Elucidation of polyethylene glycol adsorption at the solid–H2O(l) interfaces of anatase TiO2(101) using density functional theory and molecular dynamics simulations

Abstract

TiO₂ is the most successful oxide that has been used in a wide range of applications in the industrial, agricultural, pharmaceutical, and medical fields over decades. The technology for grafting polyethylene glycol (PEG) onto the TiO₂ surfaces has attracted considerable attention, and a commercial process, called PEGylation, has been intensively used in the heterogeneous catalyst, pharmaceutical, and biomedical areas. However, an accurate measurement of grafted PEG density is still challenging due to the complexity and diversity of polymers and solvent molecules. In this work, we theoretically study the adsorption of brush-type PEG on the anatase TiO₂(101) surface in the presence of solvent water molecules using the systematic free energy analysis of a combined density functional theory (DFT) and classical molecular dynamics (MD) simulation. The PEG head group adsorption, chain–chain interaction, and PEG-solvent solvation enthalpies were calculated using the ab initio molecular dynamics (AIMD) simulation. Also, the entropy change of PEG from the gas phase to the TiO₂ surface was evaluated using the MD simulation. Furthermore, the impact of the PEG molecular weight on its surface coverage was investigated. Our simulation reveals that the most stable coverage of PEG with 10 carbons on anatase TiO₂(101) is 0.75 monolayer (ML), and such a value declines as the degree of polymerization increases (0.75 → 0.65 ML for PEG 10C → 30C) due to the severe entropic loss of PEG. Our results provide a fundamental understanding of the PEG grafting mechanism on TiO₂, which can contribute to the experimental design of advanced TiO₂ suspensions for target applications.