrationConcentration (M)(M) MSAMSA ConcentrationConcentration (M)(M) FigureFigureFigure 3. 3. ((AA)) Viscosity;Viscosity; andand and ((B (BB)) ) Walden’sWalden’s Walden’s ProductProduct Product ofof of MSAMSA MSA // PyrrTFSI PyrrTFSI solutionssolutions at at room roomroom temperature, temperature, ~23~23 °C.◦°C.C.

TheTheThe Raman Raman spectra spectra of ofof MSA MSA in in PyrrTFSI PyrrTFSI are are shown shown in in FigureFigure 4 4.4.. The TheThe spectrum spectrumspectrum of ofof pure purepure PyrrTFSI PyrrTFSIPyrrTFSI 1 (bottom(bottom(bottom trace) trace) clearly clearly exhibits exhibits a a peakpeak atatat 740740740 cmcmcm−−−11.. ThisThis was was assigned assigned to to a aa TFSI TFSITFSI molecular molecular vibration vibration thatthatthat is isis sensitive sensitivesensitive to toto intermolecular intermolecularintermolecular interactions interactionsinteractions and andand IL ILIL solvation solvationsolvation structures. structures.structures. In InIn all allall spectra, spectra,spectra, this thisthis peak peakpeak is isis unchangedunchangedunchanged (and (and all all other other PyrrTFSI PyrrTFSI peaks peaks are are unchanged), unchanged), which which indicates indicates that that the the PyrrTFSI PyrrTFSI structure structure isisis unaffected unaunaffectedffected by by MSA.MSA. AsAs thethe MSA MSA concentration concentrationconcentration inincreasesincreasescreases toto 5.05.0 M, M, thethe appearance appearanceappearance ofof a aa Raman RamanRaman band bandband 1 aroundaroundaround 765 765 cm cm−−11 cancancan bebe be identifiedidentified identified asas arisingarising fromfrom the the MSA MSA C-S C-S stretch stretch mode mode by byby comparison comparison to toto the the 1 RamanRamanRaman spectrumspectrum ofof of purepure pure MSA,MSA, MSA, shownshown shown asas as thethe the uppeuppe upperrr trace.trace. trace. OtherOther Other thanthan than aa slightslight a slight shiftshift shift fromfrom from 763763 cm 763cm−−11 cm (low(low− 1 1 MSA(lowMSA MSA concentration)concentration) concentration) toto 767767 to cmcm 767−−11 cm(high(high− (high MSAMSA MSA concentration)concentration) concentration) toto 770770 to cmcm 770−−11 cm (pure(pure− (pure MSA),MSA), MSA), thethe MSAMSA the MSA C-SC-S stretchC-Sstretch stretch bandband band isis mostlymostly is mostly unchangedunchanged unchanged fromfrom thethe from purepure the MSAMSA pure spectrum.spectrum. MSA spectrum. MSAMSA deprotonationdeprotonation MSA deprotonation wouldwould resultresult would inin significantlyresultsignificantly in significantly largerlarger shiftsshifts larger inin thethe shifts C-SC-S in stretchstretch the C-S baba stretchnd.nd. TheThe band. RamanRaman The spectraspectra Raman hencehence spectra indicateindicate hence thatthat indicate thethe MSAMSA that structurethestructure MSA structureisis notnot affectedaffected is not byby aff ectedthethe PyrrTFSI,PyrrTFSI, by the PyrrTFSI, andand thethe slight andslight the shiftsshifts slight inin shifts thethe C-SC-S in thestretchstretch C-S frequency stretchfrequency frequency cancan bebe attributedcanattributed be attributed toto differencesdifferences to differences inin solvationsolvation in solvation andand solvensolven and solventtt dielectricdielectric dielectric fields.fields. fields. TheThe MSAMSA The behaves MSAbehaves behaves likelike aa like non-non- a electrolytenon-electrolyteelectrolyte moleculemolecule molecule inin thethe in solutionsolution the solution andand no andno ioio nonizationnization ionization oror protonproton or proton transfertransfer transfer cancan be canbe observed.observed. be observed.

MSAMSA S-C S-C stretch stretch ** A)A) PurePure MSA MSA B)B)

750750 1000 1000 1250 1250 1500 1500 700700 713 713 725 725 738 738 750 750 763 763 775 775 788 788 800 800 -1-1 -1 RamanRaman ShiftShift (cm(cm )) RamanRaman ShiftShift (cm(cm-1))

FigureFigureFigure 4. 4. Raman Raman spectra spectra ofof MSA MSA inin in PyrrTFSIPyrrTFSI PyrrTFSIshown shown shown with with with increasing increasing increasing MSA MSA MSA concentration concentration concentration from from from 0 to0 0 to 5.0to 5.0 5.0 M. M.TheM. The The top top top trace trace trace is the is is the the Raman Rama Rama spectrumnn spectrum spectrum of pureof of pure pure MSA. MSA. MSA. (A) ( Full(AA)) Full Full spectra; spectra; spectra; (B) ( Expanded(BB)) Expanded Expanded region region region between between between 700 1 700and700 andand 800 800 cm800− cmcmto−−11 correspond toto correspondcorrespond to the toto thethe region regionregion discussed discusseddiscussed in the inin the text.the text.text.

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3.2. MSA in 0.4 M and 2.0 M HTFSI/PyrrTFSI HTFSI/PyrrTFSI HTFSI solutions in PyrrTFSI show very interesting conductivity change behaviors. The solution conductivity increases at lower concentrations andand decrease at higher concentrations; a maximum conductivity value can be observed at ~1 M HTFSI [[6].6]. The HTFSI partially ionizes in the solution and a N-H-N “bridged” structure may be formed in the solution, in which a proton is solvated by two TFSI anions. In In this this work, two HTFSI/PyrrTFSI HTFSI/PyrrTFSI solutions with different different concentrations, 0.4 M (lower than 1.0 M) and 2.0 M (higher than 1.0 M), were used to dissolve MSA to form a series of MSAMSA/HTFSI/PyrrTFSI/HTFSI/PyrrTFSI solutions.solutions. Their Their conductivity conductivity and and viscosity viscosity values values are are shown shown in Figure in Figure5. In both5. In solutions,both solutions, the conductivity the conductivity increased increased with thewith increase the increase of MSA of concentration,MSA concentration, which which is similar is similar to the resultsto the results in PyrrTFSI in PyrrTFSI solutions solutions without without HTFSI. HTFSI. On theOn otherthe other hand, hand, the the viscosity viscosity increased increased first first to to a maximuma maximum value value at 0.5at 0.5 M MSAM MSA and and then then decreased decreased after after the peaks.the peaks. Meanwhile, Meanwhile, the dissolution the dissolution of MSA of inMSA the HTFSIin the/ PyrrTFSIHTFSI/PyrrTFSI is also exothermic, is also exothermic,∆H = 0.20 ΔH0.15 = − kJ0.20/mol ± when 0.15 MSAkJ/mol and when HTFSI MSA/PyrrTFSI and − ± areHTFSI/PyrrTFSI mixed at equal are moles, mixed whichat equal may moles, also which indicate may slightly also indicate stronger slightly intermolecular stronger intermolecular interactions in theinteractions solutions in when the MSAsolutions is added. when The MSA conductivity is added. andThe viscosity conductivity data overand aviscosity temperature data range over ofa temperature range of 23–78 °C are plotted (Figure 6A,C), and the results are fitted with the VFT 23–78 ◦C are plotted (Figure6A,C), and the results are fitted with the VFT equations for conductivity andequations viscosity, for conductivity respectively. and The viscosity, values of respective the parametersly. The from values the of fitting the para aremeters shown from in Figure the fitting6B,D. Theare shown VFT equation in Figure for 6B,D. viscosity The VFT is presented equation as: for viscosity is presented as: 𝜂 = 𝜂 𝑒B (2) T T η = η0e − 0 (2) where 𝜂0, B, and T0 are constants of equation that do not depend on the concentration and wheretemperature.η0, B, and T0 are constants of equation that do not depend on the concentration and temperature. The results for both conductivity and viscosity showed relatively stable σ𝜎0,, 𝜂η0, and T0 values, while the BB valuesvalues changed changed more more significantly significantly with with an anincrease increase in the in thelower lower concentration concentration range range and anda decrease a decrease at higher at higher concentrations. concentrations. This Thisincrease increase and anddecrease decrease in the in B the valuesB values (proportional (proportional to the to theactivation activation energy energy of the of themotion motion of ofparticles particles in inmechanical mechanical share share force force field field for for viscosity viscosity and and in electrical field field for conductivity, respectively) may explain the increase and decrease of the viscosity of the solutions solutions when when the the concentration concentration increases. increases. On On the the other other hand, hand, in the in thecase case of conductivity, of conductivity, the theincrease increase of the of thenumber number of ofcharge charge carriers carriers (𝜎 (σ)0 )maymay be be more more dominant dominant in in the the determination of conductivity. TemperatureTemperature course course results results in in the the 2.0 2.0 M HTFSIM HTFSI/PyrrTFSI / PyrrTFSI solutions solutions showed showed similar similar results and,results therefore, and, therefore, are not are shown not inshown this paper.in this paper.

6.5 90 4.5 80 A B 6.0 75 80 4.0 5.5 70 3.5 70 65 5.0 3.0 60 4.5 60

55 4.0 2.5 50

Viscosity (cP) 50 3.5 2.0 (cP) Viscosity

Conductivity (mS/cm)Conductivity 40 45

3.0 Conductivity (mS/cm) 1.5 40 2.5 30 1.0 35 0123456 01234 MSA Concentration (M) MSA Concentration (M)

Figure 5.5. ConductivityConductivity ( (□)) and and viscosity viscosity (∆ )(Δ of) MSAof MSA solutions solutions in HTFSI in HTFSI (0.4 M) (0.4/PyrrTFSI M)/PyrrTFSI (A), and (A HTFSI), and (2.0HTFSI M) /(2.0PyrrTFSI M)/PyrrTFSI (B), at room(B), at temperature. room temperature.

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B 2.6 A 1200 Parameter σ 2.4 0 1000 Parameter B Parameter T 2.2 0

2.0 800

1.8 600 (mS/cm)

σ 1.6 400

Log 1.4 Parameter Value Parameter 0M of MSA in 0.4M HTFSI/PyrrTFSI 1.2 0.5M of MSA in 0.4M HTFSI/PyrrTFSI 200 1.0M of MSA in 0.4M HTFSI/PyrrTFSI 1.0 2.0M of MSA in 0.4M HTFSI/PyrrTFSI 4.0M of MSA in 0.4M HTFSI/PyrrTFSI 0 6.0M of MSA in 0.4M HTFSI/PyrrTFSI 0.8 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0123456 Temperature (1/K) MSA Concentration (M)

90 1200 C D 80 Parameter η 0 1000 Parameter B 70 0M of MSA in 0.4 HTFSI/PyrrTFSI Parameter T 0 0.1M of MSA in 0.4 HTFSI/PyrrTFSI 60 0.5M of MSA in 0.4 HTFSI/PyrrTFSI 800 1.0M of MSA in 0.4 HTFSI/PyrrTFSI 50 2.0M of MSA in 0.4 HTFSI/PyrrTFSI 4.0M of MSA in 0.4 HTFSI/PyrrTFSI 600 6.0M of MSA in 0.4 HTFSI/pyrrTFSI 40 400

Viscosity (cP) Viscosity 30 Parameter Value

20 200

10 0 0 20 30 40 50 60 70 80 0123456 o Temperature ( C) MSA Concentration (M) Figure 6. Conductivity (A) and viscosity (C) of MSA in HTFSI (0.4 M)/PyrrTFSI solutions at different Figure 6. Conductivity (A) and viscosity (C) of MSA in HTFSI (0.4 M)/PyrrTFSI solutions at different temperatures and the value of parameters in the VFT equations fitted with the experimental data: temperatures and the value of parameters in the VFT equations fitted with the experimental data: (B) (B) fitted from conductivity data, and (D) fitted from viscosity data. fitted from conductivity data, and (D) fitted from viscosity data. The Raman spectra of MSA in the HTFSI/PyrrTFSI solutions are shown in Figure7. The relevant The Raman spectra of MSA in the HTFSI/PyrrTFSI1 solutions are shown in Figure 7. The relevant spectral region is between 700 and 800 cm− , so only this region is shown. A doublet Raman spectral region is between 700 and 800 cm−1, so only this region is shown. A doublet Raman signal signal observed for the TFSI molecular vibrational mode was used to identify the HTFSI-PyrrTFSI observed for the TFSI molecular vibrational mode was used to identify the HTFSI-PyrrTFSI bridged- bridged-proton structure [6]. In Figure7, the TFSI molecular vibration exhibits a single Raman proton structure 1[6]. In Figure 7, the TFSI molecular vibration exhibits a single Raman band at 740 band at 740 cm− , indicating that the bridged-proton structure is not formed with 0.4 M HTFSI. cm−1, indicating that the bridged-proton structure is not formed with 0.4 M HTFSI. The MSA C-S1 The MSA C-S stretch mode appears with increasing MSA concentration as a Raman band at ~765 cm− . stretch mode appears with increasing MSA concentration as a Raman band at ~765 cm−1. This position This position is nearly identical to the band position in the MSA/PyrrTFSI solutions (Figure7A) and is nearly identical to the band position in the MSA/PyrrTFSI solutions (Figure 7A) and suggests that suggests that the MSA does not interact with the HTFSI or PyrrTFSI molecules, and does not form the MSA does not interact with the HTFSI or PyrrTFSI molecules, and does not form a bridged-proton a bridged-proton structure. With 2.0 M HTFSI in PyrrTFSI (Figure7B), the doublet TFSI molecular structure. With 2.0 M HTFSI in PyrrTFSI (Figure 7B), the doublet TFSI molecular vibration band is vibration band is observed, confirming the presence of the HTFSI-PyrrTFSI N-H-N “bridged” structure. observed, confirming the presence of the HTFSI-PyrrTFSI N-H-N “bridged” structure. With With increasing MSA concentration, this doublet is unchanged and the MSA C-S stretch band appears increasing MSA concentration, this doublet is unchanged and the MSA C-S stretch band appears mostly unchanged relative to the pure MSA spectrum (top trace). This suggests that the MSA does not mostly unchanged relative to the pure MSA spectrum (top trace). This suggests that the MSA does alter the structure or stability of the HTFSI-PyrrTFSI interactions. not alter the structure or stability of the HTFSI-PyrrTFSI interactions. The DFT-computed structures of MSA in the HTFSI/PyrrTFSI solutions are shown in Figure8. The DFT-computed structures of MSA in the HTFSI/PyrrTFSI solutions are shown in Figure 8. Figure8A shows the HTFSI-TFSI N-H-N “bridged” structure surrounded by pyrrolidinium cations and Figure 8A shows the HTFSI-TFSI N-H-N “bridged” structure surrounded by pyrrolidinium cations a non-interacting MSA molecule while 8B shows MSA interacting with and disrupting the HTFSI-TFSI and a non-interacting MSA molecule while 8B shows MSA interacting with and disrupting the interaction. These calculated clusters are most relevant to the high-concentration samples which HTFSI-TFSI interaction. These calculated clusters are most relevant to the high-concentration samples have high-concentration MSA and HTFSI relative to the PyrrTFSI. The simulated Raman and IR which have high-concentration MSA and HTFSI relative to the PyrrTFSI. The simulated Raman and spectra are compared to the experimental spectra in Figure9. The relative ∆G values from the DFT IR spectra are compared to the experimental spectra in Figure 9. The relative ΔG values from the DFT calculations are provided in kJ/mol. The N-H-N “bridged” structure is that shown in Figure 9A and

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ChemEngineering 2019, 3, x FOR PEER REVIEW 9 of 14 calculations are provided in kJ/mol. The N-H-N “bridged” structure is that shown in Figure9A and the N-H-Othe N-H-O “bridged” “bridged” structure structure is that is shownthat shown in Figure in Figure9B. The 9B. N-H-N The N-H-N structure structure is more is favorablemore favorable at about at 20.6about kJ /20.6mol, kJ/mol, which iswhich a large is enougha large numberenough tonumber suggest tothat suggest the N-H-O that the “bridged” N-H-O “bridged” structure would structure not bewould observed not be experimentally. observed experimentally. This is consistent This is with consistent the spectroscopic with the spectroscopic experiment-theory experiment-theory comparison. −1 comparison. Previously, we have shown that the vibrational frequencies1 around 750 cm−1 are Previously,comparison. we Previously, have shown we that have the vibrationalshown that frequencies the vibrational around 750frequencies cm− are underestimatedaround 750 cm in ourare DFTunderestimated calculations in [6 ],our and DFT in thatcalculations context, the[6], N-H-Nand in simulatedthat context, spectra the N-H-N are closer simulated to the experiments. spectra are Figurecloser 9to, therefore,the experiments. provides Figure strong 9, evidence therefore, that provides the N-H-N strong “bridged” evidence structure that the isN-H-N most consistent“bridged” withstructure the 5.0 is most M MSA consistent and 2.0 Mwith HTFSI the 5.0 in PyrrTFSI.M MSA an Togetherd 2.0 M HTFSI with Figures in PyrrTFSI.8 and9, Together we can conclude with Figures that the8 and MSA 9, we does can not conclude interact that with the the MSA HTFSI-TFSI does not structure interact with and is the present HTFSI-TFSI in solution structure as a non-interacting and is present molecularin solutionsolute. as a non-interacting molecular solute.

A) B) Pure MSA Pure MSA

705 720 735 750 765 780 795 700 725 750 775 800 -1 -1 Raman Shift (cm ) Raman Shift (cm-1)

Figure 7. Raman spectra of MSA/HTFSI/PyrrTFSI solutions shown as a function of increasing MSA Figure 7. Raman spectra of MSA/HTFSI/PyrrTFSI solutions shown as a function of increasing MSA concentration in constant HTFSI concentrations. (A) In 0.4 M HTFSI/PyrrFTSI solutions; (B) in 2.0 M concentration in constant HTFSI concentrations. (A) In 0.4 M HTFSI/PyrrFTSI solutions; (B) in 2.0 M HTFSI/PyrrFTSI solutions. HTFSI/PyrrFTSI solutions.

Figure 8. B3LYP/6-31+G* structures of MSA/HTFSI/PyrrTFSI AILs. For computational simplicity, Figure 8. B3LYP/6-31+G* structures of MSA/HTFSI/PyrrTFSI AILs. For computational simplicity, the theFigure cation 8. B3LYP/6-31+G* is 1,3-dimethylimidazolium. structures of MSA/HTFSI/PyrrT (A) N-H-N “bridged”FSI AILs. structure, For computational in which simplicity, the HTFSI the is cation is 1,3-dimethylimidazolium. (A) N-H-N “bridged” structure, in which the HTFSI is hydrogen- hydrogen-bondedcation is 1,3-dimethylimidazolium. to a TFSI anion while(A) N-H-N the MSA “bridged” is not involvedstructure, in in hydrogen which the bonding; HTFSI is (hydrogen-B) N-H-O bonded to a TFSI anion while the MSA is not involved in hydrogen bonding; (B) N-H-O “bridged” “bridged”bonded to structure,a TFSI anion in which while the the MSA MSA is is hydrogen not involved bonded in hydrogen to a TFSI anion.bonding; (B) N-H-O “bridged” structure, in which the MSA is hydrogen bonded to a TFSI anion.

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Figure 9. The experimental Raman spectrum (A) and IR spectrum (B) are compared to simulated Raman spectra (A) and simulated IR spectra (B) for the N-H-N “bridged” structure from Figure 8A Figure 9.9. The experimental Raman spectrum ( A)) and IR spectrum (B) are comparedcompared toto simulatedsimulated and the N-H-O “bridged” structure from Figure 8B. In both panels A and B, the top trace is the Raman spectraspectra (A(A)) and and simulated simulated IR IR spectra spectra (B ()B for) for the the N-H-N N-H-N “bridged” “bridged” structure structure from from Figure Figure8A and 8A experimental result and the bottom two traces are simulations. theand N-H-O the N-H-O “bridged” “bridged” structure structure from Figure from8 B.Figure In both 8B. panels In both Aand panels B, the A topand trace B, the is the top experimental trace is the result and the bottom two traces are simulations. 3.3. CSAexperimental in ILs and result HTFSI/IL and the bottom two traces are simulations. 3.3. CSA in ILs and HTFSI/IL 3.3. CSACAS in is ILs another and HTFSI/IL organic sulfonic acid with pKa of 1.2 and limited solubility in ILs. It is weaker than CASMSA, is but another still stronger organic than sulfonic most acid of the with carboxylic pKa of 1.2 acids. and When limited CSA solubility is dissolved in ILs. in ItBMIBF is weaker4, the CAS is another organic sulfonic acid with pKa of 1.2 and limited solubility in ILs. It is weaker thanconductivity MSA, but of stillthe solution stronger decreases, than most as of shown the carboxylic in Figure acids.10A; the When “dilution” CSA is effect dissolved that will in BMIBFincrease4, than MSA, but still stronger than most of the carboxylic acids. When CSA is dissolved in BMIBF4, the the conductivityconductivity of is thenot solution observed. decreases, In the same as shown IL, the in addition Figure 10 A;of HTFSI the “dilution” up to 0.8 eff Mect demonstrated that will increase no conductivity of the solution decreases, as shown in Figure 10A; the “dilution” effect that will increase thesignificant conductivity effects is on not the observed. solution In conductivity, the same IL, Fi thegure addition 10B, probably of HTFSI becaus up to 0.8e of M the demonstrated balance of two no the conductivity is not observed. In the same IL, the addition of HTFSI up to 0.8 M demonstrated no significanteffects of HTFSI, effects which on the are solution increasing conductivity, the ion migration Figure 10 activationB, probably energy because and ofthe the number balance of ofcharge two significant effects on the solution conductivity, Figure 10B, probably because of the balance of two ecarriersffects of that HTFSI, will cause which the are conductivity increasing the change ion migration to the opposite activation directions. energy When and the both number CSA and of chargeHTFSI effects of HTFSI, which are increasing the ion migration activation energy and the number of charge carrierspresent thatin the will BMIBF cause the4 solutions, conductivity a synergistic change to theeffect opposite that increases directions. the When solution both CSAconductivity and HTFSI is carriers that will cause the conductivity change to the opposite directions. When both CSA and HTFSI presentobserved, in theFigure BMIBF 11.4 Atsolutions, low HTFSI a synergistic concentrations effect that (<0.4 increases M), the the conductivity solution conductivity of the solution is observed, also present in the BMIBF4 solutions, a synergistic effect that increases the solution conductivity is Figuredecreased 11. Atwhen low CSA HTFSI was concentrations added to the solution. (<0.4 M), Ho thewever, conductivity the conductivity of the solution increased also decreased when CSA when was observed, Figure 11. At low HTFSI concentrations (<0.4 M), the conductivity of the solution also CSAadded was to addedBMIBF to4 containing the solution. 0.4 However, M or 0.8 the M conductivityHTFSI. The maximum increased when conductivity CSA was was added obtained to BMIBF at a decreased when CSA was added to the solution. However, the conductivity increased when CSA was4 containingCSA concentration 0.4 M or 0.8of 0.05 M HTFSI. M in 0.4 The M maximum HTFSI/BMIBF4 conductivity and at was CSA obtained concentration at a CSA of concentration 0.1 M in 0.8 M of added to BMIBF4 containing 0.4 M or 0.8 M HTFSI. The maximum conductivity was obtained at a 0.05HTFSI/BMIBF M in 0.4 M4. HTFSI/BMIBF4 and at CSA concentration of 0.1 M in 0.8 M HTFSI/BMIBF . CSA concentration of 0.05 M in 0.4 M HTFSI/BMIBF4 and at CSA concentration of 0.1 4M in 0.8 M HTFSI/BMIBF4. 5.0 A B 3.6 5.0 B A 4.5 3.53.6 4.5 3.43.5 4.0

3.4 4.0 3.3 3.5

3.23.3 3.5 Conductivity (mS/cm) Conductivity (mS/cm) 3.0 3.2 3.1 Conductivity (mS/cm) Conductivity (mS/cm) 3.0 2.5 3.1 0.00 0.05 0.10 0.15 0.0 0.2 0.4 0.6 0.8 CSA Concentration (M) 2.5 HTFSI Concentration (M) 0.00 0.05 0.10 0.15 0.0 0.2 0.4 0.6 0.8 FigureFigure 10.10. ConductivityCSA Concentration ofof CSACSA (M) solutionssolutions inin BMIBFBMIBF44 (A) and HTFSIHTFSI solutions Concentration inin BMIBFBMIBF 4(M) (B).

Figure 10. Conductivity of CSA solutions in BMIBF4 (A) and HTFSI solutions in BMIBF4 (B).

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5.5 [HTFI]= 0 0.1 M 5.0 0.2 M 0.4 M 0.8 M 4.5

4.0

3.5 Conductivity (mS/cm)

3.0 0.00 0.05 0.10 0.15 CSA Concentration (M)

Figure 11. Conductivity of CSA solutions in HTFSI/BMIBF4. Figure 11. Conductivity of CSA solutions in HTFSI/BMIBF4. 4. Conclusions 4. Conclusions MSA and CSA are both relatively strong organic acids. MSA is very soluble in ILs and CSA has a solubilityMSA and of about CSA 0.15are both M. When relatively MSA strong is dissolved organic in acids. ILs, itMSA increases is very the soluble solution in ILs conductivity and CSA has and adecreased solubility theof about solution 0.15 viscosity, M. When while MSA the is CSAdissolved has the in oppositeILs, it increases effects. the It has solution been found conductivity that the MSAand decreasedcan dilute the the solution IL and release viscosity, more while “free” the ions CSA or has ion the clusters, opposite while effects. it can It also has increasebeen found the “activitythat the MSAenergy” can ofdilute the migration, the IL and which release means more stronger “free” ions intermolecular or ion clusters, interactions. while it This can isalso also increase reflected the by “activitythe exothermic energy” eff ofect the of dissolution.migration, which After the means addition stronger of the intermolecular MSA, the Raman interactions. spectra of theThis solutions is also reflectedindicate by that the there exothermic is not observable effect of dissolution. ionization orAfter proton the addition transfer inof thethe ILMSA, solutions the Raman with orspectra without of thethe solutions existence indicate of strong that acid there HTFSI is not in observable the solution. ioni Atzation high or concentrations, proton transfer HTFSIin the IL can solutions form dimers with orthrough without N-H-N the existence “bridge” of hydrogenstrong acid bonds HTFSI between in the solution. the TFSI At and high the concentrations, proton. The MSA HTFSI added can into form the dimerssolution through is non-interactive N-H-N “bridge” and cannot hydrogen affect bonds the N-H-Nbetween “bridged” the TFSI and TFSI the dimers. proton. On The the MSA other added hand, intothe CSAthe solution decreased is non-interactive the solution conductivity and cannot that affe mayct the indicate N-H-N a “bridged” stronger solvation TFSI dimers. by the On ions. the Inother one hand,of our the previous CSA decreased reports, we the found solution that conductivity the active hydrogen that may of indicate MSA can a reactstronger with solvation acetate ion by inthe ILs ions. and Inproduce one of our acetic previous acid, which reports, is a we proton found transfer that the reaction active between hydrogen a Brønstedof MSA can acid react and awith base. acetate However, ion inin ILs TFSI and− or produce BF4− ILs, acetic the anions acid, which are too is weak a proton as proton transfer acceptors, reaction and between therefore, a theBrønsted ionization acid ofand MSA a - − base.is not However, observed in in TFSI these or systems. BF4 ILs, Overall, the anions in the are TFSI too −weakor BF as4− protonILs, the acceptors, sulfonic acidsand therefore, do not ionize the ionizationsignificantly of MSA nor a isffect not the observed solvation in these structures systems. of the Overall, strong in acid the HTFSI. TFSI- or BF4− ILs, the sulfonic acids do not ionize significantly nor affect the solvation structures of the strong acid HTFSI. Author Contributions: Experimental measurements, A.T.T., J.T., P.H.L., A.D.M., D.J.W., and O.C.; computational Authorsimulation, Contributions: B.L.S.; writing, Experimental L.Y., T.D.V.; supervision,measurements, L.Y., A.T.T., T.D.V.; J.T., project P.H.L., administration, A.D.M., L.Y.,D.J.W., T.D.V.; and funding O.C.; computationalacquisition, L.Y., simulation, T.D.V. B.L.S.; writing, L.Y., T.D.V.; supervision, L.Y., T.D.V.; project administration, L.Y., T.D.V.;Funding: fundingThis researchacquisition, was L.Y., funded T.D.V. by NSF of the US, grant number CHE-1362493 and O.C. was funded by the American Chemical Society Project SEED program administrated by the South Jersey Local Section. Funding: This research was funded by NSF of the US, grant number CHE-1362493 and O.C. was funded by the AmericanConflicts Chemical of Interest: SocietyThe authors Project declareSEED program no conflict administrated of interest. by the South Jersey Local Section.

Conflicts of Interest: The authors declare no conflict of interest. References

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