Electro-osmosis in a nanometer-scale channel studied by atomistic simulation

Abstract

An atomistic simulation of an electro-osmotic flow in a 65.3 Å-wide channel is performed to study its physical details and evaluate continuum models. The working fluid is a 0.01 M solution (at midchannel) of Cl− in water. For simplicity and computational efficiency, only negatively charged ions are included. The water is modeled by the SPC/E potential and the Cl− are modeled as point charges plus an established Lennard-Jones potential. The channel walls are fixed lattices of positively charged Lennard-Jones atoms. In one case an appropriate fraction of the wall atoms is given elementary charges; for comparison, another case is simulated with uniformly distributed partial charges on the wall atoms. For the distributed elementary charge case the Cl− concentration at the wall is 80 percent higher than predicted by the Poisson–Boltzmann theory. It is over 100 percent higher for the uniformly charged wall case. In both cases, the waters in the 10 Å closest to the walls are preferentially oriented. Their respective orientations are similar except in the first monolayer. However, the effect of this orientational bias on the permittivity and subsequently the Cl− distribution is shown to be minor by Monte Carlo simulations, which predict an ion distribution in agreement with the dynamic simulation using only ε=80 to model the water. Computed, one-dimensional self-diffusivities of the waters match accepted values greater than 10 Å from the walls, but decay significantly close to the walls. The decay is not monotonic for the wall-normal diffusivity. The atoms near the walls are not fixed in a Stern layer, as typically assumed in models, but the viscosity is found to increase by over a factor of 6 in the 10 Å closest to the wall.