The zeta potential of a dispersed system is a key parameter that influences the stability of colloidal dispersions, such as suspensions, emulsions, and foams. It is a measure of the electrostatic potential at the slipping plane, which is the boundary between the stationary layer of fluid attached to the dispersed particle and the bulk fluid that surrounds it. The zeta potential is affected by various factors, including the pH of the system, the concentration of ions, and the nature of the dispersing medium.The effect of pH on the zeta potential of a dispersed system can be understood by considering the relationship between pH, surface charge, and zeta potential. The surface charge of a particle in a dispersed system is determined by the adsorption of ions on its surface. When the pH of the system changes, the degree of ionization of functional groups on the particle surface also changes, which in turn affects the surface charge.A specific example from surface chemistry to illustrate this relationship is the case of metal oxide particles, such as titanium dioxide TiO2 or silica SiO2 , dispersed in an aqueous medium. These particles have hydroxyl groups OH on their surfaces, which can ionize depending on the pH of the system:1. At low pH acidic conditions , the surface hydroxyl groups can be protonated, resulting in a positively charged surface: TiO2-OH + H+ TiO2-OH2+ SiO2-OH + H+ SiO2-OH2+2. At high pH alkaline conditions , the surface hydroxyl groups can be deprotonated, resulting in a negatively charged surface: TiO2-OH TiO2-O- + H+ SiO2-OH SiO2-O- + H+3. At the isoelectric point IEP , the surface charge is neutral, and the zeta potential is zero. The IEP is the pH at which the positive and negative charges on the particle surface are equal.As the pH of the system changes, the surface charge of the metal oxide particles changes, which in turn affects the zeta potential. When the zeta potential is high either positive or negative , the electrostatic repulsion between particles is strong, and the dispersed system is more stable. However, when the zeta potential is close to zero near the IEP , the electrostatic repulsion is weak, and the particles are more likely to aggregate, leading to instability in the dispersed system.In summary, the pH of a dispersed system has a significant effect on the zeta potential and the stability of the system. By understanding the relationship between pH, surface charge, and zeta potential, it is possible to control and optimize the stability of colloidal dispersions in various applications, such as drug delivery, coatings, and water treatment.