Analysis of the role of the pH in the anchoring of alkylphosphonic acid on a TiO2 surface: A DFTB study

The reactivity of a (110) rutile titanium dioxide surface functionalized with neutral, anionic and di-anionic forms of dodecyl-phosphonic acid was studied by Density Functional based Tight Binding theory to simulate different pH conditions. Functionalization of this surface is relevant for at least two reasons: a) to protect the surface against external agents (e.g., by preventing the proliferation of bacteria in medical implants) and b) to use these organic-inorganic hybrid materials to facilitate the anchoring of other molecules. Comparing the re-sults obtained in the gas phase and in water, experimental findings are better modelled by considering the hy-dration energy of the acids and the solvation-desolvation process involving the acids and the surface. In water, in all protonation states, acid molecules interact with the hydrated surface as a mono-negative charged species due to proton transfer before the grafting process. The formation of bi-dentate, di-anionic acid species, due to a proton transfer process or a change of pH, is favoured by anchoring alkylphosphonic acid to the rutile.