STRUCTURE OF ADSORPTION LATTICES OF META-POLYOXOTUNGSTATE ON A MERCURY ELECTRODE

Polyoxotungstate anions in aqueous solutions are known to form adlayears at different metal electrodes mercury which reveal remarkable barrier properties in heterogeneous electron transfer processes, as well as selectivity for different redox-couples. These phenomena were investigated basically by electrochemical measurements so far, while microscopic information on such interfaces is still lacking. In this work we address the structure of the adlayers formed by W12O40H26-●6Na+ (meta-polyoxotungstate, m-W) at a mercury/water interface by means of density functional theory and classical molecular dynamics (MD) simulations. First, we found the optimized geometry of m-W and their detailed charge distribution. These data are used to devise the MD force field. The interaction energy of the anions adsorbed in different orientations with the mercury surface is calculated using DFT in the framework of cluster model. The partial charge transfer from the adsorbate to the electrode is small (ca 0.3 e). MD simulations are performed for two assumed types of the adlayer packing (square and rhombic) using experimental values of the surface coverage at low electrode potentials; solvent environment is addressed by 500 water molecules, Na+ serve as a counter ions. Both adlayers differ in the disposition of cations and the m-W hydration. It is argued that the rhombic lattice is slightly more probable from the viewpoint of thermodynamics. In such a lattice all Na+ ions are incorporated into the adlayer, while in the case of square packing one cation resides in the solution. The cations within the adlayer form contact ionic pairs with m-W; their life time ranges from 0.3 to 0.4 ns. The interionic interactions in the adlayer cannot be solely responsible for its stabilization; the direct m-W-mercury interaction plays an important role. Our analysis of rotation mobility of the m-W anions in the adlattices points to the small probability of an assumed “gear-like” mechanism of the penetration of redox-active reactants through the barrier layer. Most likely they approach the electrode surface via steric “windows” between the m-W molecules adsorbed. Due to the “dipole” character of distribution of the Na+ ions near the adsorbed m-W anions the electrostatic field of adlayer from the solution side might favour penetrating positively charged reactants but impedes the approach of anionic species. The results obtained elucidate qualitatively available experimental data.