(458d) Effect of Ions on the Aqueous-Phase Adsorption of Organics on Silver

The aqueous phase adsorption of organics to surfaces is often much weaker than the gas phase due to organic solvation and the energy required for water displacement from the surface upon organic adsorption. Ions in solution can change the free energy of adsorption (ΔG_ads) of organics by affecting these two factors, but it is unclear which factor is most affected and by how much. We perform molecular dynamics (MD) to study the effect of 0.2 to 1.0 M LiCl, NaCl, CsCl, and NaClO₄ on the solvation free energy (ΔG_solv) of phenol (Fig. 1b), catechol, benzene, guaiacol, and benzyl alcohol and their on Ag(111) at the potential of zero charge (pzc) and 298 K (for phenol in Fig. 1a), as well as the solution adhesion energy (ΔG_adh). We compare the changes in the simulated ΔG_ads to what we would predict based on changes in ΔG_solv and ΔG_adh. At the pzc, MD simulations predict that ions have a minor impact, strengthening by an average of −3.6 kJ mol⁻¹. By temperature-dependent simulations we find these changes are roughly equally from enthalpy and entropy contributions. Based on ΔG_solv and ΔG_adh changes, we predict ΔG_ads to only change by ±2 kJ mol⁻¹ (for phenol in Fig. 1c) with the solvation and solution adhesion having similar impact. However, from experimental contact angle measurements on silver where the potential is more positive than the pzc and thus there is an electric field, we predict ΔG_adh is much stronger than at the pzc (for phenol in Fig. 1d). While the ion effects on are small at pzc, the ion effects at potentials away from pzc are larger due to the increased effect from ΔG_adh.