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.