¹H NMR Relaxometry, Viscometry, and PFG NMR Studies of Magnetic and Nonmagnetic Ionic Liquids

Abstract

A study is presented of the molecular dynamics and of the viscosity in pure [Aliquat][Cl] ionic liquid and in a mixture of [Aliquat][Cl] with 1% (v/v) of [Aliquat][FeCl4]. The (1)H spin-lattice relaxation rate, R1, was measured by NMR relaxometry between 8 and 300 MHz. In addition, the translation self-diffusion, D, was measured by pulse field gradient NMR. The ILs' viscosity was measured as a function of an applied magnetic field, B, and it was found that the IL mixture's viscosity decreased with increasing B, whereas the [Aliquat][Cl] viscosity is independent of B. All experimental results were analyzed taking into account the viscosity's magnetic field dependence, assuming a modified Stokes-Einstein diffusion/viscosity relation. The main difference between the relaxation mechanisms responsible for R1 in the two IL systems is related to the additional paramagnetic relaxation contribution associated with the (1)H spins-[FeCl4] paramagnetic moments' interactions. Cross-relaxation cusps in the R1 dispersion, associated with (35)Cl and (1)H nuclear spins in the IL systems, were detected. The R1 model considered was successfully fitted to the experimental results, and it was possible to estimate the value of D at zero field in the case of the IL mixture which was consistent with the values of D measured at 7 and 14.1 T and with the magnetic field dependence estimated from the viscosity measurements. It was observed that a small concentration of [Aliquat][FeCl4] in the [Aliquat][Cl] was enough to produce a "superparamagnetic"-like effect and to change the IL mixture's molecular dynamics and viscosity and to allow for their control with an external magnetic field.

1H NMR Relaxation Study of a Magnetic Ionic Liquid as a Potential Contrast Agent

“…The difference between IL and MIL R 11 and R 12 behaviors at high frequencies is similar to that reported previously for the IL/MIL [Aliquat][Cl] based system. 24 In that work this difference was possibly associated to the paramagnetic relaxation contribution due to the presence of the magnetic ions in the MIL system. Such strong spin-lattice relaxation increase at high frequencies due to paramagnetic relaxation was also reported for liquid mixtures with contrast agents of super-paramagnetic particles.…”

“…35 The details of the experimental techniques used with each equipment can be found in the literature. 24,34,[36][37][38] Viscosity measurements…”

“…24,29 The experiments were conducted with and without applied magnetic field to analyze the influence of the magnetic field on the ionic liquids' viscosity.…”

“…[20][21][22][23] The influence of the magnetic field on the physicochemical properties of MILs might be explained by the dependence between the magnetic field intensity and the structural arrangement of the ionic paramagnetic network and a peculiar coupling between magnetic moments of the paramagnetic ions. 24 The previous 1 H-NMR study, by Daniel et al, 24 solutions, and the profiles reported for other contrast agents systems is presented.…”

Synthesis and Characterization of Novel Ion‐Conducting Membranes Based on Poly(ether sulfone) and Protic Ionic Liquid

“…Time-domain nuclear magnetic resonance (TD-NMR) is an important technique in the evaluation of physical and chemical properties in a variety of porous materials, such as plaster slurry or cement and carbonate rocks. [48] Recently, it is being employed in spin diffusion (self-diffusion) and spin-lattice relaxation by fast field cycling for viscosity and molecular dynamics in ionic liquids [49] or yet in the observation of molecular arrangements and rotational correlation time calculations from longitudinal relaxation. [35][36][37][38]50] The dynamics of the confined PIL in the polymer matrix was probed through the interpretation of the transversal relaxation time (T 2 H).…”