Methods to monitor the polyelectrolyte adsorption employing monolayers of ionic amphiphiles as model surfaces

Monolayers of dioctadecyldimethylammonium bromide (DODA) at the air/water interface were used as a model for a charged surface to study the adsorption of a poly(p-phenylene sulfonate) (PPPS). Utilizing the Fromherz technique it is possible to spread the amphiphile on a pure water surface and subsequently transfer it onto a polyelectrolyte solution. The ensuing adsorption process was monitored by recording the change in the surface pressure, the change in the reflectivity at the Brewster angle, or by UV/Vis reflection spectroscopy. The first two methods give curves having induction periods, in which no change of the monolayer properties can be observed. However, when UV/Vis reflection spectroscopy is used, no induction period is detected. The induction period is explained to be a result of structural rearrangement processes at the interface due to the adsorption which are a prerequisite for the change in surface pressure and Brewster angle reflectivity.