The zeta potential of a colloidal solution of iron oxide nanoparticles varies with pH due to the changes in surface charge of the nanoparticles as a result of protonation and deprotonation reactions occurring at the surface. Iron oxide nanoparticles typically consist of either magnetite Fe3O4 or hematite -Fe2O3 , and their surface chemistry is influenced by the pH of the surrounding solution.At low pH values, the surface of iron oxide nanoparticles is protonated, resulting in a positive surface charge. As the pH increases, the surface becomes deprotonated, leading to a decrease in the positive surface charge. At a certain pH value, known as the isoelectric point IEP or point of zero charge PZC , the surface charge becomes neutral, and the zeta potential is zero. Beyond the IEP, the surface becomes negatively charged due to further deprotonation, and the zeta potential becomes increasingly negative with increasing pH.For magnetite Fe3O4 nanoparticles, the IEP is typically around pH 6-7, while for hematite -Fe2O3 nanoparticles, the IEP is generally higher, around pH 8-9. However, these values can vary depending on factors such as particle size, shape, and surface functionalization.In summary, the zeta potential of a colloidal solution of iron oxide nanoparticles varies with pH due to changes in surface charge resulting from protonation and deprotonation reactions. The zeta potential is positive at low pH values, negative at high pH values, and zero at the isoelectric point, which depends on the specific type of iron oxide and other factors.