Monte Carlo simulations of electrical double-layer formation in nanopores

Abstract

The formation of the electrical double layer (EDL) in an aqueous solution in contact with the charged solid surfaces of a slit-type nanopore has been simulated by grand canonical Monte Carlo (GCMC) and canonical Monte Carlo (CMC) methods. In the GCMC simulations, a primitive EDL model in which water is considered as a continuum and the ions are considered as hard spheres is used. The results are found to be slightly different from those predicted by the Gouy–Chapman model at low electrolyte concentration and low surface charge density. The GCMC results were then used as an initial condition for the CMC simulations of a nonprimitive model in which the EDL is composed of molecular water (four-point transferable intermolecular potential, together with fluctuating charge model), cations (Na+), and anions (Cl−). The nonprimitive model provides a very different view of the EDL at the atomic level. For example, a single layer of water molecules, instead of counterions, is strongly adsorbed on negatively charged surfaces. This feature cannot be observed in the Gouy–Chapman or in the primitive model. Moreover, because the charged surfaces are covered by water molecules, the maximum counterion concentration in a nanopore occurs at the center, instead of on the surfaces, of the pore. This phenomenon is also supported by experimental data and theoretical calculations found in the literature.