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

Materials with high dielectric constant, ε, are desirable in a wide range of applications including energy storage and actuators. Recently, zwitterionic liquids have been reported to have the largest ε of any liquid and, thus, have the potential to replace inorganic fillers to modulate the material ε. Although the large ε for zwitterionic liquids is attributed to their large molecular dipole, the role of chemical substituents attached to the zwitterion cation on ε 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 (50 < ε < 300 at room temperature). Dielectric relaxation spectroscopy reveals that molecular reorientation is the main contributor to the high ε. 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 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.