A mechanistic explanation of the peculiar amphiphobic properties of hybrid organic-inorganic coatings by combining XPS characterization and DFT modeling

We report a combined X-ray photoelectron spectros- copy and theoretical modeling analysis of hybrid functional coatings constituted by fluorinated alkylsilane monolayers covalently grafted on a nanostructured ceramic oxide (Al2O3) thin film deposited on aluminum alloy substrates. Such engineered surfaces, bearing hybrid coatings obtained via a classic sol−gel route, have been previously shown to possess amphiphobic behavior (superhydrophobicity plus oleophobicity) and excellent durability, even under simulated severe working environments. Starting from XPS, SEM, and contact angle results and analysis, and combining it with DFT results, the present investigation offers a first mechanistic explanation at a molecular level of the peculiar properties of the hybrid organic−inorganic coating in terms of composition and surface structural arrangements. Theoretical modeling shows that the active fluorinated moiety is strongly anchored on the alumina sites with single Si-O-Al bridges and that the residual valence of Si is saturated by Si− O−Si bonds which form a reticulation with two vicinal fluoroalkylsilanes. The resulting hybrid coating consists of stable rows of fluorinated alkyl chains in reciprocal contact, which form well-ordered and packed monolayers.