Aqueous NaCl Solutions within Charged Carbon-Slit Pores: Partition Coefficients and Density Distributions from Molecular Dynamics Simulations

Molecular dynamics simulations have been employed to study the structural properties of aqueous electrolytes confined within graphene pores. The effects of pore size and graphene surface charge density were quantified by calculating ionic density profiles within the pores and pore-bulk partition coefficients. Carbon-slit pores of width 0.9, 1.2, and 1.6 nm were considered. The graphene surfaces were charged with densities ranging from 0 (neutral pore), 20, 30, and 40 μC/cm 2, simulating various applied voltages. Aqueous solutions of NaCl at 1.5–1.6 M concentrations were considered at ambient conditions. When the graphene sheets are neutral, most electrolytes remain outside of the pores. The few sodium and chloride ions that are found within the pores remain preferentially at the center of pores, where they can be hydrated. As the graphene surface charge density increases, more Na + and Cl – enter the pores. At the maximum graphene surface charge density considered (40 μC/cm 2 ) the ionic concentration within the pores can be ∼10 times as high as that outside of the pores, with the maximum partition coefficient obtained when the pore width is 1.2 nm. In all pores, when the surface charge density is 40 μC/cm 2 the ions move toward the charged graphene surfaces because of counterion condensation effects, at the expense of losing part of their hydration shells. In some instances our results reveal the formation of multiple layers of adsorbed electrolytes near a charged graphene surface. These layers appear to form because of a number of effects including surface–ion electrostatic interactions, hydration phenomena, and also ion–ion correlations, especially at the maximum surface charge densities considered within the 1.2 nm wide pores. The results presented are useful for designing graphene-based electric double layer capacitors.