Diffusion, Ion Pairing and Aggregation in 1‐Ethyl‐3‐Methylimidazolium‐Based Ionic Liquids Studied by ¹H and ¹⁹F PFG NMR: Effect of Temperature, Anion and Glucose Dissolution

Abstract

In this work, using H and F PFG NMR, we probe the effect of temperature, ion size/type and glucose dissolution on the rate of transport in 1-ethyl-3-methylimidazolium ([EMIM] )-based ionic liquids by measuring self-diffusion coefficients. Using such data, we are able to establish the degree of ion pairing and quantify the extent of ionic aggregation during diffusion. For the neat 1-ethyl-3-methylimidazolium acetate ([EMIM][OAc]) a strong degree of ion pairing is observed. The substitution of the [OAc] anion with the bis{(trifluoromethyl)sulfonyl}imide ([TFSI] ) anion reduces the pairing between the ions, which is attributed to a lower electric charge density on the [TFSI] anion, hence a weaker electric interaction with the [EMIM] cation. The effect of glucose, important for applications of ionic liquids as extracting media, on the strongly paired [EMIM][OAc] sample was also investigated and it is observed that the carbohydrate decreases the degree of ion pairing, which is attributed to the ability of glucose to disrupt inter-ionic interactions by forming hydrogen bonding, particularly with the [OAc] anion. Calculations of aggregation number from diffusion data show that the [OAc] anion diffuses as a part of larger aggregates compared to the [EMIM] cation. The results and analysis presented here show the usefulness of PFG NMR in studies of ionic liquids, giving new insights into ion pairing and aggregation and the factors affecting these parameters.

Impact of Frictional Interactions on Conductivity, Diffusion, and Transference Number in Ether- and Perfluoroether-Based Electrolytes

“…23 Measurements of ion self-diffusion coefficients, D self,+ and D , self,-by pulsed-field gradient NMR (PFG-NMR) have provided valuable insight into the underpinnings of ion transport. [24][25][26][27][28][29][30][31][32] In ideal, dilute electrolytes, wherein the activity coefficients of the ions are unity, the relationships between self-diffusion coefficients and ion transport coefficients are relatively simple. In this limit, ionic conductivity is given by the Nernst-Einstein equation, which is often modified to give Eq.…”

Solvent Effects in the Homogeneous Catalytic Reduction of Propionaldehyde with Aluminium Isopropoxide Catalyst: New Insights from PFG NMR and NMR Relaxation Studies

“…Pioneering work from Pregosin has shown that through suitable analysis of diffusion data obtained by pulsed‐field‐gradient (PFG) NMR measurements it is possible to understand ion pairing and aggregation in reactions catalysed by transition metal salts. This methodology was recently used to study the effect of temperature, type of anion and presence of additives onto ion pairing and aggregation of imidazolium‐based ionic liquids as well as for studies of deep eutectic solvents . Similar studies on polyaromatic hydrocarbon aggregation, in particular on the effect of attached functional groups on aggregation, have recently been reported .…”

Nanoscale Clustering of Alcoholic Solutes in Deep Eutectic Solvents Studied by Nuclear Magnetic Resonance and Dynamic Light Scattering

“…Previously, the tendency for selfaggregation has been observed also in several ILs. 14,23,24 Analysis of the diffusion data in Figure for phenol show an upper critical solution temperature at 340 K and 33.9 wt% phenol, which is the critical point for the mixture where the two liquids become in one phase and completely miscible. 25 Comparing the diffusivity of Ch + OH and Ch + aliphatic, it can be seen that as fluidity increases the difference between the Ch + OH and Ch + aliphatic diffusion coefficients increases (see ESI).…”

“…Previously, PFG NMR has showed its usefulness in characterizing the mobility of different species, and examining solvent -solvent and solvent -solute interactions in ionic liquids (ILs) and DESs. [11][12][13][14][15] In this study, PFG NMR, 2D nuclear Overhauser effect spectroscopy (NOESY) NMR and dynamic light scattering (DLS) are used to study the solvent -solute interactions and phase behavior of three alcohols, namely glucose, 1-pentanol and phenol, in Ethaline. Previous studies showed that a low surface tension increased extraction efficiency for thiophene and related compounds.…”