An XPS investigation of the adsorption of aminosilanes onto metal substrates

Spectroscopic evidence has been obtained showing that γ-aminopropyltriethoxysilane (γ-APS) is partially adsorbed onto the oxidized surfaces of certain metals through the amino group. Results obtained from X-ray photoelectron spectroscopy (XPS) indicate that the amino groups on γ-APS are protonated by interaction with hydroxyl groups on the surfaces of silicon, iron, titanium, aluminum, nickel, and magnesium. The extent of protonation is greatest on silicon, aluminum, and titanium; intermediate on iron and nickel; and least on magnesium; and thus correlates well with the predicted isoelectric points of the oxides. The extent of protonation also depends on the pretreatment of the substrate. Thus, the extent of protonation of the amino groups is greater on aluminum surfaces that have been treated by the Forest Products Laboratory (FPL) etching process than on aluminum surfaces that have been mechanically polished. γ-APS interacts with the oxidized surface of copper by formation of complexes in which the lone pair of electrons on the amino nitrogen atoms is coordinated to copper ions.