Ion Enrichment on the Hydrophobic Carbon-based Surface in Aqueous Salt Solutions due to Cation-π Interactions

By incorporating cation-pi interactions to classic all-atoms force fields, we show that there is a clear enrichment of Na+ on a carbon-based p 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--pi 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+, Mg2+, Ca2+, Fe2+, Co-2+, Cu2+, Cd2+, Cr2+, and Pb2+) have similar adsorption behaviors. For comparison, as in usual force field without the proper consideration of cation-pi interactions, the ions near the surfaces have a similar density of ions in the solution.