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.