Effect of divalent ions on electroosmotic transport in a sodium chloride aqueous solution confined in an amorphous silica nanochannel*

A critical enabling technology for the next generation of nanoscale devices, such as nanoscale ``lab on a chip'' systems, is controlling electroosmotic flow (EOF) in nanochannels. In this work, we control EOF in an aqueous sodium chloride (NaCl) solution confined in a silica nanochannel by systematically adding different amounts of divalent ions. Multivalent ions have a different affinity for the silica surface and different hydration characteristics in comparison to monovalent ions. Therefore by adding Mg$^{++}$ and Ca$^{++}$ to the sodium chloride solution, the electroosmotic velocity and the structure of the electrical double layer will be modified. The effects of adding Mg$^{++}$ and Ca$^{++}$ will be compared using non-equilibrium molecular dynamics simulations of the EOF at different electric fields of a NaCl solution in a silica nanochannel with different fractions of Ca$^{++}$ and Mg$^{++}$ ions. In general, the wall zeta-potential magnitude, and hence the EOF velocity, decreases as the Ca$^{++}$ or Mg$^{++}$ concentration increases. The system responds linearly with electric field. We will compare the computational results with the experimental data of Cevheri and Yoda (2012).