Ab Initio Investigation of Polyethylene Glycol Coating of TiO₂ Surfaces

In biomedical applications, TiO₂ nanoparticles are generally coated with polymers to prevent agglomeration, improve biocompatibility, and reduce cytotoxicity. Although the synthesis processes of such composite compounds are well established, there is still a substantial lack of information on the nature of the interaction between the titania surface and the organic macromolecules. In this work, the adsorption of polyethylene glycol (PEG) on the TiO₂ (101) anatase surface is modeled by means of dispersion-corrected density functional theory (DFT-D2) calculations. The two extreme limits of an infinite PEG polymer [-(OCH₂CH₂) ], on one side, and of a short PEG dimer molecule [H(OCH₂CH₂)₂OH], on the other, are analyzed. Many different molecular configurations and modes of adsorption are compared at increasing surface coverage densities. At low and medium coverage, PEG prefers to lay down on the surface, while at full coverage, the adsorption is maximized when PEG molecules bind perpendicularly to the surface and interact with each other through lateral dispersions, following a mushroom to brush transition. Finally, we also consider the adsorption of competing water molecules at different coverage densities, assessing whether PEG would remain bonded to the surface or desorb in the presence of the aqueous solvent.