The zeta potential of a colloidal solution is a measure of the electrostatic potential at the slipping plane, which is the boundary between the immobile fluid attached to the particle surface and the mobile fluid in the bulk solution. It is an important parameter in determining the stability of colloidal dispersions, as it provides information about the repulsive or attractive forces between particles. The zeta potential is influenced by factors such as pH and ionic strength of the solution.1. Effect of pH:The zeta potential of a colloidal solution is highly dependent on the pH of the solution. This is because the surface charge of the colloidal particles is determined by the ionization or adsorption of functional groups on the particle surface. As the pH of the solution 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, in the case of metal oxide particles, the surface can have acidic e.g., hydroxyl or basic e.g., oxide functional groups. At low pH, the acidic functional groups are protonated, resulting in a positive surface charge and a positive zeta potential. As the pH increases, the acidic functional groups become deprotonated, and the basic functional groups become ionized, leading to a negative surface charge and a negative zeta potential. The pH at which the zeta potential is zero is called the isoelectric point IEP .Experimental evidence: A study by Kosmulski et al. 2002 investigated the zeta potential of various metal oxide particles as a function of pH. They found that the zeta potential changed from positive to negative values as the pH increased, with the IEP depending on the specific metal oxide.2. Effect of ionic strength:The ionic strength of a solution also affects the zeta potential of colloidal particles. As the ionic strength increases, the thickness of the electrical double layer EDL surrounding the particles decreases due to the increased screening of surface charges by counterions. This results in a decrease in the zeta potential, as the potential at the slipping plane is reduced.Furthermore, increased ionic strength can lead to the compression of the EDL, which can cause a shift in the slipping plane, further affecting the zeta potential. In general, an increase in ionic strength leads to a decrease in the absolute value of the zeta potential, which can result in reduced repulsive forces between particles and decreased colloidal stability.Experimental evidence: A study by Zhang et al. 2008 investigated the effect of ionic strength on the zeta potential of TiO2 nanoparticles. They found that as the ionic strength of the solution increased, the absolute value of the zeta potential decreased, indicating a reduced electrostatic repulsion between particles.In summary, the zeta potential of a colloidal solution is influenced by both pH and ionic strength. Changes in pH affect the ionization of functional groups on the particle surface, leading to changes in surface charge and zeta potential. Changes in ionic strength affect the thickness and structure of the electrical double layer surrounding the particles, leading to changes in the zeta potential and colloidal stability.