The Zeta potential of colloidal particles is a measure of the electrostatic repulsion or attraction between particles in a colloidal system. It plays a crucial role in determining the stability of colloids, as it influences the degree of aggregation or dispersion of particles. The Zeta potential is affected by various factors, including the pH and ionic strength of the surrounding medium.1. Effect of pH on Zeta potential:The pH of the medium affects the Zeta potential of colloidal particles by altering the surface charge of the particles. This occurs due to the ionization or deprotonation of functional groups present on the particle surface. As the pH changes, the degree of ionization of these functional groups also changes, leading to a change in the surface charge and, consequently, the Zeta potential.For example, consider a colloidal particle with acidic functional groups e.g., -COOH on its surface. At low pH, these groups will be protonated -COOH , resulting in a positively charged surface and a positive Zeta potential. As the pH increases, the acidic groups will deprotonate -COO- , leading to a negatively charged surface and a negative Zeta potential. The pH at which the Zeta potential is zero is called the isoelectric point IEP .A study by Kosmulski et al. 2001 investigated the Zeta potential of metal oxide particles such as alumina, titania, and silica as a function of pH. They found that the Zeta potential of these particles changed significantly with pH, with the isoelectric point occurring at different pH values for different metal oxides.2. Effect of ionic strength on Zeta potential:The ionic strength of the medium also influences the Zeta potential of colloidal particles. As the ionic strength increases, the concentration of ions in the medium also increases. These ions can interact with the charged surface of the colloidal particles, leading to a decrease in the magnitude of the Zeta potential.The presence of counterions ions with opposite charge to the particle surface in the medium can cause a phenomenon called "charge screening." This occurs when counterions surround the charged particle surface, reducing the effective charge of the particle and, consequently, the Zeta potential. As the ionic strength increases, the charge screening effect becomes more pronounced, leading to a decrease in the magnitude of the Zeta potential.A study by Zhang et al. 2007 investigated the effect of ionic strength on the Zeta potential of polystyrene latex particles. They found that as the ionic strength of the medium increased, the magnitude of the Zeta potential decreased, indicating the charge screening effect.In summary, the Zeta potential of colloidal particles is influenced by both the pH and ionic strength of the surrounding medium. Changes in pH can alter the surface charge of particles by affecting the ionization of functional groups, while changes in ionic strength can lead to charge screening effects. Understanding these relationships is essential for predicting and controlling the stability of colloidal systems in various applications.References:1. Kosmulski, M., Gustafsson, J., & Rosenholm, J. B. 2001 . The zeta potentials of some metal oxides in aqueous solutions of high ionic strength. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 195 1-3 , 153-162.2. Zhang, Y., Chen, Y., Westerhoff, P., Hristovski, K., & Crittenden, J. C. 2007 . Stability of commercial metal oxide nanoparticles in water. Water Research, 42 8-9 , 2204-2212.