ABSTRACT
Dilute mixtures of the water soluble polymer polyethyleneimine (PEI) and the cationic surfactant cetyltrimethylammonium bromide (CTAB) form mesostructured thin films at the air/solution interface. In this paper we show that these films form spontaneously, reaching an equilibrium composition. When the film is removed, a new solid film rapidly reforms, with a similar morphology when inspected by Brewster angle microscopy (BAM). Successive removal of films lead to a series of structurally similar films, until the amount of surfactant in solution approaches the lower limit of film forming concentration. The results obtained using surfactant-selective potentiometry suggest that the incipient polymer:surfactant aggregates are in a position to migrate to the surface rapidly after film removal, prior to mesostructure formation. X-ray reflectivity indicates that films formed at different PEI:CTAB compositions retain mesophase regular structures with the same d-spacing feature, equal to 52.2A. Grazing incidence diffraction measurements indicate that films are composed of small crystallites arranged in a random powder fashion, developing a rough surface morphology evidenced by BAM. The central finding is that PEI:CTAB films form when the amount of CTAB in solution is higher than a critical film formation concentration, very close to the critical aggregation concentration (CAC), allowing the formation of several equilibrated films from the same growing solution.
