Quantifying Specific Ion Effects on the Surface Potential and Charge Density at Silica Nanoparticle-Aqueous Electrolyte Interfaces

The surface charge density and surface potential of oxide surfaces are to a large extent regulated by the co(-) and counterion distributions in the electrical double layer. Here we study the effect of different anions (Cl , Br , I , HCOO , and NO3 ) in sodium electrolytes and different cations (Li+, Na+, K+, and Cs+) in chloride electrolytes on the surface charge density and relative surface potential of colloidal nanometer sized silica (SiO2) as a function of electrolyte concentration and bulk pH using potentiometric titrations, attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) from a liquid microjet. Our results reveal that the identity of the anion has no significant effect on the nanoparticles surface charge density and relative surface potential at bulk pH where silica is negatively charged, consistent with textbook arguments of electrostatics and their exclusion from the region of the electrical double layer that regulates these properties. By contrast, the identity of the cation in solution is significant on both the surface charge density and surface potential. Specific cation effects are rationalized by their respective distance of closest approach to the nanoparticles surface-the hydrated cation diameter sets the outer Helmholtz plane and in turn the capacitance of the Stern layer.