Theoretical and experimental studies of water interaction in acetate based ionic liquids

Abstract

Water interactions in 1-ethyl-3-methylimidazolium acetate ([emim][CH₃COO]) were studied utilizing classical and ab initio simulation methods. The self-diffusivities for water and the ionic liquid (IL) were studied experimentally using pulse field gradient NMR spectroscopy and correlated with computational results. Water forms hydrogen bonding networks with the ionic liquid, and depending on the concentration of water, there are profound effects on the self-diffusivities of the various species. Both simulation and experiments show that the self-diffusivities for species in the water–[emim][CH₃COO] system exhibit minima at 40–50 mol% water. Water interaction with the [CH₃COO]⁻ anion predominates over the water–water and water–cation interactions at most water concentrations. Simulations further indicate that decreasing water–[CH₃COO]⁻ interaction will increase the IL and water self-diffusivities. Self-diffusivities in the water–IL systems are dependent upon the cation in a complex way. Water interactions with [P₄₄₄₄][CH₃COO] are reduced compared to [emim][CH₃COO]. The [P₄₄₄₄]⁺ cation is bulkier than the [emim]⁺ cation and has a smaller self-diffusivity, but when water was introduced to [P₄₄₄₄] [CH₃COO], the water–[CH₃COO]⁻ hydrogen bonding network in the [P₄₄₄₄][CH₃COO] was much smaller than the one observed in [emim][CH₃COO].