PARTIAL CHARGE-TRANSFER OF THE IODIDE-ION NEAR A WATER/METAL INTERFACE

We have studied theoretically the partial charge transfer (PCT) between a hydrated iodide ion and a neighboring platinum (100) surface by an approach that is a combination of quantum chemical cluster calculations, the Newns-Anderson theory of chemisorption, and molecular dynamics simulations. The PCT from the bare ion has been calculated from quantum chemical cluster calculations as a function of the distance from the metal surface. The hydration energy of the undischarged iodide ion has been calculated by molecular dynamics simulations at different ion-metal distances. Hydration and the surface dipole layer lead to a shift in the highest occupied energy level of the adsorbate, which results in a significant reduction of the PCT compared with the situation in vacuum. The equilibrium hydration energy of the ion as a function of its charge and as a function of ion-metal distance was calculated on the basis of a Born-like approximation. The hydration energy of the partially discharged ion is combined with the direct ion-surface interactions and the desorption energy of water molecules to produce the total energy profile for iodide adsorption. Two minima separated by a small barrier are observed, one corresponding to a contact-adsorbed, partially discharged, and partially dehydrated ion and the other one corresponding to a fully hydrated iodide ion at larger distances.