Characterization of the surface modification process of Al2O3, TiO2 and ZrO2 powders by PFAS molecules

Abstract

Four types of perfluoroalkylsilanes (PFAS) molecules with various lengths of fluorocarbon chains were used to modify Al2O3, TiO2 and ZrO2 powders. The surface grafting process was examined by 29Si NMR, TGA and FT-MIR techniques. Results reveal three possible types of bonding of the hydrophobic chains to the metal oxide surface. Moreover, it was established that the type of PFAS molecules and the type of modified surface have a strong influence on the mode and amount of hydrophobic molecules anchored to the surface. It was observed that alumina oxide is modified with the highest efficiency. The fraction of OH surface groups utilized during grafting of Al2O3 by C6, C8 and C10 are similar (64–68%), but for samples grafted by C12 are much lower (40%). The PFAS molecules with shorter fluorocarbon chains (C6 and C8) are attached mainly by siloxane (T3) and geminal silanol bonds (T2), whereas longer chains (C10 and C12) are attached generally by single (T1) and geminal silanol (T2) bonds. The hindrance steric effect is observed for powder samples grafted by C12 molecules.Additionally the influence of contact with water and butanol on the conformational orientation of PFAS was evaluated. It was found, that in contact with water molecules, PFAS chains are in a tangled form, whereas in contact with butanol molecules the grafted chains are straightened.