Atomistic description of pressure-driven flow of aqueous salt solutions through charged silica nanopores

Lab-on-a-chip devices and nanoscale porous membranes made from silica substrates are currently of significant interest for application to water desalination, biosensing, and chemical separation technologies. In each of these applications, the interaction between ionic solutions and the silica interface is key to developing design rules. In this paper, we present the results of extensive all-atom molecular dynamics simulations of the streaming current flow of 0.5 M NaCl and 0.5 M CaCl2 ionic solutions through cylindrical charged silica nanopores with diameters of 1.5, 2.0, 2.5, and 3.0 nm. We present the results from these simulations which provide a detailed description of the ion transport through the pores. We investigate the effect of pore size on ion pairing, ion hydration, and the structure of ions within the aqueous solutions. We also present the flux of the various species of the solutions through the various pores and the average current that is carried by the ions through the pores. In the 0.5 M NaCl systems, we observe that as the pore diameters increase the average current becomes increasingly positive, while for the 0.5 M CaCl2 system, the average current becomes increasingly negative as the pore diameter increases. This difference is a result from the fact that charge inversion is present in the CaCl2 systems but not in the NaCl systems