Role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on steel: An XPS study

The role of pH and calcium ions in the adsorption of an alkyl N-aminodimethylphosphonate on mild steel (E24) surfaces was investigated by XPS. Fe 2p3/2 and O 1s spectra show that the oxide/hydroxide layer developed on the steel surface, immersed in the diphosphonate solution (7 ≤ pH ≤ 13, without Ca2+) or in a filtered cement solution (pH 13, 15.38 mmol l−1 of Ca2+), consists of Fe2O3, covered by a very thin layer of FeOOH (goethite). The total thickness of the oxide/hydroxide layer is ∼3 nm and is independent of the pH and the presence/absence of Ca2+. In the absence of Ca2+ ions, the N 1s and P 2p spectra reveal that the adsorption of the diphosphonate on the outer layer of FeOOH takes place only for pH lower than the zero charge pH of goethite (7.55). At pH 7, the adsorbed diphosphonate layer is continuous and its equivalent thickness is ∼24 Å (monolayer). In the presence of Ca2+ ions, the C 1s and Ca 2p signals indicate that calcium is present on the steel surface as calcium phosphonate (and Ca(OH)2, in very small amount). The adsorption of the diphosphonate molecules on the steel surface is promoted in alkaline solution (pH > 7.55) by the doubly charged Ca2+ ions that bridge the O− of goethite and the P-O− groups of the diphosphonate molecules. The measured values for the Ca/P intensity ratio are in the range 0.75-1, which suggests that the diphosphonate molecules are adsorbed on steel forming a polymer cross-linked by calcium ions through their phosphono groups. In the presence of Ca2+ ions in alkaline solution, the adsorbed diphosphonate layer is discontinuous and the surface coverage is found to be ∼34%.