Partition and Structure of Aqueous NaCl and CaCl₂ Electrolytes in Carbon-Slit Electrodes

We report molecular dynamics simulation results obtained for aqueous NaCl and CaCl₂ solutions used as electrolytes in model electric double layer capacitors. The electrodes are carbon-slit pores of widths 0.65, 0.7, 0.79, 0.9, 1.2, and 1.6 nm. The applied voltage is represented as a uniform surface charge density on the pore surfaces. Toward replicating experimentally relevant conditions, the surface charge densities span between 0 (neutral pore) and 15 μC/cm² (both positive and negative). Charge localization on pore entrances is not considered. As the neutral pores are charged, we monitor the accumulation of the ions from the bulk (at ∼1.8 M ionic strength) to the pores. Our results show that the ionic concentration inside the pores increases as the surface charge density increases, as expected. More interestingly, the surface charge density at which the ions begin to penetrate the pores increases as the pore width decreases and as the ion size and the ion hydration strength increase. The pore width at which the maximum partition coefficient obtained at the largest surface charge density considered varies with the ion type (0.65 nm pores for Na⁺, 0.9 nm pores for Ca²⁺, and 0.79 nm pores for Cl^– ions). The density distribution of electrolytes within the charged pores depends on the water structure and on the hydration structure of the ions under confinement, which is ion-specific.