Spin probing of supramolecular structures in 1-butyl-3-methyl-imidazolium tetrafluoroborate/water mixtures

Abstract

We have systematically studied micro-heterogeneous mixtures of the room temperature ionic liquid 1-butyl-3-methyl-imidazolium tetrafluoroborate ([bmim⁺][BF₄⁻]) and water using continuous wave electron paramagnetic resonance (CW EPR) and the spin-probing methodology. Using cryo TEM, a mesoscopic picture of the micro-heterogeneous mixtures could be revealed. Six spin-probes differing in polarity, charge and Lewis basicity have been used to map the dependence of the micro-polarity and rotation motion of the probe on the ionic liquid (IL) concentration. The electron paramagnetic resonance (EPR) spectra of all probe molecules have been found to be unimodal. The critical aggregation concentration has been determined and the local water concentration sensed by the probes extracted from the Mukerjee hydrophilicity index. Surprisingly, four probes of the piperidinoxyl type were found to sense very similar local water concentrations irrespective of the substituent, monovalent ionic or H-bonding, at the 4-position. Using a simple geometrical model of the primary IL aggregates observed in the micro-heterogeneous concentration range, we show that these probes cannot be statically located within the aggregates on the EPR timescale. Instead, diffusive trajectories of the probe molecules extend into the aqueous phase, i.e. aqueous and [bmim⁺][BF₄⁻]-rich phases are sampled by the probes in swift succession. No details of the internal structure of the [bmim⁺][BF₄⁻] aggregates can in general be elucidated by the spin-probing methodology under these conditions. On the other hand, the dianionic Fremy's salt binds to the surface of the IL aggregates thereby sampling predominantly the aqueous phase. At large IL concentrations, a micro-viscosity drastically smaller than the macroscopic viscosity is typically observed.

Supplementary Materials (Figures S1–S5) for this paper are available online on the journal website.

Keywords:

• spin probing
• mesostructures
• IL/water mixtures

Notes

The cybotactic region is the part of a solution in the vicinity of a solute molecule in which the solvent molecules are typically more ordered.

The system is in the fast-motion regime if the inverse correlation time of rotational diffusion, τ_c⁻¹, significantly exceeds the spectral anisotropy, Δω, i.e. if τ_cΔω << 1. Here, Δω is the maximum frequency change induced by the motion. At X-band frequencies, one finds that Δω is larger for the I = –1 (high-field) hyperfine component, the spread of Δω amounting to approximately 100 MHz (47 MHz for the low-field component). Thus, for CAT-1 in neat [bmim⁺][BF₄⁻] at 20°C, we find 2πΔAτ_c ∼ 0.50, whereτ_c = 1/(6 (D_∥D_⊥²)^1/3) has been used.