Theoretical and experimental analysis of superhydrophilic and superhydrophobic nanostructured surfaces with tunable zeta potentials*

We present a theoretical and experimental analysis of the zeta potential of superhydrophilic and superhydrophobic nanostructured surfaces with different heights (60 to 200 nm) over a range of pH (4 and 8) and concentrations (.3 to 30 mM) of aqueous KCl and NaCl solutions. The experimental determination of the zeta potential via macroscale electrokinetic flow measurements is enabled by a surface fabrication protocol based on block-copolymer self-assembly that is suitable for producing conical nanopillars of precisely controlled height and period over macroscale surface areas. Surfaces fabricated on silicon display superhydrophilicity and superhydrophobicity is attained after coating with octadecyltrichlorosilane (OTS). Negative zeta potentials are reported for all the studied surfaces. While varying the nanostructure height can suppress the zeta potential of hydrophilic surfaces at certain pH and electrolyte concentrations, the hydrophobic OTS coating generally enhances the zeta potential. Our experimental results are accounted for by a site-dissociation model for the surface charge and a hydrodynamic model for the slip length The studied surfaces are relevant for applications in energy conversion/storage and membrane-based separation.