Interactions within a [Ionic Liquid + Poly(ethylene glycol)] Mixture Revealed by Temperature-Dependent Synergistic Dynamic Viscosity and Probe-Reported Microviscosity

Abstract

Mixtures of ionic liquid (IL) with poly(ethylene glycol) (PEG) may afford media with favorable properties. Dynamic viscosities of mixtures of a common and popular IL 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) with PEGs of average molecular weight (MW) 200 (PEG200), average MW 400 (PEG400), number-average MW M(n) 570-630 (PEG600), and number-average MW M(n) 950-1050 (PEG1000) over a complete composition range at 10° intervals in the temperature range of 10-90 °C are measured. The temperature dependence of the dynamic viscosity shows ([bmim][PF(6)] + PEG) mixtures to behave as Newtonian fluids and is found to follow Arrhenius-type behavior. In the IL-rich region, excess logarithmic viscosities for the ([bmim][PF(6)] + PEG200) mixture are found to be negative and independent of the temperature. Mixtures of ([bmim][PF(6)] + PEG600) and ([bmim][PF(6)] + PEG1000) show rare and unusual viscosity "synergism" or "hyperviscosity" in the sense that the mixture viscosity is observed to be significantly higher than the viscosity of both the neat components forming the mixture, giving rise to large positive excess logarithmic viscosities. These positive excess logarithmic viscosities decrease with increasing temperature. Formation of extensive H-bonding between the IL and PEG more than compensates for the losses in Coulombic attractive and van der Waals interactions within [bmim][PF(6)] and PEG600/PEG1000, respectively, giving rise to viscosity synergism. This compensation is not enough for ([bmim][PF(6)] + PEG200) and ([bmim][PF(6)] + PEG400) mixtures. The evidence for H-bonding in the mixtures is provided by FTIR absorbance data. The product of the monomer-to-excimer emission intensity ratio and the lifetime of the intramolecular excimer fluorescence of a microfluidity probe, 1,3-bis(1-pyrenyl)propane (BPP), is used as a reflection of the microviscosity of the mixture at different temperatures. The microviscosity shows synergistic effects in all four ([bmim][PF(6)] + PEG) mixtures. The contribution of H-bonding to the microviscosity reported by BPP is observed to be more then that as compared to contributions of Coulombic and van der Waals interactions. Synergism in the dynamic viscosity and microviscosity of ([bmim][PF(6)] + PEG) mixtures is a complex interplay of inter- and intramolecular H-bonding as well as Coulombic and van der Waals type interactions.

Intramolecular Excimer Formation Dynamics of 1,3-Bis-(1-pyrenyl)propane within 1-Butyl-3-methylimidazolium Hexafluorophosphate and Its Polyethylene Glycol Mixtures

“…The viscosity dependence of the systems investigated on the temperature affords interesting outcomes, providing insights to the intramolecular excimer formation kinetics of 1Py(3)1Py. Within neat PEGs, the trend in E a , in general, is similar to that observed for the activation energy of the viscous flow (E a,η ) 30 (order of E a PEG400 > PEG600 > PEG200 > PEG1000 versus order of E a,η : PEG400 > PEG200 > PEG600 > PEG1000), the E a,η values being lower than the corresponding E a values (E a > E a,η for PEGs). In complete contrast, within [bmim][PF 6 ], while E a,η are reported to be higher than those within PEGs, 30 Careful examination of the aforementioned analysis clearly indicates that for the isoviscous PEG systems, the higher the average MW of the PEG, the higher the value of k a .…”

“…30 Temperature Dependence of Rate Constants of 1Py(3)1Py Intramolecular Excimer Formation. For the systems where single-exponential decay best describes the [I M (t)] data, under the DMD scheme the thermal dissociation of the two excimers back to the monomer may be considered negligible, i.e., k d1 ≪ 1/τ′ 01 and k d2 ≪ 1/τ′ 02 .…”

“…When[I M (t)] better fits to a double-or tripleexponential decay model, k a could not be calculated due to a larger number of unknowns than the number of recovered parameters (e.g., seven unknowns with only six independently recovered quantities for the best fit of [I M (t)] to a tripleexponential decay function). 6 ] + PEG mixtures) where [I M (t)] best fitted to a single-exponential decay function, k a was estimated using eq 5 (k a are listed in Table 3; corresponding dynamic viscosities, η, of the systems can be found in ref 30). The k a (with error ≤ 5%) thus obtained varies between ∼0.51 × 10 6 and ∼14.2 × 10 6 s −1 for neat liquids.…”

“…17,18 Similarly, the dynamic viscosity of certain ([bmim][PF 6 ] + PEG) mixtures mixture was found to be higher (hyper viscosity) than that of either of the components also exhibited rare "hyperviscosity". 30 These interesting and unusual properties of ([bmim] [PF 6 ] + PEG) mixtures may afford unconventional and hard-to-predict results in important chemical reactions and processes.…”

Spectroscopic Evidence for Unusual Microviscosity in Imidazolium Ionic Liquid and Tetraethylene Glycol Dimethyl Ether Cosolvent Mixtures

“…52−54 In addition, a microviscosity enhancement was recently observed in the mixture of imidazolium-based ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim]-[PF 6 ]) and poly(ethylene glycol) (PEG) in comparison to the individual solvent. 55 The formation of a hydrogen-bonded network by [bmim] [PF 6 ] with PEG had been considered to be the main reason for the observed microviscosity enhancement. 55−57 Anyway, most of these previous studies mainly paid attention to microviscosity changes of cosolvents which mainly affect the polar domains of ionic liquid through the hydrogen-bonding effect involved in cations and/or anions of ionic liquid and cosolvent mixtures.…”