Implausibility of bidentate bonding of the silanol headgroup to oxidized aluminum surfaces

The question of how siloxane based sol–gel coatings adhere to the oxidized aluminum surface is addressed by means of density-functional-theory (DFT) calculations. To this end, the adsorption of a model silanol molecule, CH₃Si(OH)₃, is studied on four chemically and structurally diverse surface models, i.e.: (i) hydroxylated ultra-thin oxide film supported on Al(111), (ii) boehmite γ-AlOOH(010), (iii) hydroxylated α-Al₂O₃(001), and (iv) gibbsite Al(OH)₃(010). We show that a silanol unit can form at most one strong SiO−Al chemical bond with the surface via the condensation mechanism. In particular, the formation of the first SiO−Al chemical bond is exergonic and exothermic on all considered surface models, whereas the formation of a second SiO−Al bond within the silanol unit is endergonic and endothermic on all of them. The reason for the inferiority of the latter is attributed to the strained configuration the structure has to adopt in the bidentate bonding mode. In particular, the distance between the O atoms involved in the two SiO−Al bonds is much smaller compared to the respective distance between the two OH groups on the pristine substrate. Because the considered surface models are chemically as well as structurally diverse, current results imply that the occasionally used scheme of silanol–surface bonding, where a monomeric silanol subunit in a polymer binds to the surface via two SiO–Al bonds, is most likely inappropriate and needs to be reconsidered.