Dipolar Self-Consistent Field Theory for Ionic Liquids: Effects of Dielectric Inhomogeneity in Ionic Liquids between Charged Plates

Abstract

We studied ionic liquids confined between charged plates to develop a dipolar self-consistent field theory (DSCFT) for incompressible states and a hybrid of DSCFT combined with Monte Carlo simulation for compressible states. Our theory, which has no adjustable parameters, accounts for the differences between the dipole moments and the molecular volumes of the cation and anion, the hard-core nature of the ions, and the electrical double layer formed through the strong association of the ions with the electrodes. We illustrate that a spatial change in the distributions of cations and anions with different dipole moments causes a significant spatial change in the dielectric function and hence gives rise to spatial asymmetry in the electrostatic field between the charged plates. Notably, this effect can be comparable to that caused by an asymmetry in the molecular volumes of the cation and anion. Moreover, the hard-core nature of ionic liquids causes oscillations in the density profile near the charged plates. We also demonstrate that a contrast in the dielectric values of the cations and anions in ionic liquids causes a substantial decrease in capacitance as the applied voltage is increased. The magnitude of this variation can be noticeably altered by changing the dipole moments of the cation and anion.