Stability and rheological behaviors of silica suspensions

Abstract

In this thesis, the stability of colloidal silica was achieved by three methods, such as electrostatically, sterically, and electrosterically. The model silica particles were synthesized by the sol-gel method and used for the electrostatic and steric stabilization test. First, Na$^+$ and K$^+$ ions were used for the study on the electrostatic stabilization. Two ions illustrated different stability behaviors against colloidal silica. Silica particles generate the negative charge in the basic media due to the dissociation of silanol group and are stable for a long period by the electrostatic repulsion force. When the cations are added to the solution, the particles undergo the coagulation. The stability of silica with addition of Na$^+$ was deteriorated as the pH increased, while with K$^+$ ions the stability exhibited a local minimum at pH8.5. The critical coagulation concentrations of Na$^+$ and K$^+$ ions were obtained from the aggregate size of particles using photon correlation spectroscopy. Secondly, colloidal silica can be stabilized by the adsorption of polymeric surfactant. The polymer used in this study was PVA with different molecular weights. The adsorbed amount of PVA reached the maximum adsorption at pH3 and decreased with rise in pH. However, the polymer yielded similar adsorption at pH3 and pH9 because the effective surface area of primary particles at pH9 was larger than that of flocculated particles at pH3. The adsorption of PVA of high molecular weight was saturated at lower concentration of polymer, which was due to the larger sites for hydrogen bonding between silica and PVA. Finally, the silica particles at pH2~3 were stabilized by adsorption of xanthan gum as a polyelectrolyte. Xanthan gum has been known for an effective stabilization in suspension and emulsion, but few investigations for real suspension and emulsion have been reported. Therefore, in this study, we investigated the stability of silica/xanthan gum system. Colloidal ...