Molecular level interactions in binary mixtures of 1-ethyl 3-methylimidazolium tetrafluoroborate and water

Abstract

In this study, structures of different water species present in binary mixtures of an ionic liquid (IL), 1-ethyl 3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]) and water of varying compositions have been investigated by near-infrared (NIR) spectroscopy and for the first time aggregation behavior and interactions between different species in the system have been inferred from the analyses of combination bands of NIR spectra. The relative population of different water species as well as the strength of hydrogen bonding was obtained from the analyses of peak area and peak positions of the deconvoluted –OH band of water, respectively. The ratio of the population of associated water to the population of water species bonded to IL increases sharply with increasing mole fraction of IL at X_IL > 0.40. Consequently, the excess molar volume vs. X_IL profile of the mixtures passes through a maximum. For the water species bonded to IL, analysis of the peak position showed that the strength of hydrogen bonding shows a maximum suggesting the formation of two different types of aggregates of IL below and above X_IL ≈ 0.26. The DLS measurements show that in the water-rich region of the binary systems, aggregates with hydrodynamic diameter of 354–590 nm are formed. The size of the aggregates has been found to increase with increasing amount of IL and decrease with increasing temperature. Finally, the properties of the IL–water binary mixtures have been found to be governed by the strength of IL–water bonds and population of IL–IL and water–water clusters.