The electrostatic structure at the interface is a major factor controlling the stability and chemical or photochemical reactivity of colloidal particles in aqueous solution. However, the details of the particle/aqueous interface are still poorly understood and its microscopic description often rely on the use of simple electrostatic models. Besides, in many cases the surface properties of a colloidal system cannot be inferred from studies of the corresponding planar surface. We recently showed the applicability of polarimetric angular-resolved second harmonic scattering (AR-SHS) to study the electrical double layer (EDL) of TiO2 and SiO2 particles of 100 nm-300 nm diameter. This nonlinear optical technique selectively probes the interfacial region with micromolar sensitivity. AR-SHS scattering patterns are used to determine both the particles surface potential as well as the interfacial water structure in a single experiment, without the use of labeling molecule nor a priori models for the electrostatic structure of the interface. Here we report the effect of NaCl, RbCl and CaCl2 on the surface potential and the surface susceptibility - describing average interfacial water orientation - of 100 nm SiO2 and TiO2 colloids. We show that these two parameters can be used to establish the ion adsorption mechanism in a low ionic strength regime (&lt; 1 mM added salt). At the lowest ionic strengths, all the investigated cations undergo inner-sphere adsorption, partially compensating the negative surface charge on SiO2 and TiO2 surfaces. With increasing ionic strength, the gradual formation of a hydrated condensed layer of charges can be inferred from our data. The relative differences between salts as a function of the ionic strength further 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. Our results complement the understanding of the EDL on a microscopic level by providing insight into ion adsorption at colloidal oxide surfaces with micromolar sensitivity.

Surface characterization of colloidal oxide nanoparticles with second harmonic scattering: Ion-specific effects

ACS Spring 2022

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