Specific Ion Adsorption at Hydrophobic Solid Surfaces

Molecular dynamics simulations of ions at a hydrophobic self-assembled monolayer with polarizable force fields for water and ions are used to extract potentials of mean force for ${\mathrm{Na}}^{+}$ and the halide ions ${\mathrm{Cl}}^{\ensuremath{-}}$, ${\mathrm{Br}}^{\ensuremath{-}}$, and ${\mathrm{I}}^{\ensuremath{-}}$. Similar to the air-water interface, the large halide ions are attracted to the surface, which is traced back to surface-modified ion hydration. The total effective interaction is parametrized and used within Poisson-Boltzmann theory to calculate surface potentials and interfacial tensions at finite ion concentration in qualitative agreement with experiments.