Size-dependent interactions of silica nanoparticles with a flat silica surface

We have investigated the surface chemistry of SiO2 nanoparticles (NPs) with different sizes and their corresponding interactions with a flat substrate of surface curvature ∼0. As the size of the NPs increases, the SiO2 surface is increasingly covered with H-bonded silanol groups, thereby increasing the ζ-potential and shifting the isoelectric point higher in pH. Interactions between the SiO2 NPs and the flat SiO2 surface were analyzed in situ using quartz crystal microbalance with dissipation (QCM-D) method, and the results were interpreted based on an extended Derjaguin-Landau-Verwey-Overbeek theory. At very low ionic strength (1mM NaCl), there was no particle adsorption onto the surface due to the highly repulsive energy barriers to this interaction. On the other hand, QCM-D results showed that the significant adsorption of SiO2 NPs onto a flat SiO2 surface occurred under conditions of high ionic strength (100mM NaCl). Interestingly, the adsorption behaviors of three different-sized SiO2 NPs on the surface varied considerably with size. SiO2 NPs with small size have high adsorption affinity with the flat SiO2 surface due to an extremely low energy barrier for the interactions, whereas relatively large SiO2 NPs have very weak adsorption affinity with the flat surface due to the repulsive energy barrier formed by the increase in the electrostatic and hydration repulsion energy.