Generalization of the Kirkwood - Fröhlich theory of dielectric polarization for ionic fluids

Effect of chemical substituents attached to the zwitterion cation on dielectric constant

Materials with high dielectric constant, ε_s, are desirable in a wide range of applications including energy storage and actuators. Recently, zwitterionic liquids have been reported to have the largest ε_s of any liquid and, thus, have the potential to replace inorganic fillers to modulate the material ε_s. Although the large ε_s for zwitterionic liquids is attributed to their large molecular dipole, the role of chemical substituents attached to the zwitterion cation on ε_s is not fully understood, which is necessary to enhance the performance of soft energy materials. Here, we report the impact of zwitterionic liquid cation chemical substituents on ε_s (50 < ε_s < 300 at room temperature). Dielectric relaxation spectroscopy reveals that molecular reorientation is the main contributor to the high ε_s. The low Kirkwood factor g calculated for zwitterionic liquids (e.g., 0.1-0.2) suggests the tendency for the antiparallel zwitterion dipole alignment expected from the strong electrostatic intermolecular interactions. With octyl cation substituents, the g is decreased due to the formation of hydrophobic-rich domains that restrict molecular reorientation under applied electric fields. In contrast, when zwitterion cations are functionalized with ethylene oxide (EO) segments, g increases due to the EO segments interacting with the cations, allowing more zwitterion rotation in response to the applied field. The reported results suggest that high ε_s zwitterionic liquids require a large molecular dipole, compositionally homogeneous liquids (e.g., no aggregation), a maximized zwitterion number density, and a high g, which is achievable by incorporating polar chemical substituents onto the zwitterion cations.

Are Room-Temperature Ionic Liquids Dilute Electrolytes?

An important question in understanding the structure of ionic liquids is whether ions are truly "free" and mobile, which would correspond to a concentrated ionic melt, or are rather "bound" in ion pairs, that is, a liquid of ion pairs with a small concentration of free ions. Recent surface force balance experiments from different groups have given conflicting answers to this question. We propose a simple model for the thermodynamics and kinetics of ion pairing in ionic liquids. Our model takes into account screened ion-ion, dipole-dipole, and dipole-ion interactions in the mean-field limit. The results of this model suggest that almost two-thirds of the ions are free at any instant, and ion pairs have a short lifetime comparable to the characteristic time scale for diffusion. These results suggest that there is no particular thermodynamic or kinetic preference for ions to reside in pairs. We therefore conclude that ionic liquids are concentrated, rather than dilute, electrolytes.

The ion speciation of ionic liquids in molecular solvents of low and medium polarity

The ion speciation of ionic liquids dissolved in molecular liquids of low and medium polarity is studied experimentally and theoretically. The ion speciation in some representative systems is characterized by dielectric relaxation spectroscopy and electrical conductance measurements. A corresponding-states approach is used to compare the experimental results with predictions for a reference system of charged hard spheres in a dielectric continuum. Topics of special interest are the formation of ion pairs and larger ion clusters in dilute solutions and their fate at high concentrations, where they have to redissociate in the excess ionic liquid to form the charge-ordered structure of the fused salt. In solvents of low polarity this transition leads to electrical conductance minima and liquid-liquid immiscibilities.