Second Harmonic Scattering Reveals Ion-Specific Effects at the SiO₂ and TiO₂ Nanoparticle/Aqueous Interface

Ion-specific effects play a crucial role in controlling the stability of colloidal systems and regulating interfacial processes. Although mechanistic pictures have been developed to explain the electrostatic structure of solid/water colloidal interfaces, ion-specific effects remain poorly understood. Here we quantify the average interfacial water orientation and the electrostatic surface potential around 100 nm SiO₂ and TiO₂ colloidal particles in the presence of NaCl, RbCl, and CaCl₂ using polarimetric angle-resolved second harmonic scattering. We show that these two parameters can be used to establish the ion adsorption mechanism in a low ionic strength regime (<1 mM added salt). The relative differences between salts as a function of the ionic strength demonstrate cation- and surface-specific preferences for inner- vs outer-sphere adsorption. Compared to monovalent Rb⁺ and Na⁺, Ca²⁺ is found to be preferentially adsorbed as outer-sphere on SiO₂ surfaces, while a dominant inner-sphere adsorption is observed for Ca²⁺ on TiO₂. Molecular dynamics simulations performed on crystalline SiO₂ and TiO₂ surfaces support the experimental conclusions. This work contributes to the understanding of the electrostatic environment around colloidal nanoparticles on a molecular level by providing insight into ion-specific effects with micromolar sensitivity.