EFFECT OF ADSORPTION ON THE SURFACE-STRUCTURE OF SODIUM ALUMINOSILICATE GLASSES - A MOLECULAR-DYNAMICS SIMULATION

Classical molecular dynamics (MD) computer simulations were used to study adsorption of model metals onto sodium alumino-silicate glasses and the effect these adsorbates have on the surface structure of the glass substrates. Multibody potentials were used to describe the substrate-substrate interactions; Lennard-Jones potentials were used to describe the adsorbate-adsorbate and adsorbate-substrate interactions. Lennard-Jones parameters and adsorbate mass were chosen to model Pt as the adsorbate. The adsorbate atoms penetrated 5-6angstrom into the glass, with multilayer coverage eventually occurring during deposition. The substrate showed a slight compression of the surface due to the presence of the adsorbate film. There was also a pronounced shift to smaller bond angles in the distribution of siloxane bond angles at the bridging oxygen. This redistribution was predominantly caused by a compression of siloxane bonds in 5 and 6 membered rings, although there was a partial rearrangement of ring sizes. Finally, Na at the surface were displaced by the adsorbate atoms such that Na were observed in the adsorbate film and at the film/vacuum interface.