Structural and thermodynamic properties of the electrical double layer in slit nanopores: A Monte Carlo study

Grand canonical Monte Carlo (GCMC) simulation techniques at a constant electrode-electrolyte potential drop are employed to study the differential capacitance of a planar electric double layer in slit nanopores. According to the technique, a single randomly selected ion is exchanged between a simulation box and a reservoir. The probability of this step is given by the GCMC algorithm. To preserve the electroneutrality of the system after the ion exchange, the electrode charge is adequately modified, which produces electrode charge fluctuations. The charge fluctuations are used to calculate the differential capacitance of the double layer. Results for the ion distributions, electrode surface charge density, and differential capacitance in slit nanopores are reported for a symmetric system of +1:-1 ionic valences with a common ionic diameter of 0.4 nm at electrolyte concentrations of 0.2M, 1.0M, and 2.5M, pore widths of 0.6 nm, 0.8 nm, and 1.2 nm, a potential drop of 0.05 V, a relative permittivity of 78.5, and a temperature of 298.15 K. These results are compared with the corresponding data for a +1:-2 valence asymmetric system and a size asymmetric system with ionic diameters of 0.4 nm and 0.3 nm. The results show that with increasing electrolyte concentration, the range of confinement effects decreases. For divalent anions, the width dependence of electrode charge and differential capacitance reveals a maximum. The differential capacitance curves show a camel shape to bell shape transition as the electrolyte concentration increases. Asymmetry in both ionic valences and diameters leads to asymmetric capacitance curves.