Molecular Surface Chemistry Drives Anomalous Clustering of Ultrasmall Silica Nanoparticles

Silica nanoparticles are expected to remain stable at alkaline pH because deprotonated silanol groups lead to a strong electrostatic interparticle repulsion. Here, we show that this mean-field expectation fails for ultrasmall silica nanoparticles. Electron microscopy, dynamic light scattering, and small-angle X-ray scattering (SAXS) show that 9 nm silica nanoparticles form finite equilibrium clusters at pH 8.7, whereas larger particles remain dispersed under identical conditions. Analysis of SAXS profiles demonstrate that interactions between smaller nanoparticles cannot be described by typical screened electrostatics alone and require an additional short-range attractive contribution. Surface-sensitive measurements show that decreasing particle size increases hydroxylated silanol groups, alters interfacial charge density, and enhances the contribution of higher-pK_a proton-active sites. The cluster size reaches a maximum when pH approaches the higher-pK_a, identifying surface protonation state as a key descriptor. These results show that molecular surface chemistry drives pH-dependent attractions in ultrasmall silica nanoparticles beyond mean-field electrostatics.