Carla I. Daniel

In this work, ¹H NMR relaxometry and diffusometry as well as viscometry experiments were carried out as a means to study the molecular dynamics of magnetic and nonmagnetic ionic liquid-based systems. In order to evaluate the effect of a cosolvent on the superparamagnetic properties observed for Aliquat-iron-based magnetic ionic liquids, mixtures comprising different concentrations, 1% and 10% (v/v), of DMSO-d6 were prepared and studied. The results for both magnetic and nonmagnetic systems were consistently analyzed an suggest that, when at low concentrations, DMSO-d6 promotes more structured ionic arrangements, thus enhancing these superparamagnetic properties. Furthermore, the analysis of temperature and water concentration effects allowed to conclude that neither one of these variables significantly affected the superparamagnetic properties of the studied magnetic ionic liquids.

A proton nuclear magnetic relaxation dispersion ¹H NMRD study of the molecular dynamics in mixtures of magnetic ionic liquid [P₆₆₆₁₄][FeCl₄] with [P₆₆₆₁₄][Cl] ionic liquid and mixtures of [P₆₆₆₁₄][FeCl₄] with dimethyl sulfoxide (DMSO) is presented. The proton spin–lattice relaxation rate, R₁, was measured in the frequency range of 8 kHz–300 MHz. The viscosity of the binary mixtures was measured as a function of an applied magnetic field, B, in the range of 0–2 T. In the case of DMSO/[P₆₆₆₁₄][FeCl₄] the viscosity was found to be independent from the magnetic field, while in the case of the [P₆₆₆₁₄][Cl]/[P₆₆₆₁₄][FeCl₄] system viscosity decreased with the increase of the magnetic field strength. The spin–lattice relaxation results were analyzed for all systems taking into account the relaxation mechanisms associated with the molecular motions with correlation times in a range between 10^–11 and 10^–7s, usually observed by NMRD, and the paramagnetic relaxation contributions associated with the presence of the magnetic ions in the systems. In the case of the DMSO/[P₆₆₆₁₄][FeCl₄] system the R₁ dispersion shows the relaxation enhancement due to the presence of the magnetic ions, similar to that reported for contrast agents. For the [P₆₆₆₁₄][Cl]/[P₆₆₆₁₄][FeCl₄] system, the R₁ dispersion presents a much larger paramagnetic relaxation contribution, in comparison with that observed for the DMSO/[P₆₆₆₁₄][FeCl₄] mixtures but different from that reported for other magnetic ionic liquid system. In the [P₆₆₆₁₄][Cl]/[P₆₆₆₁₄][FeCl₄] system the relaxation enhancement associated with the paramagnetic ions is clearly not proportional to the concentration of magnetic ions, in contrast with what is observed for the DMSO/[P₆₆₆₁₄][FeCl₄] system.

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 ¹H 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 ¹H spins–[FeCl₄] paramagnetic moments’ interactions. Cross-relaxation cusps in the R₁ dispersion, associated with ³⁵Cl and ¹H 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.