pH Dependent Electronic and Geometric Structures at the Water–Silica Nanoparticle Interface

Electronic and geometric structures at the water-amorphous silica nanoparticle (NP) interface are determined as a function of suspension pH using a combination of X-ray photoelelectron spectroscopy (XPS) from a liquid microjet, solid-state nuclear magnetic resonance (NMR), and density functional theory (DFT). We provide direct spectroscopic evidence of the existence of (de)­protonated silanol groups at the liquid–NP interface and give a microscopic description of the interface structure. The (de)­protonated silanol groups, Si–OH₂⁺ and Si–(OH)­(OH₂⁺) in acidic suspension and Si–O^– and Si–(OH)­(O^–) in basic, give rise to well-resolved peaks in the Si 2p spectra that allow their identification and subsequent assignment by DFT. The change in surface potential at the silica NP surface as a function of pH can be directly measured by XPS and allows for an estimate of the fraction of silanol groups that become (de)­protonated at the pH of the experiments. In agreement with DFT calculations, NMR is unable to directly identify the (de)­protonated silanol species. DFT calculations, including solvent effects indicate that protonation of bridging O atoms can compete with protonation of silanol groups, and that (de)­protonation strongly affects the local geometry and stability of the silica network.