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 dissolutio

In this work, using 1H and 19F 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.