The results from zeta potential measurements for pure P25 and modified nanoparticles are presented in Fig. 4. The zeta potential values of non-modified TiO2 vary from +15 to −23 mV and the isoelectric point was measured at RGFP966 ∼7.3–7.4. It can be observed that after modification with AAPTMS surfactant the surface character of nanoparticles changed drastically. Specifically, only negative surface charge of −45 to −10 mV was recorded over the measured pH range. The increase of the negative surface charge enhances the electrostatic repulsive forces between electrical double layer around the nanoparticles and the aqueous medium which override attractive Van Der Waals forces between the nanoparticles. Thus, the agglomeration of the nanoparticles is suppressed and the hydrodynamic diameter is reduced . The sample A6 exhibited the strongest negative charge and isoelectric point for this sample was not observed. Although a clear isoelectric point can not be seen for the samples A9 and A18, it could be assumed that the isoelectric point of these samples is positioned around the pH 6.8 since the Z-potential curves exhibit maximum at this pH and the measured Z-potential values are close to 0.