Figure 5: Simplified scheme of the protonation and deprotonation of hydroxylated TiO2 surface leading to positive and negative net charge at the surface, respectively. The isoelectric point (pHIEP) of TiO2 is typically ca. 5.8–7.5 [125, 164167]. The 𝑝 𝐾 𝐴 values of monodentate and bidentate OH groups are reported to be 12.7 and 2.9, respectively [166].