Towards the Elucidation of the Competing Role of Evaporation and

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Towards the Elucidation of the Competing Role of Evaporation and Thermal Decomposition in Ionic Liquids: A Multitechnique Study of the Vaporization Behaviour of BMImPF (1-Butyl-3Methylimidazolium Hexafluorophosphate) Under Effusion Conditions 6

Valeria Volpe, Bruno Brunetti, Guido Gigli, Andrea Lapi, Stefano Vecchio Ciprioti, and Andrea Ciccioli J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b08523 • Publication Date (Web): 18 Oct 2017 Downloaded from http://pubs.acs.org on October 25, 2017

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The Journal of Physical Chemistry

Towards the Elucidation of the Competing Role of Evaporation and Thermal Decomposition in Ionic Liquids: a Multitechnique Study of the Vaporization Behaviour of BMImPF6 (1-butyl-3-methylimidazolium hexafluorophosphate) under Effusion Conditions V. Volpe†, B. Brunetti#, G. Gigli†, A. Lapi†,$, S. Vecchio Ciprioti§, A. Ciccioli†,* † #

Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5, I-00185, Rome, Italy

Istituto per lo Studio dei Materiali Nanostrutturati, CNR, c/o Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5, I-00185, Rome, Italy

$

Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro 5, I-00185, Rome, Italy §

Dipartimento S.B.A.I., Sapienza Università di Roma, via del Castro Laurenziano 7, I-00161, Rome, Italy

* Corresponding author. E-mail address [email protected]

Abstract The evaporation/decomposition behaviour of the imidazolium ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate (BMImPF6) was investigated in the overall temperature range 425 - 551 K by means of the molecular-effusion-based techniques Knudsen Effusion Mass Loss (KEML) and Knudsen Effusion Mass Spectrometry (KEMS), using effusion orifices of different size (from 0.2 to 3 mm in diameter). Specific effusion fluxes measured by KEML were found to depend markedly on the orifice size, suggesting the occurrence of a kinetically delayed evaporation/decomposition process. KEMS experiments revealed that other species are present in the vapor phase besides the intact ion pair BMImPF6(g) produced by the simple evaporation BMImPF6(l) = BMImPF6(g), with relative abundances depending on the orifice size – larger the orifice, larger the contribution of the BMImPF6(g) species. By combining KEML and KEMS results, the conclusion is drawn that in the investigated temperature range, when small effusion orifices are used a significant part of the mass loss/volatility of BMImPF6 is due to molecular products formed by decomposition/dissociation processes rather than to evaporated intact ion pairs. Additional experiments performed by non-isothermal Thermogravimetry-Differential Thermal Analysis (TG-DTA) further support the evidence of simultaneous evaporation/decomposition, although the conventional decomposition temperature derived from TG curves is much higher than temperatures covered in effusion experiments. Partial pressures of the BMImPF6(g) species o were derived from KEMS spectra and analyzed by the second- and third-law methods giving a value of ∆evapH298 = K

145.3 ± 2.9 kJ mol–1 for the standard evaporation enthalpy of BMImPF6. A comparison is done with the behavior of the 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2) ionic liquid.

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Introduction The exceptionally low volatility compared to common molecular organic liquids is one of the most distinctive property of ionic liquids (ILs), making these substances attractive as potential substitutes of volatile solvents and in other applications.1-4 As a consequence, in recent years the determination of vapor pressure of ILs has been the subject of considerable efforts.5-11 Besides the practical interest in determining the volatility of these materials as a function of temperature, vaporization studies are also aimed at measuring the evaporation enthalpy for the more fundamental purpose of evaluating the cohesion energy in the liquid phase, so as to adjust/validate force field parameters for molecular dynamics calculations.12-18 Nevertheless, absolute vapor pressure data for ILs are still scarce and uncertain, mostly limited to few classes of stable compounds, such as those with the bis(trifluoromethyl)sulfonylimide (NTf2) anion.7-9 One of the main problems with vapor pressure measurements of ILs is that, though thermally stable compared to molecular solvents, they can start to decompose at the same temperatures where their vapor pressure become measurable.17,19-24 This has prevented vapor pressure of several ILs from being measured25,26 and probably limits the accuracy of a number of data presently available in the literature. If volatile products are formed during thermal decomposition, techniques such as thermogravimetry and transpiration, which are blind with respect to the composition of the vapor phase, might provide unreliable results even if decomposition occurs to a small extent. Moreover, the occurrence of decomposition processes can affect the accuracy of other high temperature measurements, such as those of heat capacity and other thermophysical properties of the liquid. 1-butyl-3-methylimidazolium hexafluorophosphate (in the following BMImPF6) is one of the most common aprotic ILs with an imidazolium cation. In two of the first papers on the vaporization behavior of ILs, Kabo and co-workers25,27 reported Knudsen Effusion thermogravimetric experiments that they argued to provide results unsuitable for deriving vapor pressure and evaporation enthalpy, due to the occurrence of thermal decomposition phenomena. No further


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vaporization study on this IL was carried out until last year, when Zaitsau et al.11 succeeded in measuring the vapor pressure of BMImPF6 with a Quartz Crystal Microbalance apparatus (QCM) at very low temperatures (403 – 450 K), where authors assume decomposition to be negligible. The use of mass spectrometry-based vaporization techniques can be of great help in such cases, because information on the composition of the vapor can be obtained.9,10,19,21,28-31 In this paper, the vaporization of BMImPF6 was reconsidered and new measurements were performed, by using both the Knudsen Effusion Mass Loss and the Knudsen Effusion Mass Spectrometry (KEML and KEMS, respectively) techniques, with the goal to investigate the competition between simple evaporation and thermal decomposition. Measurements were done with effusion cells having different orifice size to study the dependence of specific mass loss rate and vapor phase composition from the extent of the effusing flow and from the closeness to equilibrium conditions. The MS analysis of the vapor allowed us to study the evaporation process as a function of temperature regardless the possible simultaneous decomposition. Additional measurements were carried out by non-isothermal TG-DTA, using a protocol recently proposed by Heym,32 which is able to reveal rapidly the occurrence of thermal decomposition simultaneous to evaporation. Finally, by exploiting the thermal functions available in the literature for BMImPF6 both in the gaseous25 and in the liquid33-40 phase, an experimental value for the evaporation enthalpy of BMImPF6 is proposed, based on the third-law analysis of partial pressure data derived by KEMS. A comparison with the prototypical IL 1-butyl-3-methylimidazolium bis(trifluoromethyl)sulfonylimide (BMImNTf2), whose vaporization behaviour was recently studied by our group9 with the same techniques used here, is presented.



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Experimental

The BMImPF6 sample was purchased from Iolitec. Sample purity (99%) was checked by HNMR.

Knudsen Effusion Mass Loss (KEML) In the classic Knudsen Effusion Mass Loss (KEML) technique the mass of the sample is monitored and the mass loss rate ∆m/∆t measured at each given constant temperature. If the vapor is composed only of one species, the pressure in the effusion cell (peffusion) is evaluated from the wellknown relation41

peffusion =

∆m T 1 ∆m 2πRT 2πRT =J =K Wo Ao ∆t M M ∆t M

(1)

where T is the temperature, W o the Clausing factor (transmission probability), A o the area of the effusion hole, M the molar mass of the vapor species, and J the specific flux. K is a constant depending on the geometrical characteristics of the effusion hole and can be measured by calibration with known substances. If more than one species is present in the vapor, eq 1 gives the total pressure provided that the appropriate average value is used for M41 (see below, eq 9). Due to the presence of the hole, peffusion obtained according to eq 1 generally does not coincide with the true equilibrium pressure p. According to the analysis of Motzfeldt,42 the two quantities can be related by the following expression:

 W A  p = p effusion 1 + o o  α A 

(2)


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where A is the vaporizing surface and α is the so-called vaporization coefficient. If the vaporization process is not kinetically hindered, α is unity and a small value of the A o /A ratio makes the peffusion/p ratio unity for most purposes, regardless the orifice size. On the contrary, if a significant kinetic delay does exist, α can be much less than 1 and the peffusion/p ratio can be sizably less than unity. The apparatus used for KEML measurements is a home-modified Ugine-Eyraud B60 Setaram thermobalance described in detail in ref. 9. Briefly, an effusion cell made from alumina is inserted into a capped copper cylinder. Cell lids were made from pyrophyllite, with effusion orifices of 0.2, 1, and 3 mm in diameter. IL samples were loaded directly into the alumina cell or into a smaller Pt crucible placed inside it. A vertical hole is made on the upper border in the lateral thickness of the copper container, where a Pt100 platinum resistance thermometer is inserted from above for temperature measurement. The effusion source is surrounded by a quartz tube and externally by a tubular resistance of graphite as heating element. To test the presence of volatile impurities, all the samples were kept under vacuum at approximately 420 K for 24 h before measurements, with no significant mass loss observed. During the experimental runs, temperature was changed by a steplike program and the mass loss rates were evaluated at each constant temperature. For each cell the measurements were carried out by varying the temperature two or three times from the highest to the lowest value. Cells were calibrated by vaporizing very pure benzoic acid (Sigma-Aldrich, for calorimetrical determination), whose vapor pressure is well-known.43,44 The following values of the constant K in eq 1 have been determined for the three cells, respectively: K3 mm = (3.92 ± 0.16) 106 m-1·s-1 kg1/2 ·(mol·K)-1/2, K1 mm = (1.88 ± 0.06) 107 m-1·s-1 kg1/2 ·(mol·K)-1/2and K0.2 mm = (6.84 ± 0.16) 108 m-1·s-1 kg1/2 ·(mol·K)-1/2, where the error on the instrument constant is taken as the standard deviation (1σ) of values obtained in various calibration runs at various temperatures.



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Knudsen Effusion Mass Spectrometry (KEMS) In the Knudsen Effusion Mass Spectrometry (KEMS) technique,45 the effusing vapors are analyzed with a mass spectrometer. The apparatus used here was described in previous papers.9,46 Briefly, it is based on a single focusing 90° magnetic sector mass spectrometer. The effusing vapor species are ionised by impact with an electron beam whose energy can be changed continuously from appearance of each ion up to 80 eV. Ion current for a given m/e value (single ion mode) is measured by an electron multiplier. A movable shutter is interposed between the effusion orifice and entrance to the ionization area to subtract background contributions from the total ion current of + a given ion. The background-subtracted ion intensity of the isotope n of the species X, I n X , can be

converted into the partial pressure of X inside the cell by the equation45

PX =

Kinstr I n+X T

σ X γ n an X

(3)

X

where σX is the ionization cross section, γ n X the electron multiplier gain, a n X the isotopic abundance, and Kinstr a constant which depends on geometry of the apparatus, cell features and operating conditions. The multiplier gain was assumed to be proportional to the square root of the molecular mass, a dependence well documented in KEMS studies.45 The instrumental constant was measured ex situ by repeated vaporization experiments of pure zinc (source and purity) (σZn = 5.0 10-20 m2).45 The Knudsen source of the KEMS apparatus used in this study is heated by irradiation from a spiral-shaped tungsten resistor surrounded by several tantalum shields to keep the temperature uniform. Alumina (orifice diameter 0.5 and 1 mm) and platinum (orifice diameter 1 mm) effusion cells were used, inserted in an outer tantalum crucible. The temperature of the cell was measured with a W-Re/W-Re 5/26% thermocouple inserted in the bottom of the tantalum container. The heating system of our KE source requires rather long thermal equilibration times in


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the low-temperature regime of interest for studies on ILs. In order to avoid prolonged heating times (that would favour thermal decomposition), we performed a number of different runs on fresh sample rather than one or two prolonged runs, and relatively few data were collected for each fresh loaded sample. The intensity vs. electron energy curves (IEC, Ionization Efficiency Curves) were registered for the most important ion species to estimate their appearance energy. As a rule, measurements were carried out with an electron energy corresponding to the maximum of the ionization efficiency curve of each ion.

Thermogravimetry/Differential Thermal Analysis (TG-DTA) Simultaneous thermogravimetry and differential thermal analysis (TG-DTA) experiments were carried out following a protocol recently proposed by Heym to tackle the problem of simultaneous evaporation/decomposition.32 In general, the mass loss rate due to the simultaneous occurrence of evaporation and thermal decomposition in a liquid during a non-isothermal TG experiment under a flowing inert gas atmosphere may depend by many factors.32,47-48 If thermal decomposition obeys a 1st order reaction, the mass loss rate is directly proportional to the sample mass, and independent of the crucible’s shape, the type of gas and its flow rate. By contrast, the evaporation rate significantly depends on the size of the liquid–gas-interphase as well as on the diffusion coefficient of the vapor transported by the carrier gas. As a consequence, the type of gas used may influence the evaporation rate. On this basis, Heym and co-workers proposed recently32 a protocol for a fast check of the occurrence of evaporation with or without simultaneous decomposition. TG-DTA measurements were carried out using a Stanton-Redcroft STA1500 apparatus, operating through a Rheometric Scientific system interface controlled by the RSI Orchestrator software. Liquid samples of comparable size (about 20-22 mg) have been placed into an open cylindrical 70 µL platinum crucible using an insulin syringe, while an open empty identical crucible was used as reference. Comparable amounts of this liquid sample were subjected to two identical ACS Paragon Plus Environment


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kind of heating experiments at a low heating rate (2 K·min−1) from room temperature to 873 K using, alternatively, two different inert gas carriers at a flow rate of 50 ml min−1: Ar and N2. Three replicates were carried out using each inert gas atmosphere and very pure indium reference sample have been used for calibration of temperature, obtaining a final standard uncertainty (1σ) of ± 0.1 K.

Results and Discussion KEML experiments: mass loss rates In KEML experiments, the IL sample was placed in a platinum crucible, in turn placed in an alumina Knudsen cell. Some measurements were also done putting the sample directly in the alumina cell, but in this case creeping phenomena were observed that occasionally caused the liquid to come out from the cell. In the following, only the results obtained with the platinum crucible will be presented. The mass loss rates per unit orifice area corrected by the Clausing factor (J, specific effusion flux), i.e. the term ( A°W ° )−1 ∆ m ∆ t of eq 1 (proportional to p effusion

MT

−1

), are reported in Table 1. If no

kinetic hindrance occurs, this quantity should not depend on the effusion orifice characteristics. However, in our case a remarkable dependence is found, which calls for an explanation. By analysing Table 1 and Figure 1 it can be argued that the evaporation flux of BMImPF6 measured with the 1 mm orifice is about twice that with the 3 mm orifice and more than one order of magnitude lower than that measured with the smallest orifice of 0.2 mm. In Figure 1, the specific flux measured9 for the prototypical IL BMImNTf2 is also reported for comparison. In the latter, no orifice-dependence was observed. The specific flux of BMImPF6 measured with the 1 mm and 3 mm orifices is 10 to 30 times lower than BMImNTf2. However, the strong increase of the ACS Paragon Plus Environment

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BMImPF6 flux with the 0.2 mm orifice makes the volatility of the two compounds very similar (about 20% higher for BMImPF6) under closer-to-equilibrium conditions. Several evidences49-53 were presented in the literature that imidazolium ILs vaporize, mainly or exclusively, as individual neutral ion pairs (NIP), according to the following evaporation process:

BMImNTf2(l) = BMImNTf2(g)

(4)

The concentration of charged species in the vapor was recently confirmed to be very low.31 Although the direct experimental detection of intact ion pairs in the vapor has been reported only for few particularly stable species,52-53 and actually the existence of higher aggregates was suggested by some theoretical evaluations,50,54-55 the evaporation process 4 is commonly accepted for ILs having weakly nucleophilic anions. In the case of BMImNTf2, KEMS results are consistent with this view, as reported in our previous study9 and confirmed in the present paper, where further KEMS measurements on this benchmark IL were carried out for sake of comparison (see next section). The behaviour here observed for BMImPF6 (Figure 1) seems not consistent with this simple picture. Such a strong dependence of the effusion flux on the orifice size would indicate that the vaporization process is kinetically hindered and saturation in the gas phase is not achieved because vapor species escape from the effusion orifice with a flux higher than that leaving the evaporating surface, in accordance with eq 2. This phenomenon is unlikely to occur for the simple evaporation 4, whereas it has been reported in a number of cases where vapor species are not present as such in the condensed phase and are formed through a decomposition process.56-57 Therefore, the results of KEML measurements suggest that, in the explored temperature range and under effusion conditions, BMImPF6 undergoes thermal decomposition accompanied by the release of volatile



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products. As mentioned in the introduction, this hypothesis was already put forward in previous studies.25,27 As we will discuss in the next section, now it is also supported by mass spectrometric results. As shown by eq 1, a strictly linear trend of ln J vs 1/T is not expected a priori, more so because in the presence of decomposition + evaporation processes, M in eq 1 may depend on temperature. However, as a matter of fact, the data shown in Figure 1 display good linear trends fitted by the following equations ln J 3 mm / kg s −1m − 2 = (24 .504 ± 0 .283 ) −

ln J 1 mm / kg s −1m − 2 = (24 .220 ± 0 .499 ) −

ln J 0.2 mm / kg s −1m −2 = (21 .483 ± 0.275 ) −

(17003

± 134 ) T /K

(452-494 K)

(5)

(16536

(478-519 K)

(6)

(505-551 K)

(7)

± 248 ) T /K

(13905 ± 145 ) T /K

The slope of these lines is found to decrease with decreasing the orifice size, probably due to the superimposition of different processes with different enthalpy changes.

TG-DTA experiments Although the occurrence of decomposition in liquid BMImPF6 was already reported in the literature on the basis of non-isothermal TG measurements, the decomposition onset temperatures derived from TG-DTA curves are in the 622 – 709 K range,23-24 much higher than temperatures explored in the present study by effusion techniques. While the TG-DTA decomposition temperatures are of interest for practical applications of ILs, indicating the onset of massive decomposition, according to the results presented in the previous and in the next section of this paper, thermal degradation actually begins at much lower temperatures, making difficult to study the simultaneous non-


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decompositive evaporation by the tensimetric techniques in which the composition of the vapor phase cannot be monitored. As described in the experimental section, our TG-DTA experiments were done following the method proposed by Heym,32 in which two identical non-isothermal TG/DTA experiments are carried out using different flowing inert carrier gases (Ar and N2 in this case) at a moderate heating rate. On comparing the corresponding DTG curves (TG first derivative), two alternative conclusions may be drawn about the process which takes place: thermal degradation, if the two curves are practically superimposable, simple evaporation, if they are remarkably different. These results could be not exhaustive in the case of simultaneous and comparable evaporation/decomposition processes. The TG, DTA and DTG curves measured for BMImPF6 at a heating rate of 2 K min−1 under Ar and N2 atmospheres are shown in Figure 2. Apparently, a single step of mass loss is displayed and no difference is evident between the curves recorded using the two carrier gas up to about 473 K, i.e. within the temperature range covered in our KEML and KEMS experiments. When mass loss becomes sizeable (around 523 K both in Ar and in N2 atmospheres) a slight difference emerges in the three plots: the sample temperature in N2 is shifted towards higher temperatures up to 80% of mass loss, with extrapolated onset temperatures equal to 634.8 and 642.5 K for the TG curves in flowing Ar and N2, respectively, consistent with literature data.23,24 In the final part (from about 723 K) the two curves of each plot are practically superimposed. This behavior is consistent with the simultaneous occurrence of evaporation and thermal decomposition, with the latter more and more prevailing with increasing temperature. Although this method cannot provide information in the low-temperature range owing to the too low mass loss, the DTG curves indicate a close competition between evaporation and thermal decomposition, in accordance with the results obtained at lower temperature by KEML and KEMS, with decomposition finally prevailing at temperature above 723 K. However, in matching TG and KEML-KEMS results it should be emphasized that the experimental conditions are extremely different in the two cases. In particular, TG measurements are performed in open crucible under a flowing carrier gas atmosphere, whereas KEML-KEMS ACS Paragon Plus Environment


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experiments under high vacuum in effusion regime. From the observed dependence of the specific effusing flux on the orifice diameter, as reported in the previous section, it is reasonable to suppose that open cell conditions may affect significantly the evaporation behaviour.

KEMS experiments: mass spectra Unlikely the KEML technique, Knudsen Effusion Mass Spectrometry permits one to obtain direct information on the composition of the vapor phase and to ascertain the evaporation/decomposition processes taking place as a function of temperature. In order to get more information on the effect of the effusion orifice size, KEMS experiments were performed with orifice of 1 mm (platinum and alumina cells) and 0.5 mm (alumina). Typical mass spectra of the BMImPF6 vapor recorded with an electron energy of 20 eV are shown in Figure 3a (effusion orifice diameter 1 mm) and 3b (effusion orifice diameter 0.5 mm), where the ion currents were background-subtracted and corrected for the different multiplier gains (see eq 3) by the factor M . For sake of comparison, the spectrum of BMImNTf2 recorded at a similar temperature is also shown (Figure 3c). The spectra of the two ILs look very different. In particular, while the pattern of BMImNTf2 displays an overwhelming peak at m/e = 139, in the case of BMImPF6, the spectra are much more complex, with a number of peaks of comparable intensity. Furthermore, the relative intensity of most peaks depends markedly on the orifice size. With the smaller orifice (Figure 3b) the most intense peak is at m/e = 96, with the 1 mm orifice (Figure 3a) it is at m/e = 139. As discussed elsewhere,9 mass 139 corresponds to the BMIm+ cation. In the case of BMImNTf2 and other ILs,31 this species is assigned to the fragmentation of the unstable radical ion BMIm+NTf2· formed under electron impact from the neutral ion pair BMImNTf2(g) produced by evaporation, according to the following scheme:

BMImNTf2(l) = BMIm+NTf2¯(g) ACS Paragon Plus Environment

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BMIm+NTf2¯(g) + 1e¯BMIm+NTf2·(g) + 2e¯

(8)

BMIm+NTf2·(g) BMIm+ + NTf2·(g)

where the first equation is the evaporation process 4. Minor peaks in the spectrum of BMImNTf2 are assigned to fragments of BMImNTf2(g), as discussed elsewhere.9,29 With regard to the BMImPF6 mass spectra, a reasonable assignment of the most intense peaks is as follows: 82 = methylimidazole; 96 = ethylimidazole; 139 = BMIm+; 158 = BMImF+. Likewise in the BMImNTf2 spectrum, mass 139 is assigned to the fragmentation of the intact ion pair BMImPF6 by the very same mechanism reported in eq 8. The above assignments are supported by the analysis of the IECs (Ion Efficiency Curves, i.e. intensity vs. electron energy curves) recorded for the most important ion species detected in the experiments. The appearance energy (AE) of each ion can be deduced from these curves by appreciating the onset of intensity from the baseline at low energy. To this end, the energy scale was calibrated by the AE of mass 18 (Ionization Energy (H2O)=12.6 eV58). The AEs of the most important species are reported in Table 2, together with the value for mass 139 for BMImNTf2. The AE of mass 139 in BMImNTf2 was preliminarily reported to be 9.7 ± 0.5 eV in our previous paper9 and was re-measured in the current work, obtaining a value of 9.3 ± 0.3 eV (see Table 2), in acceptable agreement with the previous determination. It can be noted from Table 2 that AE(139) is remarkably higher for BMImPF6 than for BMImNTf2. Since the anion PF6– has a much lower polarizability compared to NTf2– (4.39 and 13.11 Å3, respectively, as computed by ab initio calculations59), this result seems consistent with the well-documented inverse correlation between ionization energy and polarizability.60 It is important to note that the AE of the species with m/e = 96 is in excellent agreement with the ionization energy reported for the ethylimidazole (8.58 eV61). This evidence suggests that this signal is due to the presence of neutral ethylimidazole in the gaseous phase, probably produced by thermal decomposition of the IL, although the formation through gas phase dissociation processes cannot be ruled out on the basis of KEMS results. As a consequence, the mass loss rates measured in KEML experiments cannot be ACS Paragon Plus Environment


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considered as representative only of the evaporation process, although with increasing the effusion orifice size the contribution of peaks other than BMIm+ should tend to become minor. The formation of ethylimidazole among the thermal decomposition products of BMImPF6 at much higher temperatures (723-823 K) was already reported on the basis of TGA-MS and pyrolysis-gas chromatography measurements.62,63 The formation of this species was reported for a number of ILs containing the EMIm+ cation and cyano-functionalised or halide anions, where it has been rationalised on the basis of a SN2 mechanism.17,21,64 On the contrary, the AE measured for mass 82 is much higher than the ionization energy reported in the literature for methylimidazole (8.66 eV65), so this ion is most probably a fragment. With regard to mass 158 (BMImF), on the basis of the AE it is not possible to assign it to a neutral BMImF(g) precursor or to a fragment ion (but see below). As apparent in Figures 3a and 3b, the orifice size was found to affect the relative intensities of the various ion species. This effect is clearly shown in Figure 4a, where the intensity ratio measured with different cells is reported for peaks at m/e = 96 and 139. While with the 1 mm orifice the peak 139 (i.e., the gaseous ion pair) is about 3 times more intense than peak 96 (ethylimidazole), with the 0.5 mm orifice the ratio is reversed, with peak 96 being the most intense. This evidence strongly supports the above hypothesis that the ion species with m/e = 96 originates from the primary ionization of ethylimidazole neutral precursor rather than from the fragmentation of the BMImPF6(g) ion pair. On the contrary, this effect is not observed for mass 158 (Figure 4b), suggesting this ion is formed by fragmentation of the BMImPF6(g) ion pair. Note that in both cases the intensity ratios show no significant dependence on temperature in the range investigated, making this criterium ineffective in distinguishing parent and fragment ions.


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KEMS experiments: evaluation of partial pressures As shown in the previous section (see Figure 3a), the mass spectrum measured with the 1 mm orifice displays the signal at m/e = 139, assigned to the BMIm+ ion, as the most intense. Since this ion is originated by electron impact from the BMImPF6(g) species, in turn formed by the simple evaporation 4, the larger orifice seems suitable to study the evaporation process, with the simultaneous decomposition/dissociation processes playing a minor role. To this end, the intensity of the main ion species was monitored as a function of temperature in the range 455-528 K by using a 1 mm diameter platinum Knudsen cell. In order to derive the evaporation enthalpy of BMImPF6, the partial pressure of BMImPF6(g) was evaluated by eq 3 from the intensity of peak 139. The main issue in using eq 3 is that a cross section value has to be estimated. To the best of our knowledge, the only approximate estimate available in the literature for the electron impact ionization cross section of an IL was provided by us9 for the BMIm+ ion formed from BMImNTf2(g) (σ = 35 ± 15 Å2). This value is larger than the typical values of molecular organic species by a factor 2-3, what could be reasonable for an ion pair. In view of the widely different electronic polarizability of the NTF2– and PF6– anions (see above),59 it is reasonable to expect a smaller cross section for the latter.66 In the lack of more information, we estimated the cross section of BMImPF6(g) by assuming for the PF6– and NTF2– anions a direct proportionality between σ and electronic polarizability, well-documented for molecular species.66 The value derived for PF6– by using the ab initio polarizabilities reported above59 is equal to 11.7 ± 1.8 Å2, to which a uncertainty of ± 15% has be associated, conservatively estimated from the known uncertainty of the polarizability vs. cross section correlation.66 The so derived partial pressures of BMImPF6(g) are reported in Table 3 as a function of temperature and can be represented by the equation ln p/bar = (27.939 ± 1.429) – (15732 ± 697)/T. It is worth noting that according to these data the vapor pressure of BMImPF6 is lower than that of BMImNTf2 by a factor ranging from 10 to



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20 in the explored temperature range. In the next section, KEMS pressures will be used to derive the evaporation enthalpy of BMImPF6. A similar analysis can be used to estimate the partial pressure of ethylimidazole, the second most abundant vapor species according to KEMS spectra (see Figure 3). The pertinent cross section may be estimated by the above-mentioned correlation with the electronic polarizability.66 The latter was in turn estimated from that reported for imidazole (7.495 Å3)67, by adding the group contribution of (CH2)2 (3.6 Å3)68. The resulting value of σ is 16.4 ± 2.5 Å2 and the partial pressures derived thereafter are also reported in Table 3. By assuming for simplicity that no other species is present, the mole fraction y(BMImPF6) in the vapor phase results to be 0.81 ± 0.04 (average value from Table 3) with no evident temperature dependence. We note that the partial pressures of 1ethylimidazole estimated by KEMS are much lower than the vapor pressure of the pure compound. By analogy with 2-methylimidazole,69 at 450 K the latter is expected to be in the order of 1 bar, so that the activity of 1-ethylimidazole in the liquid is estimated to be 10-8 or less, which should imply a very low concentration. As a consequence, we can be confident that the study of the simple evaporation of BMImPF6 was performed on a liquid phase with negligible impurity. On the basis of this roughly estimated vapor phase composition, a comparison can be attempted between KEMS and KEML results for the 1 mm effusion orifice. If more than one species is present in the vapor, eq 1 gives the total mass loss rate measured in KEML experiments provided that the following expression for the mean molecular weight is used at each temperature:41   M =  ∑ yi M i   i 

2

(9)

where yi are the mole fractions of the various vapor species in the cell and Mi the respective molecular weight. By assuming as a first approximation that the vapor is composed only by BMImPF6(g) and ethylimidazole, with y(BMImPF6) = 0.81 (see above), we obtain from eq 9 M = 240.8 g mol-1 (only slightly lower than the molar mass of BMImPF6, 284.2 g mol-1, as expected). In ACS Paragon Plus Environment

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Figure 5 we report the KEML partial pressures of BMImPF6(g) calculated according to eq 1 with the so-derived average molecular weight and the corresponding KEMS values (see Table 3). For sake of comparison, in the same figure we reported the KEML pressures measured with the largest orifice (3 mm), where the contribution of species other than BMImPF6(g) is argued to be very small, and the results recently obtained by Zaitsau et al with QCM measurements11 (the only BMImPF6 vapor pressure values currently available in the literature). Although the KEMS pressures in Figure 5 are somewhat scattered and can be affected by uncertainties in parameters of eq 3 such as instrumental constant and ionization cross sections, a very satisfactory agreement with KEML pressure data is found. Admittedly, our lowest values (KEML with 3 mm orifice) are still about 3-4 times higher than QCM results. The discrepancy could be explained by the use of open pans in the latter technique: the decreasing trend of apparent pressures with increasing the effusion hole size, described in the previous sections, could find its limiting value under open cell conditions, where simple evaporation would be the only or largely dominant process. In this view, free surface conditions seem to be the most suitable conditions under which simple evaporation may be studied without decomposition giving a significant contribution. Nevertheless, unlikely effusion conditions, free surface evaporation could not guarantee the achievement of a thermodynamic equilibrium and a properly saturated vapor phase.

Thermodynamic analysis: evaporation enthalpy of BMImPF6 In Table 3 a reasonable evaluation of the partial pressures of the BMImPF6(g) species was done from KEMS measurements using an orifice of 1 mm in diameter. In Figure 5, we showed that, in spite of the various approximations involved in the processing of KEML data, fairly consistent values are obtained by KEML under similar conditions. Since these pressures are derived from the analysis of MS signals of the BMIm+ ion formed from the BMImPF6 gaseous species, they are



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believed to provide a fairly good approximation to the true thermodynamic equilibrium vapor pressures of BMImPF6(l), regardless possible simultaneous decomposition processes. Vapor pressure data can be analysed by the so-called second- and third-law methods to derive the corresponding evaporation enthalpy. Advantages and drawbacks of the two methods were described elsewhere.9 Second-law analysis is based on evaluating the slope of the ln p vs. 1/T regression line according to the Clausius-Clapeyron equation. While this procedure does not need auxiliary thermal functions, it is known to be severely affected by the extent of dataset5 and temperature range, which are both rather limited for the KEMS data reported in Table 3 and Figure 5. The slope of the regression line of these data gives the enthalpy change at the average experimental temperature (488 o K), ∆evapH 488K = 130.8 ± 5.8 kJ mol-1, where the relatively large error is due to the low number and

high scatter of data. In order to adjust this value to the reference temperature of 298 K, the heat capacities of BMImPF6 in the liquid and in the gaseous phase are needed. Molar heat capacities of crystal and liquid BMImPF6 were measured by adiabatic calorimetry from 5 to 550 K.33. Measurements above room temperature were also carried out by several authors34-40 and, with the exception of two studies,35-36 deviation among different datasets is lower than 2%, in many cases within 1%. As far as the values at low temperature are concerned, the results by Triolo and coworkers39 agree satisfactorily with those of ref. 33. With regard to gaseous BMImPF6(g), Paulechka and co-workers25 determined the ideal gas thermodynamic functions of BMImPF6(g) neutral ion pair by means of ab initio calculations at the MP2 level of theory and standard statistical o thermodynamics. Using data from papers of refs. 33 and 25, the value ∆evapH 298K = 146.4 ± 5.8 kJ

mol-1 is obtained. Despite the above-mentioned drawbacks regarding this method applied to our KEMS dataset, the value so obtained is in excellent agreement with the only experimental value available in the literature, published last year by Zaitsau et al11 ( ∆evapH 298K = 146.5 ± 2.6 kJ mol-1, o

Table 5). Such an agreement between two so different methods gives further confidence in the approach followed in the present work to interpret KEMS results. It is worth noting that the heat ACS Paragon Plus Environment

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(

)

o o content used to adjust our result to 298 K, ∆evap H 488K − H 298K = 15.6 kJ mol-1, is only marginally

different from the value one would obtain by the semiempirical estimation used by Zaitsau et al.11,

(

)

o o o -1 ∆evap H488 K − H 298K = 14.0 kJ mol , and so a direct comparison between the ∆ r H 298K values is

feasible. The third-law analysis is based on the following equation: o o ∆r H298 K = −RT ln K p (T ) − T∆r Gef (T , Tref = 298 K )

(10)

where ∆ r Gef (T ) is the Gibbs energy function change (Gef, with reference temperature Tref equal to 298 K) o

of reaction 4, with

Gef

°

( T , T ref = 298 K) =

G o ( T ) − H o ( 298 K ) T

=

H o ( T ) − H o ( 298 K ) T

− S o (T )

(11)

for liquid and gaseous BMImPF6. As far as the liquid phase is concerned, Gefs can be easily calculated if the molar heat capacities in the entire temperature range from 0 K to the experimental temperatures are known. For the gas phase, the calculation is done using statistical thermodynamics with partition functions evaluated from spectroscopic/computational molecular parameters. The third-law method is a good alternative to Clausius-Clapeyron fits when Gefs are known with reasonable accuracy and it was successfully applied in our previous study on BMImNTf2.9 Among ILs, BMImPF6 represents one of the few cases in which such an analysis can provide results of good accuracy, because a complete derivation of thermodynamic functions was presented in the above cited papers.25,33 The third-law evaporation enthalpies calculated from the KEMS data are shown in Table 4, with the mean value and the corresponding standard deviation. On the basis of the above-mentioned comparison between different sets of heat capacities data available in the literature, we estimate a further uncertainty of about ± 1.5 kJ·mol–1 due to ∆Gef inaccuracy. Finally, an uncertainty of ± 0.5 kJ·mol–1 was considered for the aforementioned uncertainty on the cross



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o section. The resulting third-law value for the evaporation enthalpy of BMImPF6 is then ∆evapH 298K

= 145.3 ±2.9 kJ mol-1. This result is in excellent agreement with the second-law value. As a

confirmation of the good overall agreement, we note that the standard evaporation entropy change obtained as the y-intercept of the second-law fitting line ( ∆r S488K =137 ± 12 J K-1 mol-1) is consistent o

with the value (133 J K-1 mol-1) obtained from the absolute entropies of liquid and gaseous BMImPF6, obtained from calorimetry33 and theoretical calculations,25 respectively. o In selecting a final recommended value of ∆evapH 298K , we prefer to choose that derived by the third-

law method, which is less affected by the low number of data points and by possible inaccuracies due to temperature-dependent errors. This value (145.3 ±2.9 kJ mol-1) is in full agreement with the QCM result (146.5 ± 2.6 kJ mol-1).11 However, it should be considered that the latter is based on a second-law analysis, the former on a third-law treatment. Since the QCM vapor pressures11 are lower than ours (see Figure 5), if a third-law analysis were done on QCM data, a value of 151.4 ±0.2 kJ mol-1 would be obtained, not negligibly larger than ours, but not very far. A large number of molecular dynamics (MD) studies were published on BMImPF6,11,13,14,70-75 where evaporation enthalpy was estimated with different approaches and force fields. Theoretical results are reported in Table 5 together with the two experimental values now available. It is evident that most simulations significantly overestimate this property, whereas the values proposed in ref. 11 and 13 are the most effective in reproducing the experimental results. In the same table, we report also the result of two estimates based, respectively, on the Hildebrand solubility parameter δH, i.e. the square root of the cohesive energy density76 1/ 2

∆ U  δ H =  ev   V   

1/ 2

 ∆evapH − RT   =  V  

and on the empirical correlation between evaporation enthalpy and surface tension γ:6,77


(12)

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γV 2 / 3 = A + B ∆ evapH

(13)

where V is the molar volume. These methods provide a very simple way to get a rough estimate of the evaporation enthalpy. For V, the well-assessed value78 of 207.8 dm3·mol–1 was used. With regard to eq 12, we note that ∆evapH is very sensitive to small changes in the Hildebrand parameter: the values (measured by indirect methods) 30.279 29.880 and 28.0976 MPa0.5 at T = 298 result in

∆evapH298K of 192, 187 and 167 kJ·mol–1, respectively. As for eq 13, by using for A and B the values derived from a set of experimental values for ILs,77 and the value γ = 43.9 mN·m–1 given in ref. 81 (average of values measured with two independent methods at 298 K), ∆evapH298K = 149.9 kJ·mol–1 is obtained, in good agreement with the experimental value. However, other literature sources give surface tension values exceeding 43.9 mN·m–1 by up to 4 mN·m–1,81 resulting in ∆evapH298K values higher by up to 15 kJ·mol–1.

Conclusions In this study we presented the results of evaporation experiments performed on the imidazolium ionic liquid BMImPF6, aimed at studying the competition between simple evaporation and thermal decomposition. Our multi-technique approach used two techniques based on molecular effusion: Knudsen Effusion Mass Loss (in the temperature range 453 -551 K) and Knudsen Effusion Mass Spectrometry (425-528 K), complemented with non-isothermal TG-DTA experiments. While KEML measurements provided information on total mass loss as a function of temperature, the mass spectrometric analysis of the vapor phase allowed the evaporation process to be studied even in the presence of the simultaneous release of volatile decomposition products. On the basis of KEMS mass spectra and KEML mass loss rates measured with effusion orifices of different diameter, we obtained evidence for the simultaneous occurrence of simple evaporation



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(BMImPF6(l) = BMImPF6(g)) and thermal decomposition processes. Specific mass loss rates depended on the effusion orifice size, a result that may be explained by the occurrence of a kinetically delayed process. Mass spectra displayed peaks assigned to the fragmentation of the gaseous ion pair BMImPF6(g) (in particular, the peak at m/e = 139, corresponding to the BMIm+ cation), and peaks most probably due to volatile products formed by thermal decomposition/dissociation, the most intense being at m/e = 96 (assigned to ethylimidazole). These findings are consistent with the results obtained from TG/DTA experiments carried out using two different carrier gas (He and N2). The KEMS technique enabled us to monitor the evaporation process and to measure the partial pressures of the gaseous ion pair BMImPF6(g) as a function of temperature. Pressure data were processed according to the third-law method of analysis, obtaining o a ∆evapH298K value of 145.3 ±2.9 kJ mol-1, in excellent agreement with the only previous experimental

value11 recently become available in the literature. The evaporation enthalpy of BMImPF6 at 298 K is more than 20 kJ mol-1 higher compared to BMImNTf2, whose evaporation behavior is wellknown from previous studies,7-9 and vapor pressures are about 20-30 times lower in the explored temperature range.


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Acknowledgment The authors gratefully acknowledge funding from the University of Rome La Sapienza (Progetto di Ricerca di Università 2015 “Studio termodinamico dei processi di vaporizzazione di liquidi ionici aprotici”).

Supporting Information Available Ionization efficiency curves of ions with m/e = 96 and 139. This information is available free of charge via the Internet at http://pubs.acs.org

References

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Tricyanomethanide: Direct Detection of the Intact Ion Pair. J. Phys. Chem. Lett. 2012, 3, 2910-2914. (54) Ballone, P.; Pinilla, C.; Kohanoff, J.; Del Pópolo, M.G. Neutral and Charged 1-Butyl-3Methylimidazolium Triflate Clusters: Equilibrium Ionic Vapor: What Does It Consist of? J. Phys. Chem. B 2007, 111, 4938-4950. (55) Chaban, V.V.; Prezhdo, O.V. Concentration in the Vapor Phase and Thermal Properties of Nanometric Droplets. J. Phys. Chem. Lett. 2012, 3, 1657-1662 (56) Lau, K.H.; Cubicciotti, D.; Hildenbrand, D.L. Effusion Studies of the Thermal Decomposition of Magnesium and Calcium Sulfates. J. Chem. Phys. 1977, 66, 4532-4539. (57) Ganesan, R.; Ciccioli, A; Gigli, G; Ipser, H. Thermochemical Investigations in the TinPhosphorous System. Int. J. Mat. Res. 2011, 102, 93-103. (58) P.J. Linstrom and W.G. Mallard, Eds., NIST Chemistry Webbook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg MD, 20899, Http://Webbook.Nist.Gov/Chemistry/ (Retrieved October 11, 2017). (59) Izgorodina, E.I.; Forsyth, M.; Macfarlane, D.R. On the Components of the Dielectric Constants of Ionic Liquids: Ionic Polarization ? Phys. Chem. Chem. Phys. 2009, 11, 24522458. (60) Blair, S.A.; Thakkar, A.J. Relating Polarizability to Volume, Ionization Energy, Electronegativity, Hardness, Moments of Momentum, and Other Molecular Properties. J. Chem. Phys. 2014, 141, 074306/1-5 (61) Turchaninov, V.K.; Ermikov, A.F.; Shagun, V.A.; Baikalova, L.V. Inversion of the Highest Occupied Molecular Orbitals of Imidazole Derivatives. Bull. Acad. Sci. USSR, Divis. Chem. Sci. 1987, 36, 2407-2410. (62) Othani, H.; Ishimua, S; Kumai, M. Thermal Decomposition Behaviors of ImidazoliumType Ionic Liquids by Pyrolysis-Gas Chromatography. Anal. Sciences 2008, 24, 1335-1340. (63) Hao, Y.; Peng, J.; Hu, S.; Li, J.; Zhai, M. Thermal Decomposition of Allyl-ImidazoliumBased Ionic Liquid by TGA-MS Analysis and DFT Calculations, Thermochim. Acta 2010, 501, 78-83. (64) Chambreau, S.D; Schenk, A.C.; Sheppard, A.J.; Yandek, G.G., Vaghjiani, G. L.; Maciejewsi, J.; Koh, C.J., Golan, A., Leone, S. R. Thermal Decomposition Mechanism of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions, J. Phys. Chem. A 2014, 118, 11119-11132.


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(65) Ramsey, B.G. Substituent Effects on Imidazole Basicity and Photoelectron Spectroscopy Determined Ionization Energies, J. Org. Chem. 1979, 44, 2093. (66) Bull, J.N.; Harland, P.W.; Vallance, C. Absolute Total Electron Impact Ionization CrossSection for Many-Atom Organic and Halocarbon Species. J. Phys. Chem. A 2012, 116, 767777. (67) Jug, K.; Chiodo, S. Electronic and Vibrational Polarizabilities and Hyperpolarizabilities of Azoles: A Comparative Study of the Structure-Polarization Relationship. J. Phys. Chem. A 2003, 107, 4172-4183. (68) Miller, K.J. Additivity Methods in Molecular Polarizability. J. Am. Chem. Soc. 1990, 112, 8533-8542 (69) Verevkin, S.P.; Zaitsau, D.H.; Emel’yanenko, V.N. Thermodynamics of Ionic Liquids Precursors: 1-Methylimidazole, J. Phys Chem. B 2011, 115, 4404-4411. (70) Morrow, T.I.; Maginn, E.J.; Molecular Dynamics Study of the Ionic Liquid 1-n-Butyl-3Methylimidazolium Hexafluorophospate. J. Phys. Chem. 2002, 49, 12807-12813. (71) Sambasivarao, S.V.; Acevedo, O. Development of OPLS-AA Force Field Parameters for 68 Unique Ionic Liquids. J. Chem. Theory Comput. 2009, 5, 1038–1050. (72) Liu, Z.; Wu, X.; Wang, W. A Novel United-Atom Force Field for Imidazolium-Based Ionic Liquids. Phys Chem. Chem. Phys. 2006, 8, 1096-1104. (73) Mondal, A.; Balasubramanian, S. Quantitative Prediction of Physical Properties of Imidazolium Based Room Temperature Ionic Liquids Through Determination of Condensed Phase Site Charges: A Refined Force Field. J. Phys. Chem B 2014, 118, 3409-3422. (74) Deyko, A.; Lovelock, K.R.J.; Corfield, J.-A.; Taylor, A.W.; Gooden, P.N.; Villar-Garcia, I.J.; Licence, P.; Jones, R.G.; Krasovskiy, V.G.; Chernikova, E.A. et al. Measuring and Predicting ∆vapH298 Values of Ionic Liquids. Phys Chem Chem Phys 2009, 11, 8544-8555. (75) Červinka, C.; Pádua, A.A.H.; Fulem, M. Thermodynamic Properties of Selected Homologous Series of Ionic Liquids Calculated Using Molecular Dynamics. J. Phys. Chem. B 2016, 120, 2362-2371. (76) Weerachanchai, P.; Chen, Z.; Leong, S.S.J.; Change, W.W., Lee, J.-M. Hildebrand Solubility Parameters of Ionic Liquids: Effects of Ionic Liquid Type, Temperature and DMA Fraction in Ionic Liquid. Chem. Eng. J. 2012, 213, 356-362. (77) Tong, J.; Yang, H.-X.; Liu, R.-J.; Li, C.; Xia, L.-X.; Yang, J.-Z. Determination of the Enthalpy of Vaporization and Prediction of Surface Tension for Ionic Liquid 1-Alkyl-3Methylimidazolium Propionate [CnMIm][Pro] (n=4,5,6). J. Phys. Chem. B 2014, 118, 12972-12978.



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(78) Rocha, M.A.A.; Ribeiro, F.M.S.; Lobo Ferreira, A.I.M.C.; Coutinho, J.A.P.; Santos, L.M.N.B.F. Thermophysical Properties of [CN-1C1Im][PF6]. J. Mol. Liq. 2013, 188, 196-202. (79) Swiderski, K.; Mclean, A.; Gordon, C.M.; Vaughan, D.H. Estimates of Internal Energies of Vaporisation of Some Room Temeprature Ionic Liquids. Chem Commun. 2004, 21782179. (80) Lee, S.H.; Lee, S.B. The Hildebrand Solubility Parameters, Cohesive Energy Densities and Internal Energies of 1-Alkyl-3-Methylimidazolium-Based Room Temperature Ionic Liquids. Chem. Commun. 2005, 3469-3471. (81) Klomfar, J.; Součková, M.; Pátek, J. Surface Tension Measurements for Four 1-Alkyl-3Methylimidazolium-Based Ionic Liquids With Hexafluorophosphate Anion. J. Chem. Eng Data 2009, 54, 1389-1394.


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Table 1. Mass loss rate per unit surface Ja measured in KEML experiments with different effusion hole diameters (Ø). Ø = 0.2 mm

Ø = 3 mm

T /K

103 J/kg·s–1·m–2

T/K

105J/kg·s–1·m–2

T/K

106J·/kg·s–1·m–2

551.4

23.8

501.1

15.9

492.1

46.1

547.3

19.8

496.1

10.6

486.6

29.5

542.8

15.5

491.3

7.58

481.2

19.6

538.8

13.1

486.4

5.18

475.9

13.3

534.5

10.5

516.4

39.1

471.0

9.11

530.4

8.56

512.2

30.2

466.0

5.88

526.3

6.79

508.1

22.7

461.1

4.19

522.4

5.78

503.4

17.0

456.2

2.83

518.5

4.58

499.7

13.5

494.2

48.4

514.5

3.99

495.8

10.2

488.8

33.7

510.6

3.35

491.7

8.11

483.0

22.2

506.6

2.67

487.8

6.05

478.0

15.9

549.9

23.6

484.0

4.53

472.9

10.7

545.1

18.4

519.0

50.7

468.6

8.13

541.2

15.1

514.3

37.2

463.1

4.96

536.8

12.1

509.9

28.5

459.3

3.72

532.3

9.69

505.6

21.6

452.7

2.18

528.3

7.98

501.6

16.7

524.3

6.49

497.6

12.9

520.3

5.38

493.7

10.3

516.4

3.95

489.7

7.45

512.5

3.44

485.8

5.68

508.6

2.86

482.0

4.26

504.7

2.43

478.1

3.39

b/K = 527.7 a

Ø = 1 mm

b/K = 498.0

b/K = 473.6

J = (WoAo)–1 ∆m/∆t. This quantity is proportional to p effusion MT −1 (see text)

b

is the reciprocal of the mean of 1/T values



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

Page 32 of 42

Table 2. Appearance energies (AE) measured for the most abundant ion species detected during KEMS experiments on BMImNTf2 and BMImPF6

Ionic Liquid

Mass/u

Iona

AE/eV

Neutral precursor

BMImNTf2

139

BMIm+

9.3 ± 0.3

BMImNTf2(g)

BMImPF6

82

MIm+

12.0 ± 0.5

EIm(g) (?)

96

EIm+

8.3 ± 0.3

EIm(g)

139

BMIm+

11.3 ± 0.5

BMImPF6(g)

158

BMImF+

11.1 ± 0.3

BMImPF6(g)

a

Symbols B, E, M, Im stand for butyl, ethyl, methyl, imidazole/ium


Page 33 of 42

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Table 3. Partial pressuresa of the species BMImPF6 and ethylimidazole in the vapor of BMImPF6(l), as derived by KEMS experiments

T/K

BMImPF6/Pa

Ethylimidazole/Pa

495.2

2.9 10-2

6.6 10-3

514.5

9.0 10-2

2.3 10-2

474.5

6.2 10-3

1.6 10-3

455.5

1.3 10-3

3.3 10-4

493.0

2.3 10-2

5.3 10-3

527.8

1.5 10-1

3.9 10-2

482.2

9.6 10-3

2.0 10-3

468.3

3.3 10-3

6.9 10-4

509.1

4.1 10-2

9.8 10-3

458.0

1.5 10-3

3.6 10-4

504.2

2.6 10-2

5.5 10-3

a

a total uncertainty of 40% on pressure values is conservatively estimated, including the uncertainty on cross sections and multiplier gain.



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

Table 4. Evaporation enthalpy of BMImPF6(l) at 298 K ( ∆ evapH 298K ) evaluated from the third-law o

analysis of BMImPF6(g) partial pressures measured by KEMS. T/K

- ∆ evapGef (T ) a/J

o ∆evapH 298 K /kJ

K-1 mol-1

mol-1

o

495.2

165.84

144.1

514.5

164.07

144.0

474.5

167.74

145.1

455.5

169.27

145.8

493.0

166.05

144.5

527.8

162.84

144.9

482.2

167.04

145.4

468.3

168.27

145.9

509.1

164.56

146.1

458.0

169.09

146.2

504.2

165.01

146.8

Mean: a

145.3 ±0.9

From refs 25 and 33


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Table 5. Compilation of vaporization enthalpies (∆evapH298 K) proposed for BMImPF6 by different theoretical approaches, compared with the experimental values. MD = Molecular Dynamics

∆evapH298 K /kJ mol-1

Method

Reference

161

MD

70

150.6

MD

13

133.5

MD

71

190.4

MD

72

152.9

MD

14

138.1

MD

73

157

Semiempirical model

74

192.8

MD

75

144.5

MD

11

149.9

empirical correlation with surface tensiona

167-192

Hildebrand solubility parameterb

146.5 ± 2.6

Experimental (QCM)

11

145.3 ±2.9

Experimental (KEMS)

This work

a

For this estimate we used the empirical correlation given in ref 77, with surface tension values from ref 81 and molar volume from ref 78 (see text)

b

For this estimate we used the Hildenbrand parameter values taken from refs 76,79 and 80, and the molar volume from ref 78 (see text)



-2

400 K

500 K

600 K

BMImPF6 -4 -2

PF6 in Pt 0.2 mm NTf2 3 mm NTf2 1 mm

BMImNTf2

-1

-6

PF6 in Pt 3 mm PF6 in Pt 1 mm

0.2 mm

0.3 mm

ln (J/kg s m )

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

Page 36 of 42

1 mm

-8

NTf2 0.3 mm

BMImNTf2

1 mm -10

3 mm -12

BMImPF6 3 mm

-14 0,0016

0,0018

0,002

0,0022

0,0024

0,0026

K/T Fig. 1 Plot of ln J vs 1/T, where J = specific effusion flux for the ionic liquids BMImPF6 (open symbols) and BMImNTf2 (filled symbols) measured with different orifices. The uncertainty associated to datapoints is expressed by the symbol size.


Page 37 of 42

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


(a)

(b)

(c)

Fig. 2 TG (a), DTA (b) and DTG (c) curves of BMImPF6 at 2 K min–1 heating rate registered under flowing Ar () and flowing N2 (− −).



BMImPF6 1 mm effusion hole T = 495 K 120

+

BMIm 139

Normalized intensity

100

80

60

EIm

BMImF

96

40

158

MIm 20

82

0

80

100

120

140

160

m/e (u)

(a)

BMImPF6 0.5 mm effusion hole T = 488 K 120

96 EIm 100


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

Page 38 of 42

80

+

BMIm 60

139

MIm 40

82

BMImF 158

20

0

80

100

120

140

160

m/e (u)


(b)

Page 39 of 42

BMImNTf2, T = 486 K 120

+

BMIm 139

100


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


80

60

40

20

HMIm+ 83

0

80

100

120

140

160

m/e (u)

(c)

Fig. 3. Typical mass spectra for BMImPF6, with effusion hole size 1 mm (a) and 0.5 mm (b), and BMImNTf2 (c) vapors. Spectra are normalized to the most intense peak. Note that mass 139 is largely dominant in (c), whereas a far more complex pattern is observed in (a) and (b). Note also that mass 96 is the most intense peak in the BMImPF6 spectrum when the smaller orifice is used. The symbols M,E,B,Im stand for methyl, ethyl, butyl, and imidazole/imidazolium, respectively.



3

2,5

96/139 Intensity ratio

0.5 mm diameter 2

1,5

96/139 alum 1 mm 96/139 alum 0.5 mm 96/139 Pt 1 mm

1

0,5

0 420

1 mm diameter

440

460

480

500

520

540

T/K

3

2,5

158/139 Intensity ratio

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

158/139 alum 0.5 mm 158/139 Pt 1 mm 158/139 alum 1 mm

2

1,5

1 1 mm diameter and 0.5 mm in diameter 0,5

0 420

440

460

480

500

520

540

T/K

Fig. 4. Intensity ratios of KEMS peaks at m/e = 96 and 158 to the peak at m/e = 139 with effusion hole of different size. Note that the cell material has practically no effect.


Page 40 of 42

Page 41 of 42

0 BMImNTf2 (ref. 9)

-2 -4 ln (P/Pa)

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


-6

BMImPF6 this work

-8 -10 -12

P(BMImPF6) KEMS 1 mm P(BMImPF6) KEML 1 mm P(BMImPF6) KEML 3 mm P(BMImPF6) QCM (ref. 11) P(BMImNTf2) KEML (ref. 9)

-14 0,0018 0,0019

0,002

BMImPF6 QCM (ref. 11)

0,0021 0,0022 0,0023 0,0024 0,0025 K/T

Fig. 5. Comparison between the partial pressure of BMImPF6(g) evaluated by the KEMS spectra (open circles) and that measured by KEML with estimated average molecular weight (crosses), 1 mm orifice in both cases). In the plot are also reported the BMImPF6 pressures estimated by mass loss with the 3 mm orifice (neglecting decomposition), the QCM values reported by Zaitsau et al.11 and the BMImNTf2 vapor pressure.9 The uncertainty associated to each dataset is expressed by the symbol size.



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TOC GRAPHIC


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Towards the Elucidation of the Competing Role of Evaporation and

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