Abstract
Many ionic liquids (ILs) can be mixed with water, forming either true solutions or emulsions. This favors their applications in many respects, but at the same time might strongly alter their physicochemical properties. A number of methods exist for studying the macroscopic properties of such mixtures, whereas understanding their characteristics at micro/nanoscale is rather challenging. In this work we investigate microscopic properties, such as viscosity and local structuring, in binary water mixtures of IL [Bmim]BF4 in liquid and glassy states. For this sake, we use continuous wave and pulse electron paramagnetic resonance (EPR) spectroscopy with dedicated spin probes, located preferably in IL-rich domains or distributed in IL- and water-rich domains. We demonstrate that the glassy-state nanostructuring of IL-rich domains is very similar to that in neat ILs. At the same time, in liquid state the residual water makes local viscosity in IL-rich domains noticeably different compared to neat ILs, even though the overwhelming amount of water is contained in water-rich domains. These results have to be taken into account in various applications of IL-water mixtures, especially in those cases demanding the combinations of optimum micro- and macroscopic characteristics.
Highlights
Ionic Liquids (ILs) exhibit a number of unusual and advanced properties making them perspective media for many chemical processes in various fields of modern science and technology [1,2,3]
We demonstrate that the glassy-state nanostructuring of ionic liquids (ILs)-rich domains is very similar to that in neat ILs
We performed room-temperature Continuous wave (CW) electron paramagnetic resonance (EPR) studies of IL/water mixtures in liquid state, in order to compare local viscosity sensed by domain-specific spin probes in neat IL vs. IL/water mixtures (Figure 3)
Summary
Ionic Liquids (ILs) exhibit a number of unusual and advanced properties making them perspective media for many chemical processes in various fields of modern science and technology [1,2,3]. Bubbles of water in IL bulk, channels and various meshworks could be clearly identified These recent observations brought us to an interesting and potentially important question—what are the physical properties inside such well-defined heterogeneities? Taking into account all above, in this work we have designed and performed a case study of microscopic physicochemical properties in water mixtures of prototypical IL [Bmim]BF4. For this sake, we used the advanced electron paramagnetic resonance (EPR) spectroscopy and two dedicated spin probes, one of which had preferred localization in IL domains, whereas another one localized in both IL and water. Such approach allowed us to peek inside the IL-rich micro/nanodomains within IL/water mixtures, as is described in detail
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