The average concentration of ions in a liquid depends on the size of a channel if the charges on solid surface do not change. The relation between them is that the concentration of ions is inversely proportional to the channel size. When a channel decreases from a micro to a nano size, the concentration of ions will increase 1000 times. In this case, the ion’s distribution in liquid may not be considered as dilute if the charges on solid surface is large, and interactions among ions have to take into account. In this paper, molecular dynamics is applied to study the effect of extra-pairs of positive/negative ions on liquid transport properties in a nano syringe under a constant injection flow rate boundary condition. In simulations, the Coulomb’s law and 12-6 Lennard-Jones potential are used to govern the interaction between ion-ion, ion-liquid, ion-solid, liquid-liquid and liquid-solid molecules. Four different cases (no ions, counter-ions, and counter-ions combining with small and large extra-pairs of positive/negative ions in liquid) are carried out. The non-equilibrium molecular dynamics (NEMD) simulation results show that the concentration of extra-pairs of positive/negative ions has significant influence on liquid velocity profile and ion distributions. For liquid flow without ions, a quasi-parabolic velocity distribution was obtained. When the counter-ions and extra-pairs of positive/negative ions are considered, the flow approaches a plug flow as the number of extra-pair of ions increases. We also found that charges in liquid do not follow the Poisson-Boltzmann distribution, especially for the net charges which have a valley located at about 1.5 molecular sizes away from the solid surface.

This content is only available via PDF.
You do not currently have access to this content.