Molecular Simulation of Ion Transport in Silica Nanopores

Abstract

Ion distribution and transport of KCl aqueous solutions at the junction of hydrophobic and hydrophilic regions inside silica nanopores were studied by using two kinds of molecular simulation: grand canonical Monte Carlo (GCMC) simulations and nonequilibrium molecular dynamics (NEMD) simulations. The nanopores were 2 nm diameter silica pores in which surface functional groups, -SiOH, had been modified by hydrophobic surface functional groups, -SiCH(3), within three different lengths along the pore direction (z-direction), L(z0) = 0, 2, and 4 nm. If L(z0) is long enough, water could not enter the hydrophobic region, but for all L(z0) studied here, water entered the hydrophobic region. When an external electric field was applied along the z-direction, ions could not pass through the hydrophobic region when the external electric field was less than a threshold level, E(0), whereas the ionic current increased relatively linearly with increasing electric field strength above E(0). In 2 nm diameter fluidic pores, the electrical potential barrier appeared at the junction between the hydrophilic and hydrophobic regions due to the difference in dipole moment of the surface functional groups, although the overall surface charge of the pore was neutral. This junction formed an electrical potential threshold, and the ionic current could be modulated by adjusting the external electric field.