Ionic liquids (ILs) have been known as a class of green solvent and played an important role in green chemistry in recently years. Materials based on ILs, such as poly(ionic liquid) and polymer/ILs composites, are also widely investigated and possess many unique properties and applications. In this chapter, green solvent IL is reviewed and the responsive behavior of ILs based materials to external stimulus, like ultrasound (US), is introduced. Especially, it is described that FT-IR spectroscopy was effective for analyzing hydrogen bonding in ILs, polymeric ILs and PVA/IL composites. In addition, shear viscosity and viscoelasticity measurement and techniques of two-dimensional correlation and deconvolution are applied to analyze the change of interaction between IL to different substances and US responded behavior.
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The history of ILs can be dated back to 1914 (Walden, 1914) for the first IL, [EtNH3]-[NO3] (Mp: 12 °C). The IL was reported as a kind of organic salt with melting point below 100 °C and was consisted totally of cations and anions. Since some ILs are liquid in room temperature, they are also called room temperature ionic liquids (RTILs). As a well-known green solvent, ionic liquids (ILs) are actively studied in recent decades. ILs have hardly detectable vapor press as compared with the common used solvent, such as THF, DMF and chloroform and have been treated as a green solvent/medium. ILs also have other unique properties of high conductivity, negligible flammability, high thermal and electrical stability, wide electrochemical window, and the ability to dissolve many organic and inorganic compounds (Hallett & Welton, 2011; Welton, 1999). Therefore, IL was widely used in organic synthesis (Li, Yuan, Zheng, & Fang, 2012), material science (Rogers, 2007), separation process (Sun, Luo, & Dai, 2011), and electrochemical conductive layer (Lu et al., 2002). Figure 1 shows the commonly used cations and anions for the composition of ILs. One of the feature property of ILs is that different ILs could be prepared by together of different cations and anions. Therefore, the IL properties enable to changing hydrophilicity and conductivity by the combination of their ions. ILs are called as “designer solvent”. This characteristic also enables researchers to design the chemical structure based on their demand. Among the researches of ILs, the properties of ILs and the interaction of ILs with different compounds, especially for the mediums such as water, alcohol and acetonitrile (Bester-Rogac, Stoppa, Hunger, Hefter, & Buchner, 2011; Zheng, Wang, Luo, Zhou, & Yu, 2013) .
Figure 1. The structures of common used cation and anion