The influence of surface modification on the physicochemical properties of ceramic membranes

• Hydrophobic nanolayer of PFAS molecules is created on surface of TiO2 ceramics.
• Impact of grafting time and kind of PFAS on resulting hydrophobicity is observed.
• Polar component of surface free energy (SFE) is low and practically constant.
• Dispersive component of SFE is higher and declines with modification time.
• Nonlinear influence of PFAS chain length on contact angle and SFE is found.

Titania ceramic membranes were efficiently modified with perfluoroalkylsilanes (PFAS). PFAS compounds with different length of fluorine alkyl chain (in the range of 6–12 carbon atoms in chain) were used during experiments. Grafting process efficiency was determined by contact angle measurements of water and glycerol. The highest hydrophobicity was obtained for the membrane grafted by C12—after 53 h of modification. The contact angle value was equal to 136̊. The least efficient modification was obtained for the membrane grafted with C8, the contact angle was equal to 126̊ after 53 h of grafting. The contact angle measurements with bipolar substance (glycerol) were also carried out in order to apply the Owens–Wendt method for the determination of the surface free energy (SFE) values of modified membranes. The different mode of PFAS chains attachment to the membrane surface was observed. This phenomena was related to different structure of grafting molecules. The hindrance steric effect was observed for more hydrophobic molecules (C10, C12). The surface grafting process was characterized applying 29Si NMR and TGA techniques. According to obtained results, three possible types of bonding of the hydrophobic chains to the materials surface were determined. Additionally, based on the results, it was found that type of PFAS molecule has a strong influence on the mode of anchored hydrophobic molecules to the surface. It was found that PFAS molecules with shorter fluorocarbon chains are attached mainly by siloxane (T3) and geminal silanol bonds (T2), whereas longer chains are attached mainly by single (T1) and geminal silanol (T2) bonds.

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