3.3. Theoretical analysis on the stability of nanoparticles
Fig. 3 illustrates the DLVO interaction energy profiles for the
TiO2 under different conditions. Before the addition of HA into the
solution (Fig. 3a), there is no energy barrier at a pH of 6.5 whereas
the energy barriers at the pH levels of 3.0 and pH 9.0 are approximately
142.9 kT and 101.0 kT, respectively. After 24 h of adsorption
of the HA by the TiO2, the energy barrier at a pH of 6.5 increases to
approximately 215.0 kT while the energy barrier at a pH of 9.0 increases
slightly; the energy barrier at a pH of 3.0 decreases to
90.0 kT (Fig. 3b). The DLVO theory is a useful tool for describing the
interparticle interactive energies of colloidal suspensions (Hu et al.,
2010). We use the classic DLVO theory to model the aggregation
behaviors of TiO2 nanoparticles under different pH levels in the
presence and absence of HA. In the absence of HA, we conclude that
the aggregation of TiO2 is due to the absence of an energy barrierbetween the particles when the pH is close to PZC (pH of 6.5). When
the pH is far from the PZC (pH levels of 3.0 and 9.0), the higher
absolute value of the zeta potential causes a higher electrostatic
repulsive force. The higher electrostatic repulsive force causes
higher energy barriers, thus leading to a smaller aggregation size in
the solution.