Journey toward the Surface: How Glycine Adsorbs on Titania in Water Solution

The adsorption of glycine (Gly) on titania in water solution and its preferred binding modes are studied by means of classical reactive (ReaxFF) and nonreactive molecular dynamics simulations. A small cluster made of a few glycine molecules, surrounded by water molecules, is placed close to the TiO2(110) surface and its initial motion toward the substrate is described through classical reactive dynamics. Glycine appears to be less easily and strongly adsorbed on the surface in solution than in the gas phase due to its competition with the surrounding water molecules. Indeed, in line with the experimental observations, the preferential binding mode of glycine in solution is found to be a monodentate coordination of one of its carboxyl oxygens to a Ti surface site. The potential of mean constraint force (PMF) method, combined with the classical nonreactive molecular dynamics simulations, was used to calculate the change in free energy upon glycine adsorption and the results obtained through exhaustive sampling confirm the reactive dynamics view.