Dielectric properties of zwitterionic liquids

Abstract

The dielectric properties of a series of zwitterionic liquids are thoroughly examined in both the solution and melt states. In dilute solutions, the molar dielectric increment and subsequently derived dipole moment of zwitterionic liquids correlate strongly with the number of linker atoms between the ion pair, following the same trend as previously extensively studied zwitterionic solids. Further analysis indicates that the linker does not adopt a fully extended conformation but is slightly coiled. Despite its high dipole moment, the dielectric relaxation of zwitterionic liquids in the melt state resembles that of ordinary glass-forming dipolar liquids and consists of a primary α-relaxation and a high-frequency “excess wing.” The relaxation strengths of these two processes are proportional to 1/T and −1/T, respectively. On the other hand, while the α-peak broadens on the high-frequency side upon a decrease in temperature, no universal trend has been found for the low-frequency dispersion. The unexpectedly high conductivity in zwitterionic liquids appears to stem from the presence of ionic impurities rather than the “polarization current” associated with dipole rotation. Decoupling analysis shows that the dc conduction is not fully slaved to the α-relaxation and exhibits substantially weaker temperature dependence, lending support to the aforementioned argument regarding the origin of dc conductivity. Finally, the glass transition temperature of zwitterionic liquids examined in this work shows a weak negative correlation with their molecular weight, deviating from the general trend observed for ordinary dipolar liquids, whereas the fragility index increases with increasing relative polarization volume.