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 ${\mathrm{Cl}}^{-}$, ${\mathrm{Br}}^{-}$, and ${\mathrm{I}}^{-}$. 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.

Figure 1

Density profiles $\rho (z)$ of water and SAM and the electrostatic water potential ${\Phi }_W$. Triangles show the simulated ${\Phi }_W$; the line shows a heuristic fit function (see text). To the right: Snapshot of the SAM-water interface.

Figure 2

Potentials of mean force for various ions at the (a) SAM-water and the (b) air-water [24] interface. The lines show heuristic fits with the parameters in Table .

Figure 3

Potential of mean force for iodide from simulation (triangles) compared with the analytic expression (solid line) and its contributions. Poor agreement shows that surface-modified ion hydration is dominant. The ion-water van der Waals contribution [as obtained from simulations (circles)] makes the total van der Waals interaction repulsive.

Figure 4

(a) Surface potential $\Delta \Phi$ as a function of bulk salt concentration $c^0$ for various ions at the SAM-water interface (solid symbols) and the air-water interface (open symbols) as obtained from the ion density profiles in (b). (b) Rescaled cationic or anionic density profiles $c(z)/c^0$ of NaBr at the SAM-water interface and the air-water interface (inset) for infinite dilution (crosses) and different bulk concentrations $c^0$, as obtained by inserting the simulation PMFs into a Poisson-Boltzmann equation. (c) Surface-tension change as a function of $c^0$ for various ions as obtained from the Gibbs adsorption equation.