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2009, 113, 5690–5693 Published on Web 04/03/2009
Large Aggregated Ions Found in Some Protic Ionic Liquids Danielle F. Kennedy*,† and Calum J. Drummond†,‡,§ CSIRO Molecular and Health Technologies, PriVate Bag 10, Clayton MDC, Victoria, 3169 Australia, and CSIRO Materials Science and Engineering, PriVate Bag 33, Clayton MDC, Victoria, 3169, Australia ReceiVed: January 28, 2009; ReVised Manuscript ReceiVed: March 2, 2009
Large aggregated parent ions, for example, C8A7+ (C ) cation and A ) anion), have been observed within some protic ionic liquids (PILs) using electrospray ionization mass spectrometry (ESI-MS). We have shown that the formation and size of aggregates is dependent on the nature of the anion and cation. Solvent structuring in select PILs through aggregation can contribute to their classification as “poor ionic liquids” and can also strongly influence the entropic component to the free energy of amphiphile self-assembly in select PILs. Ionic liquids (ILs) are frequently described as being comprised entirely of ions in their liquid state.1 They usually consist of a relatively large organic cation and a small inorganic anion, but there are other subclasses of ILs including protic ILs which do not fit this simplified description. ILs have found application across all fields of chemical research and recently in industrial applications.1 The applications of ILs have been extensively reviewed in the literature.2-12 Protic ionic liquids (PILs) are formed by the stoichiometric reaction of a Brønsted acid and a Brønsted base. The first reported ionic liquid was, in fact, a protic ionic liquid. Ethanolammonium nitrate was reported by Gabriel in 1888,13 followed by ethylammonium nitrate (EAN) in 1914.14 EAN remains the most highly studied protic room-temperature ionic liquid. What sets PILs apart from other ILs is the presence of an available proton which is able to promote extensive hydrogen bonding. Despite extensive studies of the physicochemical properties of PILs, the bulk phase structure of the materials remains poorly understood.2,15-18 PILs have been described as “poor ionic liquids”.19 Indeed, they exhibit conductivities significantly lower than that of KCl(aq) on the Walden plot, log(equivalent conductivity) versus log(fluidity).2,19 Conductivity is a transport property which is dependent on many factors, including the number density of ions, their size and charge, as well as the viscosity of the media. One reason proposed for the relatively low conductivity exhibited by PILs is low ionization, that is, the presence of quantities of neutral acid and base.2,20 However, aggregation, the presence of net neutral complexes, or more extensive solvent structuring may also cause deviation from ideal behavior. Recently, it has been reported that many PILs mediate hydrocarbon-solvent interactions and promote amphiphile self-assembly.3,15,16,21,22 The driving force for the formation of self-assembled aggregate structures has been attributed to an entropic contribution to the free energy of association, analogous to the hydrophobic effect in water.22 This explanation suggests * To whom correspondence should be
[email protected]. † CSIRO Molecular and Health Technologies. ‡ CSIRO Materials Science and Engineering. § E-mail:
[email protected].
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that PILs that can provide a media for amphiphile self-assembly possess a degree of solvent-solvent interaction that is stronger than the solvent-hydrocarbon(amphiphile) interaction. In order to build a fundamental understanding of the solvophobic effects in PILs, it is necessary to develop an understanding of the solvent structuring in PILs. Solvent structuring within ILs may explain characteristics of many ILs, including the poor conductivity of PILs as well as their promotion of amphiphile self-assembly, solubilizing ability, and reactivity. The bulk-phase structure of the PILs, EAN, and propylammonium nitrate (PAN) was investigated using smallangle neutron scattering by Atkins and Warr.23 They found that even in these ILs with small alkyl groups that there was evidence of nanoscale heterogeneity which they attributed to a locally smectic structure. Mass spectrometry has previously been used to investigate the nature of ILs in both the gas and solution phase.24-32 Electrospray ionization mass spectrometry (ESI-MS) has been employed to investigate the species within ILs both neat and in solution.25,28,31-33 ESI is the softest ionization technique for mass spectroscopy, allowing the analysis of weakly bound species in solution. Using ESI-MS, aggregates or clusters of the constituent ions of aprotic ILs have been observed previously.25,28,31,33 The ESIMS of the neat ionic liquid [N-butylpyridinium][BF4] displayed aggregates; within the 30-100 °C temperature range, C2A+ (C ) cation; A ) anion) was the most abundant ion observed, with minor amounts of larger aggregates.25 The aggregation of [1-butyl-3-methylimidazolium][A] in solution (A ) BF4 and NTf2) was reported by Kragl et al.28 The main aggregate observed with these ILs was also C2A+, with only minor traces of larger aggregates. Dupont and Eberlin et al. have also reported the aggregation of imidazolium ILs in solution.33 The imidazolium salts investigated showed small parent aggregates C2A+ or C3A2+ and then exhibited exponential decay in the abundance of aggregates with increasing aggregate size. Also observed were the favored assemblies [(C)2(BF4)3]- and [(C)5A4]+ (C ) imidazolium ion; A ) CF3CO2-, BF4-, PF6-, and BPh4-), which were slightly more prevalent than would be predicted with the normal exponential decay. Brønsted acidic ionic liquids, SO3H functionalized aprotic ionic liquids, have also recently been
Published 2009 by the American Chemical Society
Letters
Figure 1. ESI-MS positive ion mode of anhydrous CH3CH2NH3NO3 (EAN). A series of singly positively charged aggregates is observed, with the most prominent being C8A7 (C ) cation (CH3CH2CH2NH3+) and A ) anion (NO3-)). Larger doubly charged aggregates are indicated by [ and triply charged aggregates by +.
analyzed using ESI-MS; however, no significant aggregation larger than C2A+ or C3A2+ was reported.32 Fast atom bombardment (FAB) MS has been used previously for the analysis of PILs.24,30 The method of ionization employed in this technique is rated as harsh, and any aggregates present in solution are likely to be disrupted by the process. The alkylammonium nitrates analyzed by FAB-MS did show aggregates in trace amounts, up to C5A4+ for EAN (2%) with C2A+ as the predominant aggregate.30 Here, we discuss the application of ESI-MS to the investigation of aggregation in PILs. The ESI-MS of several neat roomtemperature PILs and a series of solutions of PILs were obtained. The ESI-MS of neat ethylammonium nitrate (EAN) in the positive ion mode displayed an interesting aggregation pattern, Figure 1. Unlike the aggregate patterns previously reported for the aprotic alkyimidazolium ILs,25,28,33 EAN does not display C2A+ as the major aggregate with exponential decay in the relative intensity of aggregates with increasing aggregate size. The spectrum of neat anhydrous EAN showed a series of singly charged aggregates of the type CnAn-1+, with C8A7+ as the most prominent by close to an order of magnitude (Figure 1). Smaller and larger singly charged aggregates were also observed with only minor amounts of larger doubly and triply charged aggregates extending beyond the analysis range, 2000 m/z. The single cation C+ (m/z ) 46) and the smallest aggregate C2A+ were observed with approximately the same relative intensity when the lower mass range was analyzed; however, in the higher mass range, C2A+ is only present as a minor amount, indicating that in the neat ionic liquid, the overall amount of free C+ is small. Analysis of very similar neat PILs, propylammonium nitrate (PAN) and also butylammonium nitrate (BAN, 2 wt% H2O by KF titration), in the positive mode showed identical patterns of aggregation. These ILs differ only in the length of the alkyl chain. The major ion apparent in both of these ILs was the stable aggregate C8A7+. There were also similar relative intensities of the other smaller aggregates to those observed in EAN, indicating that the stability of C8A7+ is not dependent on the alkyl chain length in the monoalkylammonium cations investigated. The ESI-MS of neat EAN, PAN, and BAN in the negative ion mode showed a different aggregation pattern. Aggregates up to A11C10- were equally highly abundant, with smaller amounts of other aggregates present. Minor amounts of larger
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Figure 2. Stacked plot of ESI-MS spectra of CH3CH2NH3NO3 (EAN) at a series of concentrations, 1.4 (front), 3.5, 8.8, 22, and 55 mM and 0.138, 0.346, 0.863, and 2.16 M in MeOH and neat EAN (back). Spectra are offset for clarity.
doubly charged aggregates were observed, with no triply charged aggregates observed within the mass range. The negatively charged aggregates do not have a particularly stable abundant aggregate analogous to the stable C8A7+ observed in nitrate PILs in the positive ion mode. This indicates that these neat PILs consist of a polydisperse mixture of aggregated ions, with the exception of the prominent C8A7+. This is very different from previous reports for the imidazolium ILs.33 Dupont and Eberlin reported aggregation patterns in both the negative and positive ion modes for the ESI-MS of imidazolium BF4 ILs, with an exponential decay in abundance with increasing mass evident.33 These differed only in nature of the favored ion. ESI-MS conditions were varied in order to confirm that the observed stable aggregate C8A7+ was not an artifact. Varying the ESI conditions had no affect on the aggregation pattern observed; C8A7+ remained the most stable and abundant ion. The MS of MeOH solutions of EAN were obtained over a wide concentration range, Figure 2. At concentrations as low as 1.4 mM EAN, C8A7+ remained the most abundant ion; below this concentration, C+ and C2A+ were the only species observed. The only other change over the concentration range studied was a significant decrease in the abundance of the C2A+ and C3A2+ ions with increasing concentration, Figure 2. The conductivity of the EAN solutions was also measured as a means of assessing aggregation within the bulk, and significant deviation from ideal ion conductivity behavior was observed, consistent with a stepwise aggregation process (Figure 1, Supporting Information). The analysis solvent for MS was also varied. The ESI-MS of EAN was obtained in i-PrOH, H2O, MeCN, and acetone. The aggregation pattern observed for EAN in these solvents was consistent with that observed for neat EAN in all of these solvents, with the exception of acetone, in which no aggregates were observed. The lack of aggregation of EAN in acetone is considered to be due to the fact that acetone can only act as a hydrogen bond acceptor and not a hydrogen bond donor and is therefore more likely to disrupt an aggregate formed through hydrogen bonding much more severely than water or methanol. The effect of varying the anion within the PIL was also investigated. The positive and negative ESI-MS were acquired for ethylammonium formate (EAF) and ethylammonium lactate (EAL). Neither the positive nor negative ion mode for these ILs showed any aggregation. The number of nitrate fused salts which are liquid at room temperature is small, but many are liquid at slightly higher
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TABLE 1: Comparison of the Relative Intensity of Prominent Peaks in the Positive Mode ESI-MS of Various PILs in MeOH, 10 mg/0.5 mL, over Two Ranges, 30-200 and 100-1000 m/z low mass range IL
C
b
100 80 100 100 100 100 45 100 30 20
EAN PAN BAN b PeAN DEAN TEAN DEOAN TEOAN 2MEAN 22HEEAN
C2Ha 65
Os 100 70 100 Os
high mass range C2A 90 100 Os 20 Os Os 35 Os 100
C2H
100 70 22 100
C2A
C3A2
C4A3
C5A4
C6A5
C7A6
C8A7
4 5 7 2 86 100 30
15 10 9 4 100
10 7 5 3 52
8 6 5 2 6
13 7 5 4
12 6 7 3
100 100 100 100
30
10
6
3
3
2
90 60
17 10
10 10
35 30
15 10
100 5
a Os ) Off scale. b Amount: 1 mg/0.5 mL. Abbreviations: EAN, ethylammonium nitrate; PAN, propylammonium nitrate; BAN, butylammonium nitrate; PeAN, pentylammonium nitrate; DEAN, diethylammonium nitrate; TEAN, triethylammonium nitrate; DEOAN, diethanolammonium nitrate; TEOAN, triethanolammonium nitrate; 2MEAN, 2-methoxyethylammonium nitrate; 2HEEAN, 2-(2-hydroxyethoxy)ethylammonium nitrate.
temperatures;2 therefore, their aggregation patterns are also of interest. In order to investigate the influence of the structure on the aggregation pattern, the ESI-MS were obtained for a range of alkylammonium nitrate solutions, Table 1. MeOH solutions of EAN, PAN, BAN, and pentylammonium nitrate (PeAN) have C8A7+ as the most abundant ion, consistent with the neat ILs. Looking at the series EAN, diethylammonium nitrate (DEAN), and triethylammonium nitrate (TEAN), there is a decrease in the size of the favored aggregate with increasing steric bulk of the cation and also with the decreasing number of protons available for hydrogen bonding. For diethanolammonium nitrate (DEOAN) and triethanolammonium nitrate (TEOAN), the addition of an alcohol moiety to the alkyl chain appears to disrupt the aggregation of the ammonium salt with C2-H, the most prominent species. The inclusion of an ether moiety on the other hand does not disfavor aggregation, with a bimodal distribution of aggregates observed for 2-methoxyethylammonium nitrate (2MEAN), where C8A7+ and C3A2+ were both highly prominent ions. Including both an alcohol and a ether group into the ammonium salt led to the disruption of the C8A7+ aggregate but not the C3A4+ for the ionic liquid 2-(2-hydroxy-ethoxy)ethylammonium nitrate (22HEEAN). In all of the PILs studied, with the exception of DEOAN, the aggregate C9A8+ was absent. However, C10A9+ and larger aggregates were observed. Even when C9A8+ was observed in DEOAN, it was present with only a 2% relative intensity. At this stage, we are uncertain why the C9A8+ aggregate is as disfavored as the C8A7+ aggregate is favored. The negative mode MS of all of the ionic liquids studied show polydisperse mixtures of aggregates with no overstabilized negative aggregates observed. Aggregation within the PILs studied is evident from their ESI-MS. Electrostatic coloumbic interactions are presumably screened because of the high effective ionic strength of PILs. The formation of these aggregates is most probably through hydrogen bonding between the anions and cations and is therefore primarily dependent on the nature of the ions. The nitrate anion, small, tripodal, and highly charged, is suitable for forming a network of hydrogen bonds with the available protons of the ammonium cations, with PILs containing NO3capable of forming quite large aggregates. The larger formate and lactate anions with lower charge density did not support the formation of aggregates. For the monoalkylammonium nitrate salts, a polydisperse mixture of aggregates was
observed in both the positive and negative ion modes with the abundant aggregate C8A7+ identified. C8A7+ was found to be present irrespective of the length of the alkyl chain, despite the fact that you may have predicted larger aggregate sizes with increasing alkyl chain length due to stronger van der Waals interaction. Also, for di- and trialkylammonium salts, the decreased number of protons available for hydrogen bonding coupled with the increased steric bulk decreased the size of the stable aggregates observed. From the aggregates observed using ESI-MS, it can be concluded that select neat PILs consist of a polydisperse mixture of aggregated ions. Aggregates can contribute to the relatively poor conductivity exhibited in some protic ionic liquids, resulting in their classification as “poor ionic liquids”. Solvent structuring through aggregation can influence the formation of amphiphile self-assembly structures in PILs. Work is ongoing to understand the structure of these aggregates, the mechanism of their formation and whether these trends in aggregation pattern observed correlate to the structure and properties of the bulk liquids. Acknowledgment. C.J.D. is the recipient of an Australian Research Council (ARC) Federation Fellowship. This work was also partly supported by an ARC Discovery Project Grant DP0666961. Supporting Information Available: Experimental details for the synthesis and characterization of the PILs, conductivity measurement, ESI-MS methodology, and the ESI-MS for all species in Table 1. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Wasserscheid, P.; Welton, T. Ionic Liquids in Synthesis; WileyVCH: Weinheim, Germany, 2003. (2) Greaves, T. L.; Drummond, C. J. Chem. ReV. 2008, 108, 206–237. (3) Greaves, T. L.; Drummond, C. J. Chem. Soc. ReV. 2008, 37, 1709– 1726. (4) Wilkes, J. S. Green Chem. 2002, 4, 73–80. (5) Gu, Y. L.; Peng, J. J.; Qiao, K.; Yang, H. Z.; Shi, F.; Deng, Y. Q. Prog. Chem. 2003, 15, 222–241. (6) Koel, M. Crit. ReV. Anal. Chem. 2005, 35, 177–192. (7) Scammells, P. J.; Scott, J. L.; Singer, R. D. Aust. J. Chem. 2005, 58, 155–169. (8) Muzart, J. AdV. Synth. Catal. 2006, 348, 275–295. (9) Pandey, S. Anal. Chim. Acta 2006, 556, 38–45. (10) Chowdhury, S.; Mohan, R. S.; Scott, J. L. Tetrahedron 2007, 63, 2363–2389.
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