Effect of Paramagnetic Metal Ions on 1H Diffusion in Trihexyltetradecylphosphonium Benzoate Ionic Liquid
Authors: Dorai Arunkumar , Panigrahi Mrutyunjay , Kozak Dmytro , Grabda Mariusz , Shibata Etsuro , Nakamura Takashi , Kawamura Junichi 
Published: February 2020

Introduction

The high thermal stability of ionic liquids has been exploited to be used in diverse applications (e.g., energy storage devices, extraction, lubrication etc.) [1-4]. The physicochemical properties (mobility, diffusion) of ionic liquids changes when used in a wide temperature range. Several methods are available to study the diffusion and mobility of ionic liquids, of all NMR is most accurate and reliable technique to study the translational (diffusion) and rotational motions in ionic liquids [5]. Pulsed field gradient spin echo and stimulated NMR has been utilized to study the diffusion process in ionic liquids [6-7]. Studies on the diffusion process in phosphonium based ionic liquids are scarce [8]. Moreover the effects of paramagnetic metal ion on the diffusion in ionic liquid are few. In this context we studied the effect of paramagnetic metal ion (neodymium) on the translational motion (diffusion) of Trihexyl-tetradecyl-phosphonium benzoate (P66614 TTPB) ionic liquid using pulsed field gradient NMR.

Experimental Method

TTPB (C39H73O2P) (KOEI chemicals Ltd., Japan, >98.0%) was used as received. Neodymium chloride (NdCl3, Wako Pure Chemical Industries, Japan) was dissolved in de ionized millipore water. The aqueous solutions were filtered using a 25 µm sieve to avoid precipitation. To start with, 0.5 ml aqueous NdCl3 is added to 1gm of TTPB and the mixture was shaken well for 60 min using a shaker followed by centrifugation for 60 min at 1000 rpm. The concentration of metal ion was measured by ICP-AES (Optima 3300XL, Perkin-Elmer, USA). 0.3 ml of neodymium containing TTPB is added to a 5 mm NMR glass tubes for 1H and NMR measurements. JEOL ECA 300 FT-NMR spectrometer operating at a frequency of 300.5 MHz for 1H nuclei was used to study the 1H diffusion.

Results and Discussion

Neodymium ion concentration in the TTPB ionic liquid was evaluated using ICP-AES and found to be in the range 0.02 mmol/L to 1.2 mmol/L. A small amount of water is found to be incorporated into the ionic liquid along with neodymium ions. Downfield shifts are observed in the 1H 1D NMR spectra with increasing neodymium concentration. 1H diffusion coefficients of the cation and anion were measured in the temperature range 24 - 80 °C. The temperature dependence of diffusion coefficient for different neodymium concentrations are shown in Fig. 1. As observed from Fig. 1 a, at low concentrations (0.02 ~ 0.1 mmol) the diffusion coefficient is in this order DH2O > Danion > Dcation. At higher concentrations (0.4 ~ 1.2 mmol) the diffusion coefficient of the cation and anion are almost the same as observed from Fig. 1 (DH2O ~ Danion ~ Dcation).

Conclusion

The presence of a paramagnetic ion (neodymium) in the ionic liquid shifts the proton chemical shift to lower field, due to its high magnetic dipole moment. From the diffusion results, 1H diffusion coefficient of the anion is higher than the cation at low concentration of neodymium. The anionic diffusion coefficient decreases with increasing neodymium concentration and converges with the cationic diffusion coefficient. This is possibly due to the formation of clusters around the neodymium ion at high concentrations.

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Figure 1