Solvation vs concentration fluctuations: confined dilute electrolytes near demixing at constant potential

In a recent paper, the effect of concentration fluctuations in a simple fully symmetric model of dilute electrolyte, confined between two planar electrodes, was investigated using constant charge simulations in the vicinity of the solute-solvent demixing transition. In agreement with previous theoretical findings it was shown that the total capacitance was enhanced by the occurrence of concentration fluctuations. In this work we will resort to constant potential simulations to show that the same effect is to be found in the differential capacitance. Additionally, here we will consider the effects of ion solvation by a polar solvent modelled by TIP4P/2005 water. Finally, the original fully symmetric ionic solute will be replaced by a three-site coarse grained model for a well know room temperature ionic liquid, 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMI]+[BF4]−)). Our results show the presence of strong local fields within the capacitor due to incomplete local screening, and a marked asymmetry in the charge distribution. The expected capacity enhancement due to concentration fluctuations is completely mitigated by solvation effects and the response of the polar solvent to the external field.