Stabilization of water soluble surfactants at the air-water interface with Gegenion complexation

In this study the stability of Langmuir layers of tetradecyl benzoic acid (TDBA) interacting with poly(ethyleneimine) (PEI) in the subphase is analyzed as a function of PEI concentration, PEI molecular weight and surface pressure. Stable Gegenion complex, Langmuir monolayers were obtained in the PEI concentration range of 0.75 to 1.5 mM PEI monomer, and the surface pressure vs. specific area isotherms were independent of the PEI concentration. The isotherms were expanded indicating significant molecular interactions between TDBA and PEI. No effect of the molecular weight of PEI on the isotherms was observed. Outside this concentration range the Gegenion complex, Langmuir monolayers were either unstable or the isotherms were dependent on the PEI concentration. The structure of the Gegenion complex, Langmuir-Blodgett films were investigated using IR spectroscopy.