Size Dependent Surface Charge Properties of Silica Nanoparticles

Nanoparticle surface charge density plays an important role in many applications, such as drug delivery and cellular uptake. In this study, surface charge properties of silica nanoparticles with different sizes are studied using a multi-ion surface charge-regulation model. In contrast to most previous studies utilizing constant surface charge, protonation and deprotonation surface reactions are used to obtain the local surface charge, which depends on the particle size and electrolyte solution properties, including the salt concentration and pH. For a fixed particle size, the magnitude of the surface charge typically increases with an increase in pH or background salt concentration. For fixed background salt concentration and pH, the magnitude of surface charge decreases with an increase in the particle size and reaches a constant when the particle size exceeds a critical value. Size dependent surface charge is further characterized by the ratio of electrical double layer thickness to the particle diameter, and the surface charge varies significantly when this dimensionless ratio is above 0.2.