J. Phys. Chem. B 2008, 112, 14927–14936
14927
Multiprobe Spectroscopic Evidence for “Hyperpolarity” within 1-Butyl-3-methylimidazolium Hexafluorophosphate Mixtures with Tetraethylene Glycol Abhra Sarkar,† Shruti Trivedi,† Gary A. Baker,*,‡ and Siddharth Pandey*,† Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi - 110016, India, and Chemical Sciences DiVision, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: May 24, 2008; ReVised Manuscript ReceiVed: September 09, 2008
A hybrid, potentially green solvent system composed of tetraethylene glycol (TEG) and the ionic liquid 1-butyl3-methylimidazolium hexafluorophosphate ([bmim][PF6]) was investigated across all mole fractions with regard to the solvent properties of the mixture. For this purpose, a suite of absorbance- and fluorescence-based solvatochromic probes were utilized to explore solute-solvent and solvent-solvent interactions existing within the [bmim][PF6] + TEG system. These studies revealed an interesting and unusual synergistic solvent effect. In particular, a remarkable “hyperpolarity” was observed in which the ET value, comprising dipolarity/ polarizability and hydrogen bond donor (HBD) acidity contributions, at intermediate mole fractions of the binary mixture well exceeded that of the most polar pure component (i.e., [bmim][PF6]). Independently determined dipolarity/polarizability (π*) and HBD acidity (R) Kamlet-Taft values for the [bmim][PF6] + TEG mixtures were also observed to be anomalously high at intermediate mole fractions, whereas hydrogen bond acceptor (HBA) basicities (β values) were much more in line with the ideal arithmetic values predicted on a mole fraction basis. Two well-established fluorescent polarity probes (pyrene and pyrene-1-carboxaldehyde) further illustrated notable hyperpolarity within [bmim][PF6] + TEG mixtures. Moreover, the steady-state fluorescence anisotropy of the molecular rotor rhodamine 6G and the excimer-to-monomer fluorescence ratio exhibited by the fluidity probe 1,3-bis-(1-pyrenyl)propane demonstrated that solute rotation and microfluidity within the [bmim][PF6] + TEG mixture were significantly reduced compared with expectations based on simple solvent mixing. A solvent ordering via formation of HBD/HBA complexes involving the C-2 proton of the [bmim+] cation and oxygen atoms of TEG, as well as interactions between [PF6-] and the terminal hydroxyl groups of TEG, is proposed to account for the observed behavior. Further spectroscopic evidence of strong intersolvent interactions occurring within the [bmim][PF6] + TEG mixture was provided, inter alia, by substantial frequency shifts in the [PF6-] asymmetric stretching mode observed in the infrared spectra as TEG was incrementally added to [bmim][PF6]. Overall, our observations contribute to a growing literature advocating the notion that ionic liquids and certain organic solvents form ordered, nanostructured, or microsegregated phases upon mixing. Introduction Because of their remarkable properties and ubiquity in the current literature, ionic liquids (ILs) are enjoying growing application in a multiplicity of research areas.1 Although not always stated explicitly, as their popularity has increased several restrictions associated with their use as solvents have emerged.2 Among these limitations lies the fact that the solubility of many solutes is quite limited in archetypical ILs and, as a result, the ability to systematically modify the physicochemical properties relative to a given IL is vital to their continued development and expansion into additional growth areas. To better tune their physicochemical properties and hence enhance their overall utility, a number of researchers have begun to focus on hybrid systems.3 For instance, the addition of a cosolvent (preferably one that is “green”) or the mixture of two or more ILs may suitably alter the solvent features of an IL for a specified purpose. Within this context, we have investigated the effects of added water, ethanol, aqueous-ethanol, poly(ethylene glycol) * To whom correspondence should be addressed. E-mail: (S.P.)
[email protected]; (G.A.B.)
[email protected]. Phone: +9111-26596503. Fax: +91-11-26581102. † Indian Institute of Technology Delhi. ‡ Oak Ridge National Laboratory.
(PEG), carbon dioxide, and other ILs on the relevant solvent properties of ILs in terms of their interaction with model solutes.4,5 In particular, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] has frequently formed the basis for these studies, due to the vast inventory of knowledge compiled on this particular IL. It is important to consider here that, under ordinary conditions, the miscibilities of water and ethanol as cosolvents in [bmim][PF6] are limited to ∼2.1 and ∼9.7 wt %, respectively.3e,4a Consequently, the use of these cosolvents, while favorable from an environmental standpoint, allows for only slight modulation in the physicochemical properties of [bmim][PF6]. What’s more, investigations and applications employing these cosolvents are fundamentally restricted in the sense that only a partial cosolvent range can be explored. In a recent preliminary study, we noted unusual behavior for polarity-sensitive absorption probes dissolved in binary mixtures formed from PEGs and [bmim][PF6] that hint toward prominent solute-solvent and/or solvent-solvent interactions.5h In order to further elucidate this phenomenon, a binary system consisting of [bmim][PF6] and tetraethylene glycol (TEG) was investigated in the present study in considerably expanded detail. This choice was guided by the facts that TEG, like its higher molecular weight cousins the PEGs, is an odorless, nonvolatile, hydrophilic
10.1021/jp804591q CCC: $40.75 2008 American Chemical Society Published on Web 10/28/2008
14928 J. Phys. Chem. B, Vol. 112, No. 47, 2008 (hygroscopic), and environmentally friendly liquid that exhibits complete miscibility with a number of imidazolium-based ILs.6 That is, visually, [bmim][PF6] + TEG mixtures remain a single phase across the entire composition range. Furthermore, TEG has assorted applications, being an industrially important intermediate and/or ingredient in polyester resins, antifreezes/ coolants, plasticizers for nitrocellulose finishes, humectants in tobacco and textiles, lubricants for rubber, heat transfer fluids, and in gas dehydration and treatment.7 As a result, we anticipated that hybrid systems based on mixing ILs with TEG would deliver environmentally accountable, lower-cost solvent platforms with attractive physicochemical properties, including adjustable hydrophilicity. In addition, we expected that the fact that TEG has zero dispersity in molecular weight (i.e., the PEGs used prior possessed polydispersity indices above unity)5h might allow us to more readily discern trends in a mixed solvent system. In order to gather detailed information on the solute-solvent and solvent-solvent interactions existing within [bmim][PF6] + TEG, we have utilized the various Kamlet-Taft solvatochromic probes used in our earlier work,5h as well as an assortment of additional fluorescence probes sensitive to various solvent characteristics ranging from dipolarity to microviscosity.8 Corroborative evidence furnished by Fourier transform infrared (FTIR) spectroscopy also supported a molecular view of the various solute-solvent/solvent-solvent interactions occurring within the [bmim][PF6] + TEG mixture. In this study, a surprising “hyperpolarity” was observed in which the dipolarity, polarizability, and HBD acidity at intermediate compositions were atypically high, well surpassing the values determined in the pure components. The HBA basicity behavior in the [bmim][PF6] + TEG mixture was, on the other hand, not unusual. We put forward hydrogen bond interaction between the ether unit of TEG and the acidic C-2 hydrogen on the imidazolium ring as well as between the [PF6-] anion and the -OH ends of TEG in explanation of these observations. Together with our previous results on [bmim][PF6] + PEG, a self-consistent view of the behavior within the [bmim][PF6]/ polyether system is now emerging. These observations contribute to a growing literature advocating the notion that ILs and certain organic solvents spontaneously form ordered, nanostructured, or microsegregated phases upon mixing. Experimental Section Materials. The IL [bmim][PF6] (ultra pure, halide content
