Roughness Effects on the Surface Charge Properties of Silica Nanoparticles

The surface charge property of silica nanoparticles plays a key role in their function. Previous studies assumed surface charge as a homogeneously distributed constant material property, independent of the nanoparticle size and surface condition. Instead, this study considered surface chemistry as a function of local ionic conditions (Charge Regulation) to calculate the local surface charges around a rough nanoparticle, as an extension to our earlier study (J. Phys. Chem. C 2014, 118 (4), 1836-1842). For the current surface heterogeneity in the form of concave and convex circles, the surface charge showed a distinct local variation: decrease due to the electrical double layer (EDL) overlap in the valleys and increase due to curvature effects on the hills of the surface structure. The average of local surface charges decreased with the decrease of the roughness size (D-R), depending on the particle size (D-P) and pH. We characterized the variation of the average surface charge by a nondimensional group we formed as a measure for the EDL overlap and curvature effects [(D-R/lambda) x (D-R/D-P)]. Based on this, we devised a phenomenological model as an extension to the existing flat surface theory, which can successfully predict the average surface charge around a rough/patterned nanoparticle as a function of the particle size, roughness size, and pH.