Adsorption of nonionic surfactants onto polystyrene: Kinetics and reversibility

The reversible adsorption and desorption kinetics of nonionic surfactants at a hydrophobic surface have been studied by reflectometry. This enables the measurement of the adsorption isotherm with unprecedented accuracy. It is shown that the adsorption mechanism can be split into three processes. The surface is first covered by single monomers that experience a strong hydrophobic interaction. This is followed by a typically cooperative process leading to the formation of surface aggregates. At the saturation level, the surfactant molecules stagger in the interface such that there is a possible overlap of hydrophilic headgroups with hydrophobic tails. All this information comes directly out of the kinetics that can be modeled in terms of a local equilibrium.