Titanium dioxide (TiO2) nanoparticles are widely used in various industries due to their low absorption coefficient, high dielectric permeability, good biocompatibility, high hardness, and photocatalytic activity. However, the colloidal properties (size, charge, sedimentation properties) of TiO2 nanoparticles in aqueous solutions have not been sufficiently studied, and the results of studies are not comparable due to different experimental conditions and the diversity of nanoparticles produced, which limits the application of TiO2 nanoparticle suspensions.
In this work, we determined the effects of the method (ultrasonic and mechanical) and duration (up to 60 min) of mixing and the pH of the solution (from 3 to 11) on the colloidal properties of TiO2 nanoparticles in a 10 mM NaCl solution. The concentration of particles was 100 mg/L. Particle size distribution was determined using dynamic light scattering, zeta–potential was measured by using laser doppler electrophoresis, and light transmittance coefficient was estimated by spectrophotometry method. We examined two types of TiO2 nanoparticles, namely anatase and rutile particles, with average particle sizes of 18 and 83 nm and a phase composition of anatase:rutile 87:13 and 73:27 wt.%, respectively.
It was found that prolonged treatment of suspensions (>30 min) led to surface overcharging and enhanced aggregation and sedimentation of nanoparticles. For both nanoparticles in an acidic environment, suspensions with maximum resistance to aggregation and sedimentation were formed. The sedimentation curves for suspensions were well described by a first-order kinetic equation (R2>0.9). All other things being equal, the behaviour of anatase nanoparticles is more sensitive to the pH value; meanwhile, rutile nanoparticles were more affected by the method of mixing.
The effect of pH and mixing method on the electrokinetic, dispersion, and sedimentation properties of suspensions provides valuable information that can be used to distinguish the colloidal stability of particles in aqueous solutions and broaden the application of TiO2 nanoparticle suspensions.
