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

Abstract

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.