Surface activity and flocculation behavior of polyethylene glycol-functionalized silica nanoparticles

Colloidal silica nanoparticles have been functionalized with methyl polyethylene glycol silane (mPEG silane) and the PEGylated particles have been characterized with focus on exploring their surface chemical properties. The degree of surface functionalization was quantified using NMR diffusometry, and the measurements showed that the silane binds covalently to the silica surface. Samples with surface coverages ranging from 0.068 to 0.315 μmol silane/m(2) have been analyzed. The functionalized particles proved to be surface active and showed a significant reduction in surface charge and zeta potential with increasing degree of PEG functionalization. All samples showed colloidal stability at neutral pH and above within the range studied. At lower pH, the samples with low surface coverage displayed a reversible flocculation behavior, while samples with a high surface coverage and samples without functionalization remained stable. This suggests that steric stabilization is effective at low pH when the surface coverage is high enough; electrostatic stabilization is effective for samples without functionalization; and that inter-particle PEG-silica interactions cause flocculation of particles with too low degrees of PEG functionalization.