Temperature-Dependent Constrained Diffusion of Micro-Confined Alkylimidazolium Chloride Ionic Liquids

Abstract

Alkylimidazolium chloride ionic liquids (ILs) have many uses in a variety of separation systems, including micro-confined separation systems. To understand the separation mechanism in these systems, the diffusion properties of analytes in ILs under relevant operating conditions, including micro-confinement dimension and temperature, should be known. For example, separation efficiencies for various IL-based microextraction techniques are dependent on the sample volume and temperature. Temperature-dependent (20–100 °C) fluorescence recovery after photobleaching (FRAP) was utilized to determine the diffusion properties of a zwitterionic, hydrophilic dye, ATTO 647, in alkylimidazolium chloride ILs in micro-confined geometries. These micro-confined geometries were generated by sandwiching the IL between glass substrates that were separated by ∼1 to 100 μm. From the measured temperature-dependent FRAP data, we note alkyl chain length-, thickness-, and temperature-dependent diffusion coefficients, with values ranging from 0.021 to 46 μm2/s. Deviations from Brownian diffusion are observed at lower temperatures and increasingly less so at elevated temperatures; the differences are attributed to alterations in intermolecular interactions that reduce temperature-dependent nanoscale structural heterogeneities. The temperature- and thickness-dependent data provide a useful foundation for efficient design of micro-confined IL separation systems.

Does Viscosity Decoupling Guarantee Dynamic Heterogeneity? A Clue through an Excitation and Emission Wavelength-Dependent Time-Resolved Fluorescence Anisotropy Study

“…In the last 20 years or so, ionic liquids have established themselves as critical solvent media in various chemical, engineering, and technological fronts. ,− Dynamic heterogeneity is detected routinely in this class of solvents. ,,,,, To generalize our hypothesis, we took 1-ethyle-3-methylimidazolium ethyl sulfate (EMIM-ES) ionic liquid (IL) and contemplated its emission and excitation wavelength-dependent viscosity decoupling to interrogate if the system is truly dynamically heterogeneous in terms of rotational motion of C153. Similar to the previous cases, from the temperature-dependent r ( t ) of C153 at the different emission and excitation wavelengths (Figure and Table S4 of the Supporting Information) the log–log plots of ⟨τ R ⟩ and η T are constructed (Figure d,h).…”

Predicting Gaseous Solute Diffusion in Viscous Multivalent Ionic Liquid Solvents

“…The diffusion of gas species in ILs is typically attributed to a “hopping” mechanism. , However, the hopping behavior is shown to be non-Arrhenius, a fact that has been frequently identified in the IL literature. , Adding to the complexity, the amorphous nature of ILs introduces significant variability in these hopping pathways. Furthermore, gas absorption can significantly alter the properties of the IL system, such as a reduction in viscosity and other substantial changes .…”

“…56,57 However, the hopping behavior is shown to be non-Arrhenius, a fact that has been frequently identified in the IL literature. 58,59 Adding to the complexity, the amorphous nature of ILs introduces significant variability…”