Adsorption of octadecanol/1-pyrenenonanol mixed monolayers of insoluble surfactant onto Au(111): an electrochemical study

The adsorption of a mixed monolayer of octadecanol and pyrenenonanol onto a Au(111) electrode is presented. The adsorption was studied as a function of the relative components of each surfactant in the layer. The mixed layers were characterized using electrochemical methods for the metal ∣ solution (M ∣ S) interface and utilizing a Langmuir trough for the gas ∣ solution (G ∣ S) interface. The two components do not mix ideally at either interface as shown by measurements of the film pressure at the M ∣ S and G ∣ S interfaces. The presence of either component disrupts the organization of the layer at the M ∣ S interface, resulting in an increase in interfacial capacitance. This is also observed at the G ∣ S interface as a shift in the limiting mean molecular area, which is not linear with the mole fraction of either component. The deviation from ideality is similar for both interfaces even though the film pressure measured at the M ∣ S interface for pyrenenonanol is much larger than that measured for the G ∣ S interface. The correspondence between the two interfaces suggests that the state of the adsorbed layer on the M ∣ S interface was controlled by the initial formation of the layer at the G ∣ S interface, even after multiple cycles of potential induced desorption and readsorption of the layer.