Molecular Dynamics Simulation of Electroosmotic Flow in Nanotubes

Abstract

The ion distribution and electroosmotic flow of sodium chloride solutions confined in cylindrical nanochannels with different surface charge densities are studied with molecular dynamics (MD). In order to obtain simulation results corresponding to more realistic situations, the MD simulation consists of two steps. The first step is used to equilibrate the system and obtain a more realistic ion distribution in the solution under different surface charge densities; and the second step is to apply an electrical field to drive the liquid and extract the electroosmotic flow information. Simulation results indicate that a higher surface-charge density corresponds to a higher peak of the counter ion concentration. Predictions based on the continuum theory were also calculated and compared with the molecular dynamics results. Even though the continuum theory cannot reflect the molecular nature of ions and water molecules, it is found that for low surface charge densities, the continuum theory can still give reasonable results if modified boundary conditions are applied. Charge inversion under high surface charge density has been predicted and observed recently, however, the simulation results do not indicate charge inversion even for a surface density as high as −0.34 C/m2. This might be due to the fact that we perform the MD simulations with monovalent ions, which have a tendency to suppress charge inversion, as demonstrated in the recent literature.