Single Ion Studies

Alkali and halide ion adsorption near metal surfaces has been investigated by a variety of groups. A simple (9-3) Lennard-Jones potential, first used by Lee et al. 

From the positions of the maxima of the ionic density profiles relative to the minima of the ion-metal interaction potentials, they concluded that 1 is contact adsorbed and Li is not. Spohr [190] and later Perera and Berkowitz [191] obtained similar results by means of free energy calculations for 1 and simultaneous Li and 1 adsorption, respectively, on Pt(lOO), using the same interaction potentials. Eck and Spohr [77, 192] and Toth and Heinzinger [80] studied the adsorption of Li+ and several halide ions near the ab initio model of the mercury interface [40]. The liquid/ gas interface, contrary to metallic interfaces, is depleted in the interfacial region [193-195]. This is a consequence of the driving force towards fully hydrated ions. 

As an example for results obtained by the constrained MD method, we briefly discuss some of the differences between the free energy profiles of Li+, F , and I ions on the mercury surface. In this study [192], only the water-metal interactions are described by the SCF interaction energies [40]. The ions interact with the surface exclusively by means of image interactions. Ewald summation in two dimensions is used to properly describe the long range polarization effects near the interface. 

The behavior of the fluoride ion is qualitatively different. The solvent interactions stabilize the ion thermodynamically in the range from -8 to -5 A, whereas the solvent effect is mostly repulsive for (and Cl ). For F the attraction by the layer of adsorbed water (which are oriented preferentially with the hydrogen atoms pointing into the solution) appears to dominate over the steric repulsion. The distance of the minimum (at about 5.5 A) corresponds to an adsorbed species F (H20)6 or F (H20)y on top of the adsorbed layer of water molecules. A detailed analysis [196] shows that the very strong repulsion at distances below 5 A is due to hydration. 

The PMF for Li+ adsorption shows two solvent-induced local minima. In bulk solution, the hydration shell of Li+ forms a rather rigid octahedral complex. Li+ and the water molecules in its hydration shell move cooperatively. Consequently, the hydration complex is sensitive to the barriers formed by the two pronounced layers of water molecules around z = —6k. and z = —3 A. This leads to a local free energy minimum on the solution side of each of the two maxima in the oxygen density profiles. There, molecules from the water layers can be part of the hydration eomplex (see below). 

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