Understanding structure-property relationships and the dynamics of 1-ethyl-3- methylimidazolium methyl phosphonate and its mixtures with 1-n-alkylimidazole co- solvents using electrical impedance and pulse gradient spin echo NMR spectroscopy

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2023-08

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Abstract

Understanding the dynamics and structure-property relationships in ionic liquids (ILs) and IL/molecular liquid (IL/ML) mixtures is of particular importance in electrochemical energy applications where ILs have been impactful because of their chemical inertness, wide electrochemical window, and high intrinsic ionic conductivity, which obviates the need for a supporting electrolyte. The extent to which cations and anions in ILs and IL/ML mixtures are dissociated is most important for electrochemical applications. Many ILs are completely dissociated in high dielectric solvents whereas in low dielectric solvents one has ion- association. Ion-association in ILs and IL/ML mixtures can also influence chemical and physical properties, such as viscosity, density, surface tension, volatility, solubility, and chemical reactivity. The most important ones are the transport properties - conductivity, diffusion, and viscosity, when it comes to understanding the ionic structure and dynamics of ILs and IL solutions. So, many authors have tried to find answers to questions such as, ‘‘How ionic is an ionic liquid?’’ or “How does one define ionicity or ion-dissociation in ILs and IL mixtures?” Several theories have been developed to understand the correlation between ionic structure or ionic behavior/ionicity and physicochemical properties. The main goal of this project is to understand the structure, dynamical behavior, and ion-association in the imidazolium-based IL, 1-ethyl-3-methylimidazolium methyl phosphonate (neat IL) and IL/1-n-alkylimidazole mixtures, where 1-n-alkylimidazole = 1-methylimidazole, 1-ethylimidazole, 1-propylimidazole, and 1-butylimidazole, through the measurement of the density, viscosity, ionic conductivity, and self-diffusion coefficient. The correlation between transport properties (viscosity, diffusion, and molar conductivity) in the presence and absence of molecular co-solvent is analyzed using Walden plot, which links the molar conductivity and viscosity, and the Nernst-Einstein (NE) equation, which links ion self-diffusion coefficients with the molar conductivity. The deviation of the impedance conductivity from the NE equation in the neat IL can be interpreted in terms of the correlated motion of ions. The analysis of transport properties indicates the presence of neutral aggregates (i.e., ion-pairs) in the neat IL and increased ion-association in IL/1-n-alkylimidazole mixtures.

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Keywords

Ionic Liquids, Physical Properties, Pulse Gradient Spin Echo Nuclear Magnetic Resonance (PGSE-NMR), Structure-property relationships

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