Anomalous ion transport through hydrophilic and hydrophobic nanopores

Nanopores promise wide applications in biosensing, ion-sieve and supercapacitors, ect. However, when the pore diameter approaches nanoscale, ion dynamic properties would deviate from the continuum analysis. In this paper, we reported that ion mobility in a nanopore depended on the surface hydrophobicity of the nanopore material through molecular dynamics (MD) simulation methods. The surface hydrophobicity was regulated by changing the polarity of the Si-N bond and the Van der Waals coefficient. For the nonpolar bond case, the solid-liquid hydrophobic force left an area as expressway for ion and water transport. The ion velocity within 3 Â of the surface was even faster than that in the central diffusion layer. In contrast, for the polar bond case, the strong interaction between the surface charges and the counter ions resulted in the formation of the Stern layer where the velocity of ions as well as water molecules sharply deviated from pore center value. Consequently, the average Na+ velocity in the polar bond nanopore was at least 5 times slower than that in non-polar bond nanopore.