LETTER pubs.acs.org/JPCL
Binary Ionic Liquids with a Common Cation: Insight into Nanoscopic Mixing by Infrared Spectroscopy Jean-Michel Andanson, Matthias Josef Beier, and Alfons Baiker* Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences, ETH Zurich, H€onggerberg, HCI, Wolfgang-Pauli-Strasse 10, CH-8093 Zurich, Switzerland
bS Supporting Information ABSTRACT: Mixing of ionic liquids (ILs) can expand the range of properties and versatility of this new class of solvents. Exploiting this potential requires a fundamental understanding of the mixing behavior of ILs. In this study, we have investigated binary IL mixtures involving a common cation (1-butyl-3-methylimidazolium) with two commonly used anions (chloride, bromide, tetrafluoroborate, hexafluorophosphate or bis(trifluoromethanesulfonyl)imide ([NTf2])) by means of IR spectroscopy. Upon mixing the ILs, significant changes in the anion IR bands are induced, indicating mixing at the molecular level for highly symmetric anions. Furthermore, density functional theory (DFT) calculations show water to be a suitable probe molecule: IR bands of anion-1 3 3 3 HOH 3 3 3 anion-2, anion-1 3 3 3 HOH 3 3 3 anion-1, and anion-2 3 3 3 HOH 3 3 3 anion-2 are well suitable for inspection and quantification of the coexistence of each type of cluster. In contrast to some previous spectroscopic investigations, this work suggests good molecular mixing and the absence of nanosegregation in these binary ILs. SECTION: Macromolecules, Soft Matter
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n recent years, research on ionic liquids (ILs) has gained considerable attention mainly due to the potential of these materials as solvents in green chemistry. ILs are practically nonvolatile solvents due to their extremely low vapor pressures, and their structural diversity allows tailored solvent applications.1 Already in 2006, Seddon et al. observed the first example of immiscible hydrophobic and hydrophilic ILs at room temperature.2 Despite the fact that ILs have been under tremendous scrutiny, studies of binary ILs are so far very rare. One of the most interesting results covers the gas separation of CO2 and N2, which is more efficient in a 5 and 10 mol % mixture of [C2mim] (1-ethyl-3-methyl-imidazolium) [NTf2] in [C2mim][BF4] than in both pure ILs.3 More examples of binary ILs can be found in several studies47 and the references therein. Optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) of binary ILs mixtures with a common cation correspond well to the weighted sums of the spectra of individual ILs, which was interpreted as evidence for the absence of molecular interactions.8 To explain the absence of molecular interactions, the authors postulated a model with locally ordered domains with blocks similarly to block copolymers, for the mixture of Br and [NTf2] anions with [C5mim]+ (1-methyl3-pentyl-imidazolium) as a common cation.6 In other words, the probability of finding two identical anions next to each other is higher than what would be expected from statistical ideal mixing. In order to gain further insight into this mixing behavior, here we analyzed similar IL mixtures by attenuated total reflection Fourier r 2011 American Chemical Society
transform infrared (ATR-FT-IR) spectroscopy in the mid-IR range. This technique has proved to be efficient to examine molecular structures of pure ILs9 as well as the interaction between ILs and water1012 or ILs and CO2.13,14 Mixtures of [C4mim] (1-butyl-3-methylimidazolium) [PF6]/ [C4mim]Cl and [C4mim][PF6]/[C4mim][BF4] were prepared with molar ratios of 25:75, 50:50, and 75:25. The effect of the mixing on some of the strong IL vibrational bands accessible in the mid-IR range is shown in Figure 1. The band at around 1170 cm1 is mainly related to [C4mim]+,1517 while the main [BF4] band appears in the 11001000 cm1 region.9,17 The [PF6] band in the range of 900800 cm1 corresponds mainly to PF stretching modes.16,18 The [PF6] and [BF4] bands change drastically when changing the ratio of the two ILs. These results indicate that the molecular environment of the anions is altered when changing the [BF4] and [PF6] ratio. Analogous changes were observed when adding supercritical CO214,18 or solvents with different dielectric constant19 to [C4mim][PF6] or [C4mim][BF4]. The origin of this phenomenon is still under investigation. Other IR bands change linearly with the concentration of the two anions. The same experiment was also performed with [C4mim][Cl]/[C4mim][PF6] mixtures where the [PF6] band exhibited a very similar evolution (same shape Received: September 29, 2011 Accepted: November 4, 2011 Published: November 10, 2011 2959
dx.doi.org/10.1021/jz201323a | J. Phys. Chem. Lett. 2011, 2, 2959–2964
The Journal of Physical Chemistry Letters
LETTER
Figure 2. Selected IR anion bands for [C4mim][NTf2]/[C4mim][Br] mixtures with molar ratios of 0:100, 25:75, 50:50, 75:25, and 100:0.
Figure 1. Main IR anion bands of dry (
