Mass Spectrometric Analysis of Ionic Liquids: From Fundamentals and Analytical Methods to Modeling of Spacecraft Thrusters

Ionic liquids (ILs)¹ encompass great structural diversity (Figure 1a) and have gained interest for many applications (Figure 1b). Property tuning and rational design of ILs is possible due to their substitutable organic scaffolds and variety of possible anions. Understanding the properties and behaviors of ILs at a molecular level is a task for which mass spectrometry (MS) is well suited. Active areas of MS-based research include fundamental studies aimed at understanding clustering, dissociation, and decomposition; analytical method development, including for environmental contamination detection; and applications-specific studies, such as modeling electrospray spacecraft propulsion. (Figure 1c) summarizes some MS methods and applications relevant to the study of ILs.

ILs as Spacecraft Propellants

Low vapor pressure makes ILs compatible with exposure to space’s vacuum, and their ionic nature makes them efficient for electrospraying. ILs as next-generation spacecraft propellants for electrospray thrusters exploit these features. Electrospray ionization (ESI) is familiar to mass spectrometrists as an ambient ionization method (Figure 2a); ESI for spacecraft thrusters occurs in vacuum and may use a capillary (Figure 2b) or an externally wetted needle (Figure 2c). The size and stability of droplets, clusters, or ions emitted during the electrospray process have ramifications on thruster performance. Characterization of these emissions is achieved using MS detection, among other performance diagnostics.²

ILs as Potential Environmental Contaminants

While ILs were initially touted as environmentally benign and nontoxic, it has become clear that more nuance is warranted^3,4 and that different structures lead to different physical properties, biological effects, and environmental risks. Bolstering environmental concerns,⁵ imidazolium-based IL cations have been found in the environment and human serum,^6–8 and a fluorinated anion associated specifically with ILs was found during screening for organic fluorine, such as per- and polyfluoroalkyl substances (PFAS), suggesting IL species are contributing to total organic fluorine measurements.⁹ The extent and impact of environmental contamination have yet to be fully explored, and MS will play a key role in this endeavor.

Mass Spectrometric Analysis of ILs

Clusters Formed via Electrospray Ionization

Characterization of IL clustering has potential implications for understanding the role of clusters on bulk properties and for modeling electrospray thruster performance. The most straightforward experiments consist of observing and characterizing the clusters formed and relating the observed distributions to IL structure and/or experimental conditions. Additional experiments have further investigated clusters through ion spectroscopy^10,11 (for which a few citations are provided, but which is generally beyond scope) and dissociation experiments.

Using atmospheric pressure capillary ESI (Figure 2a), Eberlin and coauthors reported the extensive formation of gaseous supramolecules of imidazolium-based ILs from solution.¹² They observed both singly charged assemblies, [C_nA_n+1]^- where C = cation and A = anion, as well as higher charged assemblies, [C_nA_n+2]^-2 and [C_nA_n+3]^-3, or the positively charged analogues. While intensity tends to decrease as n increases, so-called magic number cluster sizes defied the trendline, indicating special stability with potential relevance to clustering at the mesoscale in liquids. Vacuum ESI of neat ILs (Figure 2b and c) is useful as a model of electrospray-based spacecraft thrusters, where knowledge and, ideally, tuning of the emission regime are important. Using capillary emitters, under certain conditions, the pure ion evaporation regime could be achieved¹³ and flow rate can allow cluster size tuning.¹⁴ Externally wetted electrospray emitters (Figure 2c) have attractive advantages from a thruster design perspective and have been successfully demonstrated in vacuum.¹⁵ Externally wetted sources have been shown to produce desirable purely ionic emissions for ILs that had previously only been reported to operate in the mixed droplet/ion regime from capillary sources.¹⁶ 2-hydroxyethylhydrazinium nitrate (HEHN) is an example of an energetic protic IL that has been electrosprayed from an externally wetted vacuum ESI source.¹⁷ The protic nature of this system allowed for additional clusters to be formed, such as a proton-bound dimer of the cation and a neutralized version of the cation.

Generally, aprotic IL clusters of the form [C_n+1A_n]⁺ dissociate via collision-induced dissociation (CID) by loss of neutral ion pair(s) (Figure 3a). For mixed clusters (Figure 3b), the nature of the product ion (or relative abundances, if both are produced) provides insights into which cation has the lower affinity for the anion (or vice versa in negative mode). Protic ILs can produce additional dissociation pathways through proton transfer between the cation and anion to neutralize the species (Figure 3c). As an example, HEHN clusters have been subjected to CID-MS, where loss of hydroxyethylhydrazine, nitric acid, or “ion pair” was possible.¹⁸ These insights are important for modeling such species as candidates for electrospray propulsion and for understanding the potentially increased volatility due to small molecule loss. A 2023 report by Zhou et al. on HEHN investigated additional clusters, presented additional modeling, and more directly applied data to inform potential impacts on electrospray propulsion performance.¹⁹ Providing insights in the bridging between protic and aprotic systems, alkylammonium nitrate clusters were studied as a function of ammonium substitution and cluster size.²⁰ The trend was that an increasing degree of alkylation decreases isolated loss of nitric acid or alkylammonia, suggesting ion pair loss consistent with aprotic ILs. For ethylammonium nitrate, nitric acid loss was more intense than ethylammonia loss, and loss of small neutrals was most intense for the smallest cluster, with clusters beyond 2 ion pairs predominantly dissociating via (apparent) ion pair loss.

Several studies have been reported with the aim of studying binding affinities, including determining E_1/2 values from energy-resolved CID to evaluate relative affinities of a series of anions for specific cations.²¹ Absolute binding energies have been pursued using threshold CID (TCID),^22,23 with competitive TCID improving precision.^23,24

Isolated Ion Dissociation and Characterization

Individual ions are the building blocks upon which ILs are designed. They are typically the terminal product of cluster disruption and are often the target analyte for environmental and bioaccumulation studies. Several reports specifically focusing on the CID of ILs have been published. Common imidazolium-based IL cations were studied early on,²⁵ and our group added a focus on exploring how additional functional groups on substituents affected dissociation pathways.²⁶ It is clear that functionalized substituents provide additional dissociation pathways that can be useful for the identification of the substituent, and that the N-heterocycle typically retains the charge to form terminal product ions similar to the alkyl-substituted “standard” (m/z 83 for methyl-imidazolium IL cations). Benzyl-substituted methylimidazolium deviated from this, forming the tropylium (or benzylium, m/z 91) product ion. Additional studies have focused on specific design motifs under the umbrella of imidazolium-based ILs, with examples including zwitterionic liquids²⁷ and sulfur-containing ILs.²⁸ Beyond the common imidazolium, ILs with other cation scaffolds have been investigated by CID-MS, for example, with a focus on comparing aromatic and nonaromatic N-heterocyles (NHCs).²⁹ Notably, nonaromatic N-heterocyclic IL cations produce significant cross-ring cleavage (or ring opening followed by additional dissociation), complicating the spectra but also providing structural “fingerprints” that may be useful for analytical applications. A 2005 report by Milman and Alfassi demonstrated detection and characterization of IL cations and anions by ESI-MS and tandem MS.³⁰ While the utility of ESI-MS/MS was noted, some challenges, including low surface activity and/or limited fragmentation of some species, were also highlighted.

Despite these challenges, specific applied methods with, for example, environmental or health monitoring focus have been reported with MS-based analysis of targeted ILs from specific matrices, including quaternary ammonium compounds from wastewater and lake sediments,³¹ and alkyl-imidazolium species from the tissue of marine mussels³² and from human serum.³³ A review article—which focused heavily on chromatographic methods and environmental applications—noted that there is a dearth of adequate methods for the scope of IL species and matrices of relevance.³⁴ Gaining sufficient understanding of the ESI and CID-MS behavior (as well as chromatographic performance) of IL species of diverse structures to improve analytical methods is an ongoing endeavor.

Decomposition Products

Transformation due to aging, high temperature exposure, or other stresses may affect the performance of ILs in their applications and form related species that may be environmentally released or otherwise harmful. While early reports consistently indicated very high thermal stabilities (often greater than 400 °C), additional studies focused on longer exposure to much lower temperatures showed substantial decomposition at much lower temperatures.³⁵ An effort is now underway to better characterize the stability of ILs as a function of their structure under a range of applications or storage-relevant conditions, as well as under extreme conditions. Here, we highlight several examples illustrating how MS provides crucial molecular‑level insight into the resulting decomposition products.

Thermogravimetric analysis (TGA)-MS and other complementary experiments of neat ILs indicate that alkyl-imidazolium cations paired with halides undergo nucleophilic attack of the anion on the cation at the N—C bond, to form a singly alkylated imidazole and an alkylhalide.^36–38 Our group has recently reported on the pyrolytic decomposition and complementary CID-MS of a range of nine ILs, encompassing four scaffolds (imidazolium, pyridinium, piperidinium, and pyrrolidinium) and substituents, including alkyl groups, benzyl groups, and ether groups (all with the chloride anion).³⁹ While CID-MS of the isolated cations typically became increasingly complex with functional substituents, pyrolysis predominantly proceeded through similar pathways irrespective of the functional group inclusions. This could be useful for predictive efforts. The benzyl substituent was again a notable outlier, as it produced evidence of a combination of multiple decomposition products (or reaction of a decomposition product with another cation) to form a product assigned as bibenzyl. This suggests that while general themes hold for many cases, it is worthwhile to explore across the structural landscape to find the bounds of these behaviors.

The anion also plays a crucial role in the observed bulk decomposition products. The decomposition of 1-ethyl-3-methylimidazolium (EMIM) bis(trifluoromethylsulfonyl)imide (Tf₂N) was studied using TGA-MS,⁴⁰ where it was found that the anion first decomposes, forming species such as NH₂ and F that can then attack the alkyl group of the imidazolium. In another study, the pyrolysis of imidazolium ILs paired with different anions suggests a shift in observed behavior between halides (producing alkylhalides) and other anions (BF₄, PF_6, and CF₃SO₃), which favored producing the corresponding alkenes.⁴¹

The aging of neat ILs held at both room temperature and 95 °C over 16 months was studied for four ILs with pyrrolidinium or imidazolium scaffolds, alkyl substituents, and either Tf₂N or FSI (bis(fluorosulfonylo)imide) anion using ion chromatography (IC) coupled to ESI-MS.⁴² Loss of either alkyl sidechain was observed and was consistent with other thermal decomposition results for halides, presumably due to a bromide impurity, according to the report. Decomposition products could then react with other cations to form multi-substituted species. While higher temperature conditions did lead to more significant decomposition, both temperature conditions led to some aging and were well under the reported thermal stability temperatures.

An apparatus to collect the nonvolatile and volatile products separately from thermal decomposition (up to 400 °C) of ILs for offline analysis has been reported and applied to various species, including di-imidazolium, di-pyrrolidinium, and di-phosphonium cations (with Tf₂N anions).⁴³ The apparatus and the nature of the ILs studied here are distinct. For the imidazolium-based dicationic ILs, loss of the terminal alkyl groups or cleavage of the linker was possible; for phosphonium species, additional oxidative pathways were observed; and species with PEG spacers were significantly more labile, with cleavage occurring at the oxygen.

Conclusions and Outlook

The mass spectrometric study of ILs has already made significant contributions to our understanding of IL properties, performance, and decomposition, and is poised to continue making an impact, with spacecraft thruster technologies and environmental contamination monitoring being two areas likely to lead the efforts and experience the greatest positive impact from the work. However, despite their promise, ILs remain relatively underexplored by MS compared to many other analyte classes. Some of the key areas where future advances are predicted include improved coverage of the structural diversity of ILs in MS-based studies; using molecular-level insights, fundamental principles, and molecular modeling to better predict properties, dissociation patterns, performance, and other parameters; rational development and improvement of analytical MS methods for IL detection based on such fundamental understanding; and studies directly aimed at environmental detection and environmental fate of ILs and IL decomposition species.

Acknowledgments

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial financial support of this research under a Doctoral New Investigator grant (PRF# 62556-DNI6). The authors acknowledge partial financial support under the Strategic Research Initiative (SRI) Faculty Seed Funding program of the Mississippi State University College of Arts & Sciences.

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