Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

Abstract

In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

Upgrading the Topical Delivery of Poorly Soluble Drugs Using Ionic Liquids as a Versatile Tool

“…Furthermore, the high susceptibility of ILs to structural modifications allows their properties to be tailored according to a particular application, and this may be used to further reinforce their utility in different areas [ 4 , 5 , 6 ]. Thus, different combinations of the cations and anions that constitute them, and/or the attached substituents, may lead to specific physicochemical and biological properties, including hydrophobicity, impact on solubility, and toxicity (cyto- and ecotoxicity), among others [ 10 , 11 , 13 ]. Hence, when considering the applicability of ILs towards the development of improved drug delivery systems, the strategic tailoring of these materials may be crucial to ensuring their functionality, at non-toxic concentrations.…”

Structural Reorganization of Imidazolium Ionic Liquids Induced by Pressure-Enhanced Ionic Liquid—Polyethylene Oxide Interactions

“…Thus, the imidazolium C 2 –H group may be a favorable site for the IL-PEO interactions. Based on the results of 1 H NMR and MD simulations [ 12 ], C 2 -H is most prone to the effects of the hydrogen bonding. For example, the relative distance of anion is close to C 2 -H and water for [BMIM][BF 4 ]-water mixtures [ 12 ].…”

“…The macroscopic properties of the ILs mainly depend on the interionic interactions of the cations and anions [ 17 , 18 ]. The presence of nano-segregation domains in ILs, i.e., heterogeneous microstructures, was reported in the literature [ 12 ]. The local nanoscale organization of imidazolium ILs can be described as supramolecular structures of ion pairs (cation-anion), clusters (cation-anion-cation-anion…), and/or three-dimensional network (cluster-cluster associations).…”

“…ILs are emerging green solvents owing to their low evaporation and other unique properties [ 10 , 11 , 12 , 13 , 14 , 15 , 16 ]. The anomalous behaviors and structure-property relationship of ILs were reviewed by Esperanca et al [ 11 ] and Silva et al [ 12 ], respectively, in the recent. In general, the structures of ILs containing 1-alkyl-3-methylimidazolium cations can be easily designed for specific applications in industrial processes [ 10 , 11 , 13 ].…”

Phase Transitions and Electrochemical Properties of Ionic Liquids and Ionic Liquid—Solvent Mixtures

“…The resulting ILs were easy to manipulate and awaken scientific attention due to their particular properties. The third generation of ILs exhibits tunable chemical and physical properties depending upon the desired applications [ 2 , 60 , 61 , 62 ]. Typically, ILs structure combines organic cations with either organic or inorganic anions.…”