Influence of surface conductivity on the apparent zeta potential of amorphous silica nanoparticles

Abstract

Zeta potential is a physicochemical parameter of particular importance in describing ion adsorption and double layer interactions between charged particles. However, for metal-oxide nanoparticles, the conversion of electrophoretic mobility measurements into zeta potentials is difficult. This is due to their very high surface electrical conductivity, which is inversely proportional to the size of the particle. When surface conductivity is similar to or higher than the electrical conductivity of bulk water, it can significantly lower the electrophoretic mobility of the particles. It follows that the magnitude of the apparent zeta potential determined from the Smoluchowski equation (disregarding surface conductivity) can be grossly underestimated. We use a basic Stern model to describe the electrochemical properties and to calculate the true zeta potential of amorphous silica nanoparticles immersed in NaCl solution. The parameters of our surface complexation model are adjusted by potentiometric titration and electrophoretic mobility measurements at high salinity (10(-1)M NaCl). Electrophoretic mobilities are calculated using Henry's electrokinetic transport model with specific surface conductivities and zeta potentials estimated by our surface complexation model. The very good agreement of calculated and measured electrophoretic mobilities confirms that the true zeta potential corresponds to the electrical potential at the outer Helmholtz plane (OHP). Consequently, the shear plane might be located close to the OHP. The assumption of the presence of a stagnant diffuse layer at the amorphous silica/water interface is therefore not required.

Influence of surface conductivity on the apparent zeta potential of calcite

“…where a is the mean radius (in m) and s Σ is the specific surface conductivity (in S) of the crystals, which can be computed by an electrostatic surface complexation model [25,29,36]. Eq.…”

“…The specific surface conductivity (or conductance) s Σ represents the excess electrical conductivity integrated in the vicinity of a solid surface in reference to the electrical conductivity of bulk water [23,29,48]. The specific surface conductivity is due to the electromigration (superscript "e") of charged species and the associated electro-osmotic processes (superscript "os") at the solid-water interface [18,19,36]: The specific surface conductivity can be calculated as a function of the excess of charge and ionic mobilities at the solid surface [36,49].…”

“…[ ] Hydrophobic media possessing low dielectric permittivity like Teflon [52], air [27], or amorphous silica [29] are characterized by large hydration layers at their surface, which may allow the migration of hydrated protons (hydronium ions) along the particle surface via the hydrogen bonding network. We assume here that the calcite-water interface, also characterized by large hydration layers, allows the hydrated protons to move along the particle surface.…”

“…We assume here that the calcite-water interface, also characterized by large hydration layers, allows the hydrated protons to move along the particle surface. It is interesting to note that Holmes et al [53] considered the specific surface conductivity of ionizable surface hydroxyl groups at the surface of thorium oxide, and that Revil and co-workers [23,48], Zimmermann et al [52], and Leroy and co-workers [27,29] considered the contribution of protons to the specific surface conductivity of silica, Teflon, and air and silica, respectively. (23) and (24), the ion mobility, water dielectric permittivity, and viscosity in the diffuse layer are assumed to be equal to their values in bulk water, and d ϕ is the electrical potential at the beginning of the diffuse layer.…”

Modeling the evolution of complex conductivity during calcite precipitation on glass beads

“…(10), the tangential mobility of the counterions in the Stern layer of the calcite/water interface, β b , remains unknown (Ricci et al 2013). Cations are assumed to be mostly adsorbed in the Stern layer of calcite as outer-sphere surface complexes because, similarly to the silica/water interface (Sverjensky 2001;Vaudelet et al 2011;Leroy et al 2013), the large hydration layer at the calcite surface repels hydrated cations from the surface (Stipp 1999;Heberling et al 2011Heberling et al , 2014. Nevertheless, as pointed out by Wolthers et al (2008) and Ricci et al (2013), some Na + and Ca 2+ counter-ions may also be adsorbed as inner-sphere surface complexes.…”