How does the addition of ammonia affect the properties of colloidal silica/silica sol?

The addition of ammonia plays a significant role in the synthesis and properties of colloidal silica/silica sol. Here's how it affects various properties:

pH Adjustment: Ammonia is commonly used as a pH-adjusting agent in the synthesis of colloidal silica/silica sol. By adding ammonia to an acidic silica solution, the pH is raised to alkaline conditions, typically above pH 8. This alkaline environment promotes the formation of stable colloidal silica particles.
Particle Size and Stability: The addition of ammonia helps to control the nucleation and growth of silica particles during the sol-gel process. It facilitates the hydrolysis and condensation reactions of silica precursors, leading to the formation of uniform colloidal particles with controlled sizes and distributions. The alkaline pH also stabilizes the colloidal suspension by promoting electrostatic repulsion between particles and preventing aggregation or coagulation.
Surface Charge: Ammonia affects the surface charge of colloidal silica particles by altering the ionization state of surface silanol groups (-Si-OH). At alkaline pH, ammonia reacts with silanol groups to form ammonium silicate species (-Si-O-NH4+), which impart a positive charge to the particle surface. This surface charge influences the stability, dispersion, and interactions of colloidal silica particles in solution.
Viscosity and Rheology: The addition of ammonia can impact the viscosity and rheological behavior of colloidal silica/silica sol. Alkaline conditions promote the formation of more extended silica networks through increased polymerization and cross-linking of silica particles. This results in higher viscosity and thicker sols, which may have implications for processing and application.
Gelation and Aging: Ammonia influences the gelation and aging behavior of colloidal silica sols. By adjusting the pH with ammonia, sol-gel transitions can be controlled, leading to the formation of gels with desired properties such as porosity, pore size distribution, and mechanical strength. Additionally, ammonia can affect the kinetics of aging processes, including particle aggregation, restructuring, and phase separation, which may impact the long-term stability and properties of the sol.
Overall, the addition of ammonia to colloidal silica/silica sol systems plays a crucial role in controlling pH, particle size, stability, surface charge, rheology, gelation, and aging behavior, ultimately influencing the properties and performance of the sol in various applications.