Detection of ion adsorption at solid-liquid interfaces using internal reflection ellipsometry

We use imaging internal reflection ellipsometry (IRE) in combination with a microfluidic device to study the adsorption of inorganic salt ions to silica-water interfaces. In our data analysis, the measured polarization-dependent reflectivity is compared to calculations from a layer stack model, where the electric double layer is modeled as a separate layer. Due to the high resolution of our technique, we are able to quantify the adsorption of Na⁺ and Ca²⁺ ions from aqueous solutions of their chloride salts as a function of their bulk concentrations at pH 3 and 10. Our measurements demonstrate a preferential adsorption of Ca²⁺ counterions. The experimental results are well described by calculations using a triple layer surface complexation model for the electric double layer with published equilibrium constants.