Theoretical Aspects of Ionic Liquids for Soft‐Matter Sciences

Abstract

The thermodynamic and electrochemical properties of ionic liquids produce a broad spectrum of unconventional phenomena both microscopically and macroscopically. However, despite numerous theoretical and experimental studies, the fundamental roles of the relevant interactions such as electrostatic interactions and hydrogen bonding often remain unclear at the molecular level. The complexity of the molecular interactions typically increases when ionic liquids dissolve polymers or polar substances such as water. Accordingly, recent studies have revealed new features of ionic liquids. Further insights into the role of the molecular polarity of ionic liquids are required. This article presents an overview of the important phenomena of ionic liquids concerning soft‐matter sciences based on selected experimental and theoretical studies. We focus on the effect of the dielectric response of ionic liquids to distinguish ionic liquids from common inorganic salts, such as alkali metal halides.

Polarization of ionic liquid and polymer and its implications for polymerized ionic liquids: An overview towards a new theory and simulation

“…In principle, the g‐factor can be calculated from the statistics of the orientational order, but in practice, this factor is commonly introduced as a phenomenological fitting parameter in experimental analyses. In Reference 60, we outlined the particular importance of hydrogen bonding for ILs. First, hydrogen bonding in ILs is a common consideration in experiments.…”

“…Thus, ILs can be both high and low dielectric media, and therefore, the dielectric responses of the cations and anions in ILs may be critical to understanding the thermodynamic and electrochemical properties of IL-based materials. 60 Moreover, specific interactions such as hydrogen bonding and aromatic interaction often exist in ILs and are of particular importance in the physical properties of ILs, [77][78][79][80][81][82][83][84][85][86][87] thus allowing us to distinguish ILs from inorganic salts. Indeed, Dupont proposed that 1,3-dialkylimidazolium ILs are described as hydrogenbonded polymeric supramolecules.…”

“…In this spirit, by reviewing previous experimental and theoretical works, we highlight the reasons that (1) ILs/PILs qualitatively differ from inorganic ions and common solvents in terms of statistical thermodynamic theory, (2) the theory of IL‐containing polymers may require further development in the existing theories, and (3) such a difference is likely to provide new features regarding various phenomena such as phase separation, dielectric relaxation (DR), ionic conductivity, and mechanical response. However, our group previously reviewed several experimental and theoretical works also in Reference 60 to address the issues mentioned above. Thus, we do not repeat the same features as those discussed in Reference 60, but we review the works most relevant to our objectives in this article.…”

“…10, Nevertheless, the pioneering work for a mean-field theory for the double layer of ILs by Kornyshev appears to have dispelled the fear regarding the complexity of the theoretical construction. 59 ILs near electrodes were drastically simplified as charge species with the excluded volumes, and the model is essentially analogous to the Flory-Huggins type of (or mean-field) free energy combined with the Poisson equation, 60 yet Fedorov, George, and Kornyshev showed that the theory can explain the bell-shape and camel-shape capacitances for the capacitors containing ILs. 59,[61][62][63] Later, Bazant, Storey, and Kornyshev developed the Landau theory of charged spheres with the excluded volumes for ILs near electrodes.…”

“…However, our group previously reviewed several experimental and theoretical works also in Reference 60 to address the issues mentioned above. Thus, we do not repeat the same features as those discussed in Reference 60, but we review the works most relevant to our objectives in this article. Interested readers should refer to Reference 60 to supplement the discussions in this article.…”