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+, Mg2+, Ca2+, Fe2+, Co2+, Cu2+, Cd2+, Cr2+, and Pb2+) 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.

Multiscale Simulation of the Interaction and Adsorption of Ions on a Hydrophobic Graphene Surface

“…However, the accuracy and reliability of the polarizable continuum model remain largely suspicious, especially at the aqueous interface. In their result, the computed ion adsorption energy in water is unreasonably lower than the DFT results for hydrated ions with explicit water molecules. Shi et al .…”

“…In their result, the computed ion adsorption energy in water is unreasonably lower than the DFT results for hydrated ions with explicit water molecules. Shi et al . calculated the interaction energies between hydrated ions with limited water molecules and graphene, however, this interaction is generally not able to represent the inherent ion‐π interaction.…”

“…Different efforts have been paid to resolve this issue. Two pioneered studies have been reported by employing the polarizable continuum model and limited number of water molecules to compute the adsorption energy of hydrated ions on graphene surfaces. The calculated results were then applied to optimize the interaction parameters between ion and carbon atoms of graphene.…”

Nanoporous graphene as a reverse osmosis membrane: Recent insights from theory and simulation

“…One such area is the interaction between ions in solution and graphene, since classical MD simulations only capture this interaction very roughly. Preliminarily work on this topic suggests that Na + ions are more strongly attracted to graphene surfaces than classical MD simulations would suggest [111] and that bare (unsolvated) anions can either physisorb or covalently bind to a graphene surface [37]. The implications of these ion-graphene interactions for RO performance remain to be explored.…”

Study of Water Interfaces With Phase-Sensitive Sum Frequency Vibrational Spectroscopy

“…Ion adsorption at the hydrophobic interface can greatly affect the bonded OH band through EDL that reorients interfacial water species accordingly which can be readily detected by the change of Imχ (2) S . There have been hot debating on the mechanism of ion adsorption at hydrophobic interface in both theoretical calculations [112][113][114][115][116][117][118][119][120] and experiments. [96][97][98][99][100][101]112,[121][122][123][124][125] As shown in Fig.…”