Mixed Polyelectrolyte-Surfactant Langmuir Monolayers at the Air/Water Interface

Polystyrenesulfonate acid (PSS) and alkyltrimethylammonium bromide (C_nTAB, n = 8, 14, or 18) were dissolved in a chloroform/methanol solution and cospread on an air/water interface. The surfactant−polyelectrolyte interaction leads to the formation of hydrophobic complexes which are able to spread well at the air/water interface. The effects of surfactant chain length and surfactant/polymer ratio on the characteristics of the mixed monolayers were studied in terms of surface pressure−area (π−A) isotherm, area relaxation, and hysteresis behavior as well as the surface morphology and composition of the corresponding Langmuir−Blodgett films. The mixed monolayers prepared by cospreading method are also compared with the complex monolayers prepared by preprecipitation of the surfactant−polyelectrolyte complexes from an aqueous solution. The experimental results show that the chain length of an incorporated surfactant is the main factor determining the properties of a complex monolayer. By using a longer chain surfactant to complex with polyelectrolyte, a more condensed monolayer with higher collapse pressure and stability can be obtained. For the effect of surfactant/polymer ratio (S/P), it is found that an increase of S/P ratio not only produces more complexes capable of staying at the air/water interface but also affects the incorporation of uncomplexed surfactant into the mixed monolayer. The X-ray photoelectron spectroscopy (XPS) analysis shows that the amount of uncomplexed surfactant is higher at low S/P value (0.2) and is insignificant when the S/P value increases to about 1.0 or 2.0, where a maximum amount of complexes were formed at the interface. A further increase of S/P ratio may cause additional incorporation of uncomplexed surfactant and/or micellization of surfactant around PSS cores, depending on the surfactant chain length. A model illustrating the incorporation and spreading of the surfactant−polyelectrolyte complexes at the air/water interface was proposed.