The zeta potential of silica nanoparticles is highly dependent on the pH of the surrounding medium. The zeta potential is a measure of the electrostatic potential at the slipping plane shear plane of a particle, which is the boundary between the particle's surface and the surrounding liquid. It is an important parameter in determining the stability of colloidal dispersions, such as suspensions of silica nanoparticles.The surface of silica nanoparticles is covered with silanol groups Si-OH , which can ionize in aqueous solutions, depending on the pH. At low pH values, the silanol groups are protonated Si-OH2+ , resulting in a positive zeta potential. As the pH increases, the silanol groups start to deprotonate Si-O- , leading to a negative zeta potential. The pH at which the zeta potential is zero is called the isoelectric point IEP of the silica nanoparticles, which is typically around pH 2-3.The effect of pH on the zeta potential can be accurately measured using electrophoretic mobility. Electrophoretic mobility is the velocity of a charged particle in an electric field, and it is directly proportional to the zeta potential. To measure the electrophoretic mobility, a suspension of silica nanoparticles is placed in a cell with electrodes, and an electric field is applied. The movement of the particles is then monitored, typically using a technique called laser Doppler velocimetry.To determine the effect of pH on the zeta potential, the electrophoretic mobility measurements are performed at different pH values. The pH of the suspension can be adjusted using appropriate buffers or by adding small amounts of acid or base. The zeta potential can then be calculated from the electrophoretic mobility using the Henry equation:Zeta potential = f a * * Electrophoretic mobility / where f a is Henry's function which depends on the particle's size and the thickness of the double layer , is the viscosity of the medium, and is the dielectric constant of the medium.By plotting the zeta potential as a function of pH, one can observe the effect of pH on the zeta potential of silica nanoparticles and determine the IEP. This information can be used to optimize the stability of colloidal dispersions and to control the interactions between silica nanoparticles and other components in various applications, such as drug delivery, coatings, and sensors.