Ion enrichment on the hydrophobic carbon-based surface in aqueous salt solutions due to cation-π interactions.

By incorporating cation-π interactions to classic all-atoms force fields, we show that there is a clear enrichment of Na(+) on a carbon-based π electron-rich surface in NaCl solutions using molecular dynamics simulations. Interestingly, Cl(-) is also enriched to some extend on the surface due to the electrostatic interaction between Na(+) and Cl(-), although the hydrated Cl(-)-π interaction is weak. The difference of the numbers of Na(+) and Cl(-) accumulated at the interface leads to a significant negatively charged behavior in the solution, especially in nanoscale systems. Moreover, we find that the accumulation of the cations at the interfaces is universal since other cations (Li(+), K(+), Mg(2+), Ca(2+), Fe(2+), Co(2+), Cu(2+), Cd(2+), Cr(2+), and Pb(2+)) have similar adsorption behaviors. For comparison, as in usual force field without the proper consideration of cation-π interactions, the ions near the surfaces have a similar density of ions in the solution.