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

•Internal reflection ellipsometry is capable of detecting ions adsorbed at solid–liquid interfaces.•Ellipsometric angles are converted to adsorbed amount of ions.•Theoretical calculation agrees with experimental results verifying this technique.

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.