The zeta potential and the thickness of the electrical double
layer are two of the most important properties (Morrison and Ross,
2002). An increase in both the zeta potential and the thickness of
the electrical double layer can result in an increase in the electrostatic
repulsive interaction. Several mechanisms manipulate the
surface charge of the particles, including surface ionization, ion
adsorption, and lattice ion dissolution. After the addition of HA into
the solution, the negatively charged HA molecules are imparted to
the surface of the TiO2 nanoparticles and cause relatively high zeta
potentials, which increases the net energy between the particles,
especially at a pH of 6.5. This result is due to the highly negative
zeta potentials, which lead to a high electrostatic repulsive force. It
stops the aggregation of TiO2 and evidently scatters the aggregates
(from 869.0 to 533.2 nm). For a pH of 9.0, the net energy in the
DLVO calculations increases slightly after the addition of the HA.