Ionic Vapor Composition in Pyridinium-Based Ionic Liquids Ionic Vapor Composition in Pyridinium-Based

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  • Ionic Vapor Composition in Pyridinium-Based Ionic Liquids Vitaly V. Chaban† and Oleg V. Prezhdo*,‡

    †Instituto de Cien̂cia e Tecnologia, Universidade Federal de Saõ Paulo, 12231-280, Saõ Jose ́ dos Campos, SP, Brazil ‡Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

    ABSTRACT: Strong electrostatic interactions in ionic compounds make vaporization a complex process. The gas phase can contain a broad range of ionic clusters, and the cluster composition can differ greatly from that in the liquid phase. Room-temperature ionic liquids (RTILs) constitute a complicated case due to their ionic nature, asymmetric structure, and a huge versatility of ions and ionic clusters. This work reports vapor−liquid equilibria and vapor compositions of butylpyridinium (BPY) RTILs formed with hexafluorophosphate (PF6), trifluoromethanesulfonate (TF), and bis(trifluoromethanesulfonyl)imide (TFSI) anions. Unlike inorganic crystals, the pyridinium-based RTILs contain significant percentages of charged clusters in the vapor phase. Ion triplets and ion quadruplets each constitute up to 10% of the vapor phase composition. Triples prevail over quadruples in [BPY][PF6] due to the size difference of the cation and the anion. The percentage of charged ionic clusters in the gas phase is in inverse proportion to the mass of the anion. The largest identified vaporized ionic cluster comprises eight ions, with a formation probability below 1%. Higher temperature fosters formation of larger clusters due to an increase of the saturated vapor density.

    ■ INTRODUCTION Room-temperature ionic liquids (RTILs) are salts with low temperature melting points. They are composed entirely of ions, and many of them are liquid at room conditions.1−8 In the beginning of the 21st century, RTILs rapidly became the most actively investigated group of solvents ever. Nowadays, RTILs are actively explored in physics, chemistry, and molecular biology as universal solvents, reaction media, and electrolyte solutions, for separation applications, and in chemical synthesis. Low volatility and flammability, wide liquid range, amphiphi- licity, chemical stability (in most known cases), tunability, and ionic conductivity are among the practically important characteristics of the pursued RTILs. The alkylpyridinium-based RTILs are of great value for the

    field,9−16 since precisely these compounds launched the new era of ionic liquids when aluminum(III) chloride was mixed with N-alkylpyridinium.17 Representatives of this established class of RTILs have already been successfully applied in a significant number of processes. Examples include, while not limited to, conductive liquids in electrochemistry, photo- induced electron transfer,18−21 organocatalysis for well-known reactions,22,23 and desulfurization of fuels.24−27 Furthermore, pyridinium-based RTILs exhibit an outstanding thermal stability,28 which depends primarily on the chain length of the cation and the nature of the anion. This property appears practically important, because certain chemical reactions require elevated temperatures for initiation. Using a thermogravimetric analyzer, Bittner and co-work-

    ers28 reported physical chemical properties and thermal stabilities for a number of pyridinium-based RTILs composed of the pyridinium cations with different substituents and the following anions: bis(trifluoromethanesulfonyl)imide [TFSI],

    trifluoromethanesulfonate [TF], tetrafluoroborate [BF4], and tris(pentafluoroethyl)trifluorophosphate [FAP]. The authors systematically compared the ethylpyridinium [EPY], butylpyr- idinium [BPY], and hexylpyridinium [HPY] cations. An effect of the grafted methyl group in the “2” [B2MPY], “3” [B3MPY], or “4” [B4MPY] positions of the pyridine ring was also carefully investigated. It was found that all RTILs maintain a perfect stability up to 500 K, whereas a complete thermal decomposition takes place above 900 K. A longer aliphatic chain results in a higher decomposition temperature, defined as the temperature of the maximum decomposition rate: 706, 724, and 784 K for [EPY], [BPY], and [HPY], respectively. The anion determines the thermal decomposition mechanism and the corresponding temperature. Compare, [B3MPY][TF] and [B3MPY][TFSI] are totally decomposed above 850 K. In turn, the decomposition temperature of both [B3MPY][FAP] and [B3MPY][BF4] exceeds 1000 K. While critical points for the pyridinium-based RTILs were not identified, it is clear from our simulations that they are significantly higher than the decomposition temperatures. Thus, critical temperatures of these RTILs are outside of experimental reach. While earlier RTILs were considered to be nonvolatile

    liquids, now it is recognized that distillation of certain RTILs without degradation is indeed possible.29,30 At relatively low temperatures, RTILs exhibit extremely low but still nonzero vapor pressures,31 which increase continuously upon further heating. The recent findings highlight an importance of understanding the vapor−liquid equilibria (VLE) of these

    Received: March 27, 2016 Revised: May 3, 2016 Published: May 10, 2016

    Article

    pubs.acs.org/JPCB

    © 2016 American Chemical Society 4661 DOI: 10.1021/acs.jpcb.6b03130 J. Phys. Chem. B 2016, 120, 4661−4667

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    pubs.acs.org/JPCB http://dx.doi.org/10.1021/acs.jpcb.6b03130

  • compounds. VLE provide valuable information, from a fundamental standpoint, regarding the nature of interionic interactions simultaneously in the liquid phase and the vapor phase. The VLE data are necessary to assess the potential performance of different RTILs as lubricants, coatings, and separating agents. As discussed above, the stability of the pyridinium-based RTILs allows one to measure their saturated vapor pressure, prior to decomposition, and investigate experimentally vapor composition. The aggregation state of ions in the vapor phase of RTIL is

    still debated actively.30,32−35 Much of the uncertainty arises from experimental hurdles, since the work has to be performed at high temperatures and low vapor pressures of RTILs. Trace impurities can introduce significant biases to the results. Computer simulations,36−45 such as molecular dynamics (MD), allow one to avoid the enumerated technical problems and provide an atomically precise description of the vapor phase. However, reliable interaction potentials are required, which have to work equally well over the entire temperature range of interest. An insightful simulation study was published by Chen and

    co-workers.46 Vaporization enthalpy and vapor-phase species were obtained from MD simulations and ab initio calculations of the 1,1,3,3-tetramethylguanidinium-based RTILs. According to the authors, both electrostatic and van der Waals interionic interactions contribute critically to the vaporization behavior. Additionally, changes of ionic configurations upon vaporization also appear important and should be accounted for. Chaban and Prezhdo40 reported MD simulations to observe

    ionic vapor in real time. They concluded that, while both neutral and charged species were present in the vapor phase of 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonate)- imide [EMIM][TFSI] and N-butylpyridinium tetrafluoroborate [BPY][BF4], the neutral ion pair dominated in both RTILs. Accurate MD simulations of 1-butyl-3-methylimidazolium hexafluorophosphate with an emphasis on critical parameters were contributed by Weiss47 very recently. Mass-spectrometry measurements and density functional

    theory calculations of Eberlin and co-workers48 agree with the above-mentioned theoretical results, suggesting that the vapor phase of [EMIM][BF4] contains not only a single ion pair but also dimers and trimers. Additional interesting investigations of the ionic vapor species employ analytical spectrum meth- ods.49,50

    The present work characterizes, for the first time, the vapor− liquid interfaces for the three pyridinium-based ILs, [BPY]- [PF6], [BPY][TF], and [BPY][TFSI], at relatively high temperatures, 700−850 K, using atomically precise classical MD simulations. The interfaces are explicitly implemented in the simulated MD cells (Figure 1), and the exchange of molecules between the liquid phase and the vapor phase is observed in real time. The RTILs chosen for this study represent different anion structures, responsible for varying physicochemical properties of RTILs.

    ■ METHODOLOGY Classical MD simulations in the constant temperature, constant volume ensemble were used to derive molecular trajectories corresponding to VLEs of [BPY][PF6], [BPY][TF], and [BPY][TFSI]. The constant temperature was maintained by the velocity rescaling thermostat (relaxation constant 200 fs), which generates a correct velocity distribution for a simulated ensemble of particles.51 The integration time-step was set to

    1.0−2.0 fs depending on the simulated temperature. Accord- ingly, the equations of motion were propagated for 50−100 ns. The first 5 ns of each MD simulation were regarded as equilibration and excluded from subsequent statistical analysis