Laser reflectometry: Examination of the influence of some assumptions and approximations to calculate the adsorption from the reflectometric signal

The adsorption density of a polymer was calculated from the output of a reflectometric instrument, using the model described by Dijt et al. taking into account a step layer profile of the adsorbed film, in order to weight the influence of several experimental parameters (refractive index and thickness of the substrate, concentration of the polymer solution) and hypothesis (thickness of the adsorbed layer, linear dependence of the output on the surface excess). Calculated data were backed with measurements in an impinging jet cell, using two adsorption systems: the cationic poly(allyl amine, HCl) on SiO2 and the anionic poly(styrene sulfonate) on a polyamide 6-6 covered silicon substrate. We established the limit of a linear response of the reflectometric signal versus adsorption density, which depends on the refractive index of the substrate and on the concentration of the polymer solution. Calculations showed that an increase in the assumed adsorbed film thickness between 5 and 20 nm caused a decrease of about 13% in the calculated surface excess at the plateau of the isotherm for PSS. At concentrations of the polymer solution above 1 or 2 g L−1, we found experimental circumstances where the reflectometric output was negative or null, although adsorption was taking place. This was an effect of the high refractive index of the solution.