Insight into the Bonding of Silanols to Oxidized Aluminum Surfaces

International audienceIn the context of elucidating the mechanism by which siloxane-based sol–gel coatings adhere to the surface, the adsorption of a model silanol molecule, CH3Si(OH)3, and its oligomers (up to the trimer) on oxidized and fully hydroxylated aluminum substrates is described using density functional theory (DFT). To link our calculations with the synthesis of siloxane-based sol–gel coatings, the focus is given on the condensation mechanism. We find that the formation of a monodentate bonding mode with the hydroxylated surface via the condensation mechanism is exothermic by ≥0.5 eV in all considered cases. In contrast, the formation of a bidentate bonding mode is exothermic only for the trimer. However, taking entropic contributions into account, we find that the formation of the bidentate bonding mode is exergonic already for the dimer due to favorable entropic effects of a liberated water molecule during the reaction. In contrast, the reaction entropy is unfavorable for the monodentate formation because the effects of the immobilized silanol molecule counteract and surpass those of the liberated water molecule. The monodentate to bidentate transformation is therefore determined by the interplay between entropy and energy, and we find that the longer the oligomer chain, the more likely is the bidentate formation due to increasingly favorable reaction energies. These results further reveal that for the silanol monomer, additional molecule–surface chemical bonds do not form via the condensation mechanism due to the strained configuration it has to adopt in the bidentate bonding mode