Structure of the Electrical Double Layer at the Silica Nanoparticle-Electrolyte Water Interface

Abstract

Mineral-oxide particles exhibit an amphoteric nature in aqueous solution due to ionization of surface hydroxyl groups (). The magnitude of ionization is a collective outcome of factors, such as the speciation of surface hydroxyl groups, composition and size of the particle, pH of the solution and electrolytes present in the solution. The charge distribution on and around the surface of a particle in solution affects the reactions taking place at this solid-liquid interface. The charged surface, together with the ions near the interface makes up the electrical double layer (EDL). Due to its direct implications on surface reactivity, the EDL is significant for investigations in catalysis, colloidal science, energy-storage devices, ion adsorption and toxicology. Despite the attention given to it, the exact structure of the EDL is still debatable. One of the reasons for this is the lack of a direct measurement of surface properties such as the surface potential and acid dissociation constant of the (weak) acidic surface hydroxyl groups (). This thesis uses a combination of experimental techniques and a modeling approach to understand the EDL for the silica nanoparticle (np)–electrolyte water interface. The abundance of the silica-water interface in the environment and the relatively simple surface reactions (compared to other mineral oxides) make silica an ideal choice for such EDL investigations. Potentiometric titrations (PT) are the core of the experimentation along with electrokinetic (EK) measurements, in this thesis. PT is an important and well-understood technique for the determination of surface charge density (SCD), which constitutes the EDL. Electrokinetic measurements, on the other hand, provide information on the charge cloud associated with the charged particle. The presence of electrolytes affects the SCD and restructures the EDL. Experimentally obtained charge density data is often used as input to carry out surface complexation modeling (SCM) to interpret the EDL at the silica np-electrolyte water interface. This modeling procedure is greatly simplified by using previously estimated Stern-layer capacitances as constraints in the modeling approach. Here, the Stern layer capacitances used were calculated from surface-potential estimates that were recently measured by our group using liquid-jet X-ray photoelectron spectroscopy (LJ-XPS). In this thesis, we see how a well-constrained SCM allows for the estimation of the electrolyte binding () constant and the (negative logarithm of the acid dissociation constant) for the terminal silanol groups (). A long-standing view in the colloid community is that the SCD of silica is directly influenced by the absolute concentration of electrolyte present in the solution. While this is true, it must be remarked that detailed investigations presented in this thesis suggest that the SCD of colloidal silica is directly influenced by the ratio of counterions to the surface silanol groups () and not solely on the absolute electrolyte concentration.