Since IONP is neutral the zeta potential value is higher 12.5mV which indicates stability. In MIONP, Nigella sativa seed extract is acidic (negatively charged) and the pH of methanol depends on the concentration of methanol (pH increases as the concentration increases) and the drying of the nanoparticles in a hot air oven could have decreased pH (increase in temperature reduction in pH ), would have led to reduction its zeta potential to 5.5mV.(pH in Methanol, no date)