Electric Field-Driven Long-Range Order and Enhanced Polarization Switching in High-Dipole Ionic Liquids

Ionic liquids (ILs) with significant dipole moments exhibit massive electric polarization (P) under an applied AC electric field (E) at room temperature. Among the various investigated ILs, the p-toluene sulfonate of 1-ethyl-3-methylimidazolium, [C₂mim][Tos], which has a large dipole moment in the tosylate anion (∼10.99 D), shows large hysteresis in P-E curves with a high saturation polarization (P_s ∼ 92 μC cm^-2) and a remanent polarization (P_r ∼ 68 μC cm^-2) at a relatively low electric field E (1.75 kV cm^-1). The mechanism of polarization and its switching in ILs are corroborated by the switching current peaks observed in current density (j) versus E (j-E) profiles and the change in fluorescence intensity of a cyanine dye doped in ILs. The reversible cyanine fluorescence intensity changes in response to the applied AC electric field reflect the dynamic orientational changes of the IL, synchronized with the polarization reversal. The dependence of P_s and P_r values on applied AC electric fields (E) is fitted by two straight lines with different slopes, below and above a threshold electric field E_th (∼1.25 kV cm^-1). Long-range order is not obtained below the threshold potential of E_th. Under AC electric fields above E_th, more significant polarization and cyanine fluorescence responses to the electric field are observed. The present study indicates the presence of long-range ordering of molecular dipole moments over a distance of more than 1 μm from the interfacial adsorption layers through the transition zone to the bulk zone.