A DFT Study of the Bonding of Silanol Molecules to Oxidized Aluminum Surfaces

One of the most promising novel corrosion resistant coatings, used for the protection of aluminum and its alloys, are hybrid sol-gel coatings, which combine the properties of organic and inorganic materials. Novel coatings are needed because traditional high-performance chromate conversion coatings are toxic and cancerogenic. One of the precursors for the synthesis of hybrid sol-gel coatings are silanol molecules and it is assumed that they react with the surface primarily via a condensation mechanism, as shown in figure 1a [1,2]. To shed some new light on the atomistic mechanism of bonding, the interactions of a simple silanol molecule (methylsilanetriol) with a model of the oxidized aluminum surface at the solid/vacuum interface, were studied within the framework of Density Functional Theory (DFT) utilizing periodic boundary conditions and the PBE exchange-correlation functional. The condensation reaction between two methylsilanetriols in the gas-phase (Figure 1b) was compared to the condensation reaction between the molecule and the oxidized surface (Figure 1a). According to the calculations both reaction energies are exothermic. The reaction of the molecule with the surface is by about 0.2 eV more exothermic, indicating that the bond between the molecule and the surface is slightly stronger than the bond between the silanol molecules themselves. Our calculations thus show that the currently proposed silanol–surface bonding mechanism is physically sound.