We have investigated the stability of TiO$_2$ nanoparticles in media with different dielectric constants at different pH values. The zeta potential of TiO$_2$ nanoparticle suspensions was measured by measuring the electrophoretic mobility and hydrodynamic radius using the dynamic light scattering (DLS) method. We expected a higher stability of the suspensions with a medium that has a lower dielectric constant. We proved that the opposite is true, as lowering the dielectric constant of the medium also increases the electrostatic attraction between the counterions and the nanoparticles. The counterions accumulate on the surface of the nanoparticles and shield the charge of the nanoparticle,
which is responsible for the repulsive interaction between two nanoparticles. The stability of the suspensions was higher at a pH further away from the isoelectric point of the nanoparticles, as the charge of the particles was greater and consequently the repulsion between them. In this case, the charge shielding has no significant influence on the stability as in the case of the dielectric constant. We performed five more Monte Carlo simulations, in each of which we changed the dielectric constant of the medium in the system. The simulations yielded results that agree with the experimental measurements.
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